Onion diseases and their control

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FARMERS' BULLETIN 1060

UNITED STATES DEPARTMENT OF AGRICULTURE

'W^f'» THEIR

Control

ONION SMUT kills the young seedlings in the
spring, is common in old onion sections, but can
he controlled successfully by applying a formalde-
hyde solution in the row with the seed by means of
a drip attachment on the seeder.

Onion mildew blights the leaves in midscason, but
can be held in check by rotation of crops, good drain-
age, thorough cultivation, and by spraying the foliage
thoroughly with Bordeaux mixture before the dis-
ease becomes widespread in the field.

Storage rots — smudge, ueck-rot, soft-rot, black-
mold — are controlled by protecting the crop from
moisture during and after harvest and by facilitating
as rapid and thorough curing as possible.

Bruising due to careless handling, topping, or mill-
ing should he avoided.

For storage a well-ventilated warehouse kept at
32° to 35° F. is preferable.

Contribution from Iho Bureau of Plant Industry
WM. A. TAYLOR, Chief
Washington. D. C. IS'ovcnilior, 1919

2

ONION DISEASES AND THEIR CONTROL.

J. C. Walker,

Assistant Pathologist, Office of Cotton, Truck, and Forage Crop Disease

Investigations.

CONTENTS.

Page.

Scope of this bulletin 8

Descriptive key to onion diseases 8

Diseases primarily Important In the

Held 4

Smut ; 4

Mildew (blight) 0

Leaf-mold 12

Fusarlum rot 13

l'lnk-root 13

Itoot-knot 14

Tage.

Farm practice in relation to storage

diseases 15

Diseases primarily important in stor-
age and transit 18

Neck-rot 18

Soft-rot 20

Mack-mold 21

Smudge (anthracnose black-
spot) 22

Summary of control measures 23

SCOPE OF THIS BULLETIN.

THE ONION INDUSTRY of the United States is widely dis-
tributed and of increasing importance. The chief producing
centers may be grouped into two hirge divisions, namely (1) those
in the northern and central tier of States, extending from Massachu-
setts to the Pacific coast, which produce principally a late summer or
fall erop, and (2) those in the souhern tier of States, extending
from Florida to southern California, in which a winter or spring
crop is grown. The wido range of soil and climatic conditions in
the different sections makes difficult a general discussion of onion
diseases which may apply to all regions. This bulletin is written
primarily from the standpoint of onion growing in the North Cen-
tral and Northeastern States, viz, from Iowa to Massachusetts.
Recommendations in general, however, will apply to other sections,
and at certain points special reference will be made to diseases in
the South and AVest.

DESCRIPTIVE KEY TO ONION DISEASES.

Since certain diseases of onions resemble one another rather closely,
the following descriptive key will aid in their prompt recognition.

A. Diseases primarily Important In the field.

1. Dark pustules appear within the leaves or scales and may later
split open, exposing black, powdery masses, principally on the young
seedlings. (See flg. 1) Smut, page 4

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' Bulletin 1060.

A. Diseases primarily Important in the Hold — Continued.

2. The leaves turn pale green and yellowish, become covered with
a violet furry growth, nnd finally collapse; most serious In moist
weather In mldseason or later. (See fig. 5) Mildew, page 9

3. A black, moldy growth on leaf tips or seed stalks, often follow-
ing mildew Leaf-mold, page 12

4. A rapid dying back from the lips of the leaves, accompanied

by rotting from the base of the bulb. (See fig. 0) Fusarlum rot, page 13

5. The roots turn pink In color and die; new roots are attacked as

they develop, resulting in a marked stunting of the plant— Pink-root, page 13

0. The leaves become a sickly green ; swellings form on the roots.

(See fig. 7) Uoot-knot, page 14

15. Diseases primarily Important In storage and transit.

1. A rot begins at the neck of the bulb and progresses downward ;
the tissue shrinks and collapses; a gray to brown moldy growth and
hard, bhiek kernels later appear on the surface of affected scales.

(See figs. 8 and 0) Neck-rot, page 18

2. A rot begins at harvest time or later, but differs from neck-rot In
that It is softer and more watery, usually with a very offensive odor.

(See fig. 10) Soft-rot, page 20

3. A semhvMtcry rot advancing from the base of the scale upward.

(See fig. 0) Fusarlum rot, page 13

4. TJiack, powdery masses form, not in definite pustules within the
scales, as in smut, but on the outer surface of or between the scales.

(See fig. 11) Black-mold, page 21

5. Smudgy, superficial black spots made up of fine dots, but with
no powdery masses, appear shortly before harvest time on the outer
scales; primarily on white varieties. (See fig. 12) Smudge, page 22

In addition to these specific diseases there are certain other troubles,
due to insects, with which they are liable to be confused.

(1) Thrlps Injury: Small, white, chafed spots appear on the leaves and cause
them to die prematurely; the minute pale-yellow thrlps which cause the injury
are commonly present.

(2) Onion maggots often kill seedlings by feeding upon the roots. Tlds
Injury is sometimes confused with onion smut, but the absence of pustules of
black powder distinguishes it from the latter.

DISEASES PRIMARILY IMPORTANT IN THE FIELD.

SMUT.

Onion smut appeared in the United States nearly 50 years ago,
when it was found to be doing damage in the Connecticut River
valley, and has spread into most of the large northern onion sections,
including Xew York, New Jersey, Ohio, Wisconsin, Illinois, Iowa,
sind Oregon. Whan once introduced, it becomes more serious each
year the crop is replanted on the same soil and spreads slowly to ad-
joining areas.

CHARACTERISTICS OF SMUT.

The disease appears soon after the seedlings como above ground.
Brown to black elongated blisters form within the scales or leaves,

Onion Diseases and Their Control.

5

the latter usually being- slightly thickened mid often curved down-
ward abnormally. Not uncommonly the leaf splits, exposing n pow-
dery black mass of spores in the interior of the blister (fig. 1). Many
of the young seedlings are so severely attacked that they die within
three to five weeks after germina-
tion. In others, which may sur-
vive until midseason or harvest, the
new leaves and scales continue to
he attacked as they develop. A
very few outgrow the disease. Al-
though most of the infected bulbs
are so small and imperfect that they
are thrown out at harvest time, oc-
casional ones are of sufficient size
to escape notice and thus reach the
warehouse or market. Such speci-
mens are characterized by the
slightly raised brown to black pus-
tules most prevalent near the base of
the bulbs and usually occurring as
deeply as the third or fourth scale.
Smut does not cause a storage rot,
but smutted bulbs shrink more
rapidly and are more subject to the
attack of other organisms than
healthy ones.

CAUSE OF SMUT.

Smut is due to a fungous para-
site (Vrocystis cepulae) which at-
tacks only the onion and certain
closely related speeies of plants.
The black, powdery masses which
are exposed upon the splitting of
the blisters consist of myriads of
spores, or seed bodies, which serve
to propagate the fungus. These
spores fall to the soil and over-
winter there, being very resistant to
cold. In the spring, at the time
when the onion seeds are germinating
and infect the young seedlings.

Fio. 1. — Onion smut. A young bulb
allowing unbroken blisters on scales
and leaves. Those later split opcu
and expose the black, powdery spore
masses.

the smut spores also germinate
Having established itself within the
young plant, the fungus continues to develop, taking its nourishment
from the onion seedling and again forming new spore blisters.

A peculiar and important fact about smut is that the onion plant
can be attacked by the fungus only while in the young seedling stage.
125150°— 1!)— Bull. 1000 2

G

Farmers Bulletin 1060.

After tho plants arc about 3 inches high they become immune to
further infection, and onion sets or onion seedlings 3 to 4 inches high
transplanted to smiit-infeeted soil will not contract the disease.

Smut spreads slowly in tho soil, hut an infested spot in a field will
gradually become larger and more severely diseased each year onions
are grown. Moreover, the spores are carried to other parts of tho
field on the farm implements, feet of men and animals, by surface
water, and in dust carried by tho air. It may also be carried into
new localities on onion sets containing the disease in their outer
scales. Purchasers of onion sets who arc not troubled with the smut
should bo on their guard lest they contaminate their soil by planting
smutted sets.

CONTROL OF SMUT.

Sanitary measures. — Where practicable, tho tops should be burned
after harvest, and caro should bo taken to avoid as much as possible

FlG. 2. — Onion seeder with a formaldehyde drip attachment for disinfecting the soil
at tho time ot seeding, to control onion smut. The cut-off valvo Is regulated from
tho handle. (See text for description.)

tho returning of infected onion refuse to the soil. The spreading on
the fields of waste from onion warehouses is a bad practice, since it
may contain smut or other diseases which may thus be introduced
into healthy soil.

Use of onion sets. — Where green onions are grown for early spring
markets or where an early crop of bulbs is desired, onion sets aro
commonly used. Since plants grown from sets arc not attacked by
smut, this method of propagation can be used with success on smut-
infested soil.

Onion Diseases and Their Control.

7

Use of onion seedlings from healthy soil. — Within recent years
northern market gardeners have to some extent practiced starting
onion seedlings in the hotbed or greenhouse and transplanting to
the field when 3 or 4 inches high. On southern truck farms it is tha
general custom to sow the seed in beds in the fall and transplant
during the winter. Where this method is profitable, onion smut can
be avoided by growing seedlings on healthy soil until 3 or 4 inches
high and then transplanting to the fields.

Use of formaldehyde drip. — In the large commercial onion dis-
tricts of the Northern States, where the use of sets or seedlings is

Fig. 3. — Field of onions on smut-Infested soil where formaldehyde was applied. Tho
disease has killed a large percentage of the plants in the two untreated rows In
the center, while the remainder of the field has practically a full stand.

impracticable, advantage is taken of the fact that onion smut can be
controlled by the application of a solution of formaldehyde in the
furrow with the seed. Since the young onion seedling is susceptible
to the disease for only a short time, it is necessary to use only enough
of tho disinfectant to keep the fungus in cheek in the soil close to
the seed.

Apparatus. — The apparatus for applying the formaldehyde solu-
tion consists of a galvanized iron tank with a pipe leading down to tho
furrow (fig. 2) . It can be made by any tinner. Such an attachment is
offered for sale by certain manufacturers of onion seeders. The size of
the tank will vary with the type of seeder used, but for the ordinary

8

Farmers' Bulletin 1060.

single-row seeder a 2-gallon tank is a sufficient load. It is best placed
just behind the seed box, where the extra weight will rest mainly on
the packing wheel. The pipe should be so arranged as to run tho
solution directly into the center of the furrow just before the packing
wheel closes it, and to lead it far enough down to prevent splashing
on the shoe or the packing wheel. It is essential to have a cut-off
valve in the pipe which can be controlled from the handle of tho
seeder, thus enabling the operator to shut off the flow quickly at
the end of tho row or whenever desirable. There should be about
n five-sixteenths inch flow of liquid from the tank, in order to sccuro

the proper quantity
of 200 gallons per
aero. In certain lo-
calities the attach-
ment has been modi-
fied successfully for
uso with the 8-row
gang seeder.

Strength and quan-
tity of the solution.—
Great care should bo
used in applying this
treatment, since too
strong a formalde-
hyde solution will pre-
vent tho onion seed
from germinating,
while too weak a so-
lution will not hold
tho fungus in check.
It is advantageous to
increase the strength and decrease the quantity of tho liquid as much
as possible, in order to reduce the labor of application, but this can
he done only up to a certain limit.

To each gallon of water use i fluid ounce of 37 to 40 per cent
formaldehyde solution, sometimes known commercially as formalin. 1
Apply this diluted solution at the rate of 200 gallons per acre, or 1
gallon to about 185 feet of row.

Whore several rows of seed arc sown close together for the pro-
duction of onion sets, the quantity of solution applied should bo
increased accordingly.

Cost of application. — About 12 pounds of tho 40 per cent formal-
dehyde solution arc required for each acre, which, when purchased

Via. 4. — Yield of a treated and an untreated row of onions
in the field shown In figure M. The treated rows aver-
aged 543 bushels and the untreated ones 200 bushels per
acre.

1 For larger quantities, 1 pint to 10 gallons gives the same dilution.

Onion Diseases and Their Control.

9

in wholesale quantities, can bo secured at about 25 cents per pound
(1919 prices). When a single-row seeder is used, the amount of
labor required at the time of sowing is about doubled, bringing the
entire cost of application to

about $5 per acre.
3 and 4.)

(Sec figs.

MILDEW (BLIGHT),

The first symptoms of onion
mildew, or blight, may be
found by examining the leaves
closely on a dewy morning,
when a violet furry covering
may be seen on the outer sur-
face. Within a day or two
the color of the leaves becomes
pale green and finally yellow-
ish, the furry growth becomes
more widespread, and the dis-
eased portions of the plant
eventually collapse (fig. 5).
The disease commonly starts
in the field in spots and
spreads to the surrounding
areas, its development being
greatly aided by moist
weather. Tf the weather re-
mains dry following an out-
break of the disease, the plants
send out new leaves and par-
tially recover. However, on
the return of damp conditions
the fungus revives and the
new growth becomes blighted.
The killing of leaves in this
manner, although it may not
entirely kill the plants, re-
duces growth, and the bulbs
remain small.

The onion mildew has been reported from most Stales where
onions arc grown extensively. The dcstrnctivcncss of the disease,
however, varies widely in different localities and in different years
in the same region. As in the case of a number of other downy

Fig. 5.— Onion mildew (blight). Note the
fungous growth on the dying lower leaves;
the two youngest leaves are still healthy.
The spread of the disease to new leaves de-
pends on the amount of moist weather pre-
vailing. (Photographed by the Vermont
Agricultural Experiment Station.)

10

Farmers' Bulletin 1060.

mildews, prevailing moist weather is absolutely essential for its
development in epidemic form. Heavy losses have been reported in
New York, Michigan, Oregon, California, and Louisiana. In the
two States last mentioned the chief damage is to the seed crop, the
heavy fogs which are very prevalent being especially favorable for
the development of the mildew, which attacks the seed stalks, causing
than to fall over before the seed is mature.

CAUSE OF MILDEW.

Mildew is caused by a fungus (Peronospora schleideni) and bo-
longs to the class of downy mildews, the most important among
which arc late-blight of potato, grape mildew, and cucumber mildew.
The furry masses on the affected leaves are branches of the fungus,
which bear abundant spores. These spores arc very light and arc
easily carried by the wind to healthy plants. However, they are
short lived and very sensitive to drying.

In damp weather drops of water on the plant furnish sufficient
moisture for prompt spore germination. The resulting fungus
growth enters the plant and absorbs food from it, thereby causing
its leaves to shrivel and die. The fungus thus nourished produces
more spores, which in turn are wind borne to healthy plants, whore
under favorable conditions they again produce the disease.

In the fall the fungus forms in the diseased loaves thick-walled
winter spores which are resistant to drought and cold. They germi-
nate in the spring and again start the disease in the new crop. It is
also by moans of those winter spores that the mildew may bo spread
from diseased fields to healthy fields in infested soils, in diseased
onion tops, or iu sots.

CONTROL OF MILDEW.

Sanitation and cultivation. — The successful control of onion mil-
dew depends largely on attention to cultural and sanitary measures.
Since many of the winter spores live over in dead tops, it is advisa-
ble to rake these and burn them in the fall. Thorough cultivation is
essential, in order to keep the crop as vigorous as possible, so that it
may rapidly outgrow the disease when the latter is chocked by dry
weather.

Rotation of crops. — Since the winter spores live over in the field,
it is advisable to plant infested fields to other crops for a few years.
It is not known how long the fungus can live in the soil, but it will
certainly be greatly reduced in two or throe years. It is true that
many growers prefer to grow onions for a number of years suc-
cessively on the same soil. However, it is no doubt true that a num-
ber of other cultivated crops yield high enough returns to bo con-

Onion Diseases and Their Control.

11

sidered in a rotation, especially when the general henefits of a three
or four year rotation are considered. In certain seetions farmers,
realizing the importance of this practice in the control of diseases,
are successfully rotating onions with eabbage, potatoes, and sugar
beets, and in some eases grain is ineluded.

Air and soil drainage. — Inasmuch as moist conditions greatly favor
mildew, care should bo taken to seleet fields where the air drainage
is good, so that excessive dew and fog may be avoided. Good soil
drainage also helps to reduce blight, since it reduces the moisture in
the air near the surface of the soil.

Spraying for mildew. — In the control of onion mildew, Bordeaux
mixture applied as a spray has been used to some extent, but not
with complete sueeess in all eases, since there is considerable difficulty
in making the spray stick to the leaves. If spraying is to be done,
rosin-fishoil soap should be added to the spray mixture to make it
adhere to the leaves. The spray should be applied thoroughly be-
fore the disease has beeome established in the field, and it may bo
necessary to spray the field several times during the summer, espe-
cially during the rainy season.

In the preparation of Bordeaux mixture the ingredients should be
xised in the following proportions :

Where Bordeaux mixture is to be used frequently and in reason-
ably largo quantities, it is more convenient to make up the ingredients
in concentrated stock solutions or suspensions, since these can be kept
on hand indefinitely if water lost by evaporation is replaced.

Stock solution and suspensions. — Build an elevated platform to
hold the barrels, preferably near a well or other source of water.
Suspend 50 pounds of copper sulphate, inclosed in burlap or a loosely
woven sack, so as just to dip into a 50-gallon barrel of water. Slake
50 pounds of lime in another barrel and dilute to 50 gallons with
water. Plaeo 37J pounds of rosin-fishoil soap in a barrel and slowly
add water to make 37| gallons, stirring constantly to avoid the forma-
tion of lumps. -These stock solutions and suspensions now eontain 1
pound of their respective ingredients in each gallon of liquid.

Preparation of the mixture. — Follow the directions given below
where 100 gallons of spray mixture can be made up and used at one
time; for smaller amounts adjust the quantities used accordingly.

PREPARATION OF BORDEAUX MIXTURE. 1

Copper sulphate-
Quicklime

Uosin-fishoil soap
Water to make—.

4 pounds.
4 pounds.
3 pounds.
CO gullons.

1 The section on Bordeaux mixture was prepared by Mr. W. 1!. Clark.

12

Formers' Bulletin 10G0.

Provide two 50-gallon barrels with one head removed from each
and with openings of ainplo size near the' bottom head. The flow
from the openings can bo most easily controlled by attaching to
them, with a pipe nipple, pieces of 1| or 2 inch rubber hose just
long enough to hook up to the tops of the barrels while they arc be-
ing filled and stirred. Stir the concentrated solution and suspen-
sions thoroughly and measure into one barrel 8 gallons of eopper-
sulphatc stock solution and into the other 8 gallons of the lime sus-
pension. Add to each enough water to make 47 gallons, stirring
thoroughly. Provide a trough leading under the openings of the
two barrels and emptying into the strainer of the spray tank.
Lower the free ends of both pieces of hose, as nearly as may be, so
that the diluted lime suspension and copper-sulphate solution flow
at equal rates, mixing in tho trough before entering the spray tank.
"While the mixture is flowing into the spray tank, slowly add C gal-
lons of the rosin-fishoil soap suspension by pouring it into the spray-
tank strainer in a small stream, so that the C gallons will bo well dis-
tributed throughout tho entire mixture. Do not let tho soap eomo
into contact with either of tho other two ingredients until they havo
mixed. This method gives a thorough mixture of all tho ingredi-
ents without the necessity for any hand stirring.

Do not put copper sulphate or Bordeaux mixture into tin or iron
vessels. Use wood or copper containers.

A copper or bronze strainer of 18 meshes per inch should bo used.
Bordeaux mixture should bo applied as soon as made, as it is not so
good after settling.

Spray machines to be used. — The typo of sprayer used must bo
decided for each individual ease. Hand or power sprayers ordi-
narily used for potato and other vegetable spraying can be adapted
to onions. A pressure of 100 pounds or more per square inch should
ho maintained, and a nozzle which will give a very fine spray is
preferable.

LEAF-MOLD.

Tn midseason or later dying back from the tips of the leaves com-
monly occurs. "While this injury may be due in part to insufficient
soil moisture, the trouble is often increased by a weakly parasitic
fungus {Macros porium parasltlcum) which attacks the dying parts
and later produces a black mold on the dead tissues. This fungus
also commonly attacks the seed stalks, independently or following the
mildew.

Although of widespread occurrence, the actual loss due to leaf-
mold is of minor importance, with the possible exception of tho
injury to the seed crops of Louisiana and California. There, in eon-

Onion Diseases and Their Control.

13

junction with the mildew, it may cause
severe damage. However, no satisfac-
tory method of control has been per-
fected.

FUSARIUM ROT.

A rapid dying back of the leaves
from the tips at the time when the
plants are approaching maturity is com-
monly associated with the Fusarium
rot a fleeting the bulb. (See fig. G.)

Most of the roots eventually become
rotted off, and in their place a mass of
white moldy growth is produced. The
bulbs become soft, and on cutting them
open one finds a semi watery decay ad-
vancing from the base of the scales up-
ward. The rot progresses rather
slowly, and many of the recent infec-
tions are unnoticed at harvest time.
The disease thus becomes a factor in
transit and storage, where the decay
may continue until the bulbs are en-
tirely destroyed.

Reports of this trouble have come
from Ohio, Illinois, Wisconsin, and
"Washington, and it probably occurs in
oilier States. It is caused by one or
more species of soil fungi (Fusarium
spp.), which invade the base of the
bulbs, often following maggot injury.

Strict attention to sanitary measures
as outlined later (see p. 10) and careful
sorting out of diseased bulbs at harvest
time are the chief measures to be taken
for the control of Fusarium rot.

PINK-ROOT.

In the Bermuda onion regions of
southern Texas pink-root is the most
serious disease confronting the grower.
The symptoms become manifest in the
seed bed or after transplanting by the
roots turning pink in color, after which
they shrivel and die. As the plant

Fio. 0. — An onion showing Fusa-
rium rot. Decay starting at the
bane of the scales causes the
leaves to die rapidly, and the
bulb continues to rot in storage
and in transit.

14

Farmers' Bulletin 1060.

sends out now roots they in turn eventually become diseased and f unc-
tionless. This procedure continues throughout the growing season,'
and although the affected plants seldom are killed by the disease, the
reduced nourishment results in merely scallions or small bulbs being
formed. Pink-root becomes the more destructive the longer onions

are grown on the same
field. It is caused by a
soil fungus (Fusarium
malli), which is carried
over from year to year in
the soil.

The disease is being in-
vestigated at present by
the Texas Agricultural
Experiment Station, and
recommendations for its
control must await tho
results of these studies.

ROOT-KNOT. 1

Root-knot of onion is
an celworm disease which
may be recognized by tho
spherical swellings or en-
largements of affected
roots, as shown in figure
7. Tho aboveground parts
of badly diseased plants
are dwarfed, the leaves
are a pale-green, sickly
color, and the bulbs are
reduced in size. Root-
knot, while occurring as
a serious trouble of many
wild and cultivated
plants in the southern
portion of tho United
States, has boon reported
only occasionally on onions, and fortunately there is little likelihood
that it will ever bo a major disease of this crop. It is caused by a
minute eelworm, or nematode (Tleterodera rarficicola) , which lives in
the soil and penetrates tho onion roots, where swellings arc produced.
After reaching maturity in the roots, the nematode may lay hundreds

Fig. 7. — Onion root-knot, caused by an celworm. Note
that certain roots contain spherical swellings, from
some of which small, dark masses (the eggs) arc
protruding. Many active larvai of the eelworm
escape from these egg masses Into the soil, and
render It unfit for growing onions as well as many
other crops.

1 The Hoction on root-knot was prepare*! by Dr. Li. V. Iiyars.

Onion Diseases and Their Control.

15

of eggs, which hatch into active forms (larvae) and thus complete the
life cycle.

Soil once infested with the eelworms can he freed hy the use of live
steam applied under considerable pressure. In large areas where
steaming may not be practicable, the number of nematodes can be
reduced by a proper system of crop rotation. By planting for two
or more years on infested land crops which are not subject to nema-
tode attack, the eelworms may be starved out to such an extent that ;
a susceptible crop, such as the onion, will not be damaged seriously
when planted in the soil.

To avoid root-knot it is advisable never to plant onions on land
that is known to be infested.

For further details regarding the control of root-knot, see Farmers'
Bulletin G48.

FARM PRACTICE IN RELATION TO STORAGE DISEASES.

The interval of several weeks between harvest and storage or ship-
ment is a very critical one with relation to the development of dis-
eases in the warehouse or in transit. The care which is taken with
the crop at this period may mean the difference between success and
failure in carrying it through storage or in placing it on the market
in good condition. It should be remembered that the plant at this
time has practically terminated its growth, and that on going into its
dormant state it becomes more susceptible to the attack of storage-
rot fungi and bacteria, which are continually present in the soil and
on dead refuse.

While going into dormancy the bulbs must be allowed to " sweat "
or cure preparatory to storage or shipment. For this purpose they
are ordinarily placed in crates and stacked in the field or in open
sheds, where the sunshine and air currents aid greatly in removing
the moisture which is given off. Thus, if the weather remains
warm and dry during harvesting and curing, it is the common ex-
perience of growers that the crop will go through storage with small
losses due to decay. Prevailing rainy weather at this time, however,
will almost invariably lead to heavy losses. The moisture is favor-
able for the development of the fungi and bacteria, and at the same
time it delays harvest and prevents the proper maturing of the crop.
Under such conditions certain storage diseases, such as neck-rot (see
p. 18) and soft-rot (see p. 20), make considerable progress before the
bulbs are pulled. On the other hand, high humidity of the atmos-
phere during the curing period causes the moisture given off by the
onions to accumulate in the crates, which favors the development of
decays.

16

Farmers' Bulletin 1060.

The control of storage diseases, therefore, will consist largely of
attention to cultural methods based on the knowledge of these gen-
eral facts. In view of this, the following specific recommendations
are made with regard to the handling of the onion crop.

SANITATION.

The organisms causing storage rots in general thrive on dead
vegetable matter. Onion tops and diseased bulbs left on the field and
onion refuse from the warehouse furnish excellent opportunities for
these fungi and bacteria to multiply. The spores of certain of these
organisms, especially those causing neck-rot (see p. 18), are readily
carried long distances by the wind. A pile of rotting onions near
the warehouse may thus be a means of infecting a crop a consid-
erable distance away. All onion refuse left on the field should be
raked and burned after harvest. Waste from the warehouse should
he dumped in a remote place, or if spread on the fields it should be
confined to those not to be used for the growing of onions.

HARVESTING. 1

As soon as the neck of the onion bulb loses its stiffness sufficiently
to allow the top to drop over readily the onion is ready to harvest.
It is best to allow the tops to dry out as much as circumstances will
permit before harvesting, since this will help to reduce the trouble
from storage rots. If the field matures unevenly it is well to start
pulling when most of the plants have reached this stage. An un-
usual amount of rainy weather just previous to harvest may postpone
the ripening and tend to cause an overproduction of "scallions" or
" stiff necks." These should not be placed in storage, but sorted
out and sold as soon as possible. In such event the cutting of the
roots with a wheel hoc will tend to hasten maturity. In clipping or
twisting the tops a neck 1 or 2 inches long should be left, to avoid
the exposure of the succulent tissue of the fleshy scales of the bulb.
Care should be taken to avoid bruising the bulbs and thus opening
the way for the organisms which cause decay. The milling of bot-
tom sets before storage causes a certain amount of bruising and
lowers the keeping quality.

REMOVAL OF DISEASED BULBS.

It is essential that care be taken at harvest time to throw out all
bulbs which show any signs of disease or insect injury. Although
smut (sec p. 4) and mildew (sec p. 9) do not of themselves cause
decay, bulbs which have been attacked by these fungi arc thereby

' V'or more detailed Information relative to Horticultural methods of harvesting, eivrlng,
nnd storing onions, see Farmers' Bulletin 35-t, "Onion Culture."

Onion Diseases and Their Control.

17

made more susceptible to the invasion of storage-rot organisms.
Fusarimn rot (see p. 13), on the other hand, gains a start in the field
and continues to injure the bulbs in storage. In unusually damp
weather soft-rot and neck-rot may start in the field, and it is well
to be on the watch for bulbs with softened necks at harvest time.

CURING.

Onions are sometimes allowed to cure in windrows in the field,
and if the weather is clear, yellow and red varieties can be handled
successfully in this manner. It is preferable, however, to place
them in slatted crates soon after topping and pile in open sheds or
in stacks in the field. In the latter case the piles should be covered
with temporary roofs for protection from rain. Exposure of white
varieties (see p. 18) to damp weather in the field will almost in-
variably prove fatal, and they should be placed in a curing shed,
where advantage can be taken of clear weather and protection can be
given during rainy periods.

STORING.

In the Northern States onion warehouses should be built with the
purpose in mind of keeping the temperature just above 32° F. during
severe winter weather with as little artificial heating as possible.
This necessitates walls consisting of a number of thicknesses of lum-
ber and one to several air spaces. Provision is necessary for ample
ventilation, since the bulbs are continually giving off moisture which
must bo removed. This can bo increased materially on clear days
by opening doors and windows for a few hours. In very cold
weather it is necessary to heat the house during this process, in order
to prevent the freezing of the bulbs. A steam or hot-water heating
system or stoves placed at intervals are satisfactory. Large bulbs
are stored in slatted bushel boxes or folding crates, while bottom
sets should be placed 2 to 4 inches deep in shallow crates.

Where a modern warehouse is not available, a dry cellar which can
bo held at 32° to 35° F. can be used with good results.

RELATION OF VARIETIES TO STORAGE DISEASES.

In the Northern States and on the Pacific coast, where Globe
onions are grown most extensively, yellow and red varieties are much
less susceptible to decay in storage and transit. White varieties, on
the other hand, are very subject to storage diseases, especially neck-
rot and smudge, and they require much more care during harvest and
curing in order to be handled successfully. Tn the onion bottom set-
growing sections the same is true of the White Portugal, as compared

18

Farmers' Bulletin 1060.

with the Red Wethersfiold, Yellow Strassburg, and Yellow Danvcrs.
Certain white varieties, such as Queen, Pearl, and Barletta, are such
poor keepers that they arc seldom held in storage for any length of
time.

In the onion regions of Texas, southern California, and Louisiana,
the Bermuda varieties are largely disposed of soon after harvest,
on account of their poor keeping quality and the lack of cold-storage
facilities. In Louisiana the Creole variety is the favorite because
it resists much more effectively than the Bermuda varieties the attacks
of fungi and bacteria in storage and in transit.

DISEASES PRIMARILY IMPORTANT IN STORAGE AND

TRANSIT.

NECK-ROT.

Neck-rot is a destructive and widespread disease of onions in stor-
age and in transit. During certain seasons many growers have lost
.50 per cent or more of their crop on account of this trouble. "White
varieties are especially susceptible, but considerable loss is often sus-
tained with red and yellow varieties.

CHARACTERISTICS OF NECK-ROT.

Usually there is little or no evidence of this disease up to or at
the time of harvest, but after the onions are topped and have lain
in crates for a few days the early signs appear. A softening of the
scale starts usually at the neck, more rarely at the base or at a
wound.

There is a definite margin between the healthy tissue and the dis-
eased portion, which takes on a sunken, water-soaked appearance. A
gray feltliko growth later forms on the rotting scales, which may
bo accompanied by a gray to brownish mold, consisting of the spores
(seeds) of the causal fungus, and by brown to black kernellike bodies
(sclerotia) one-eighth to one-fourth inch in diameter (figs. 8 and 9).
On red and yellow onions the pigment of the diseased portions is de-
stroyed, while in the former the rotted tissue sometimes assumes a
pinkish tint. The disease progresses rather slowly unless conditions
are very moist, several months often elapsing before the entire bulb is
destroyed. The white varieties decay most readily, while the colored
types more often escape it and when infected show a marked resist-
ance to its progress.

CAUSE OF NECK-ROT.

Neck-rot is a disease caused by one or more species of fungi
(Botrytis spp.) closely related to the common gray molds which
attack lettuce, cabbage, and numerous other vegetables. These fungi

Onion Diseases and Their Control.

19

are not vigorous parasites and seldom seriously attack actively grow-
ing onion plants. They do not ordinarily penetrate the dry outer
scale of the onion, but require a wound in order to gain entrance to the
plant tissues. Tho gray to brown moldy growth on the rotted scales
consists chiefly of the spores of the fungi, which are especially
adapted to dissemination by air eurrents. They are thus carried to
tho healthy bulbs, whore they germinate and send fungous threads
into the necks which have been wounded
by the cutting or twisting of the tops.
These threads then kill the tissue
slightly in advance of their progress
through the scale. The black, kernel-

Fig. 8. — Onion neck-rot. The softening
and shriveling of the scales begin at
the neck of the bulb, with the later
development of black, kernellike masses
on the surface.

Fio. *). — Onion neck-rot. Lon-
gitudinal section of a dis-
eased bulb, showing the outer
scales badly rotted while the
disease is just appearing on
the inner scales.

like bodies, or sclerotia, are compact masses of fungous threads,
which, being resistant to cold and drought, serve to carry the
organisms over winter.

CONTROL OF NECK-ROT.

Proper care of the crop during harvest and curing is the chief
measure of control for neck-rot (sec p. 17). Avoid exposure to damp
weather and provide cool, dry storage. White varieties should re-
ceive special attention, since they are very susceptible to tho disease.
Proper sanitation (see p. 1G) is also very important and worthy of
careful consideration in connection with this disease.

20

Farmers' Ballelin 1060.

ARTIFICIAL CURING FOR THE CONTROL OF NECK-ROT.

A rapid drying out of the nock of the bulb by moans of artificial
heat immediately after harvest is effective in the control of neck-
rot. This is accomplished by passing a current of air heated to 100°
to 120° F. over the onions in shallow crates until Ihe necks are thor-
oughly dried. The threads of the causal fungus entering after the
top of the bulb has been removed apparently require some moisture

for their development, while
thoroughly dried tissue offers
a barrier to their progress.
Experiments on a small scale
have shown this to be a satis-
factory method for the control
of this disease. To apply this
control on a commercial sealo
it is necessary to have a special
kiln or a special room in the
warehouse for this purpose.
The temperature can be raised
to 120° F. with safety. Pro-
vision should be made for a
fairly rapid air circulation to
carry off the moisture.

Experiments are under way
with the object of devising the
best methods for the applica-
tion of this principle on a com-
mercial basis.

SOFT-ROT.

Soft-rot being of bacterial
origin differs from other stor-
age rots in the absence of
fungous threads and spores,
although it is sometimes fol-
lowed by saprophytic fungi.
The tissue first becomes glassy or watersoaked in appearance, and
later disintegrates into a soft, watery mass. An offensive odor often
accompanies the rot. The organism being unable to penetrate the
unbroken scales, infection commonly takes place through the neek,
often before the crop is harvested. When the rot affects only one or
two inner scales, as is often the ease (fig. 10), the only external sign
of the disease is the lack of firmness detected on pressing the bulb
'between the fingers. Sunburn, freezing, and external bruises due to

Via. 10. — A section of nn onion bulb showing
bacterial soft-rot. This Is a watery soft-rot,
accompanied by an offensive odor, which starts
usually at the neck and invades one or more
scales, often leaving the outer scales Intact.

Onion Diseases and Their Control.

21

careless handling are followed very often by soft-rot, especially if the
surfaces of the -wounds remain moist.

CAUSE OF SOFT-ROT.

Soft-rot is due to a group of bacteria 1 which cause soft rots on
cabbage, carrot, celery, and a large number of other vegetables. As
a class they do not attack uninjured plants, and they require a wound
and sufficient moisture to gain a foothold.

CONTROL OF iSOFT-ROT.

Crop rotation and sanitation should be thoroughly practiced to
suppress tho causal organisms. Precautionary measures already
recommended (sec pp. 16 and
17) with regard to harvesting,
curing, and storing should be
followed carefully. Sort out
all affected bulbs before stor-
age or shipment.

BLACK-MOLD.

Because of its resemblance
in appearance, black-mold is
often confused with onion
smut by growers and dealers.
The chief distinguishing char-
acteristic between the two is
the fact that the blaek pow-
dery masses of spores in the
case of black-mold arc borne
on the exterior of the scales
and can be rubbed off readily
(fig. 11). It is true that the
disease is not confined to the
exterior of the bulb, but as the inner scales are separated, the black
powder will be found to exist on the exterior of the individual scales.
Onion smut, on the other hand, as seen in storage or market, is char-
acterized by oblong or linear brown or black lesions, most commonly
near the base of the bulb and as deep as the third or fourth scale.
Black-mold causes a slow shriveling of the affected scale, which
assumes a brittle texture. Moist conditions favor the disease, while
a cool, dry environment seems to check it.

Black-mold occurs to some extent in northern onion sections, but
it is of slight economic importance there. In Louisiana, Texas, and

Fia. 1 1. — Onion Mack-mold. Compare its symp-
toms with those of onion smut. Note the Ir-
regular sooty masses on tho outside of or
between the scales, accompanied by the shriv-
eling of the scales about tho neck of the bulb,
which gives them a brittle, papery texture.

1 ttacilhis carotmwruH Is an example of this group.

22

Farmers' Bulletin 10G0.

California, however, it is the most important storage and transit
disease.

CAUSE OP BLACK-MOLD.

Black-mold is caused by a fungus (Asperillus niger) which is a
common organism living on almost airy dead or dying vegetable
matter. Where it is most serious on onions, it undoubtedly grows
and multiplies throughout the year in the soil or on dead refuse.
It is present to a slight extent on the dead outer scales of the bulbs
before harvest, but is not notieeable until the onions are pulled.
Rainy weather at this critical period will result in a widespread

infeetion, which continues to develop
in storage or transit.

CONTROL OF BLACK-MOLD.

General sanitary measures and
protection from moisture after har-
vest are essential. The bulbs should
be thoroughly dry before they are
shipped, since moisture favors the
rapid development of the disease in
transit. Dealers in northern mar-
kets receiving infected lots to be held
any considerable length of time be-
fore consumption should transfer
them to cold storage in order to hold
the disease in cheek.

SMUDGE (ANTHRACNOSE, BLACK-
SPOT).

Onion smudge is confined largely
to white varieties and appears in the
Fio. 12.— Onion smudge. Note the fi e hi j us t hefore harvest time, often

smudgy spots made up of small black , J . . '

dots. The disease is confined largely continuing to develop during the
to the outer scales of white varieties, storage period. It is characterized
by small dark-green to black dots which appear on the outer scales.
These small dots may be grouped together in various ways and are
often arranged in coneentric rings, giving a smudgy, unsightly ap-
pearance to the white bulbs (fig. 12). The fungus ordinarily attacks
the fleshy scales only mildly and in such cases does not cause any ap-
preciable shrinkage in storage, its chief damage being the reduction
of the market quality of the crop. However, after rainy weather dur-
ing harvest, when the bulbs are crated and stored without being dried
and cured thoroughly, the disease causes considerable loss.

Onion Diseases and Their Control.

23

CAUSE OF SMUDGE.

Smudge is caused by a fungus (Colletotrichum {Vermicularia)
circinans), which lives over winter on onion scales in the soil or on
bulbs in the warehouse, and consequently it increases in amount where
onions are grown on the same fields year after year. It is widely
distributed through the trade on white-onion sets, and by this means
is introduced into soil new to onions. Under favorable conditions
the fungus attacks the outer scales and forms many small black dots
on which myriads of minute spores are produced (fig. 12). These
spores may be carried away in drops of water to other onion scales,
where they germinate within a few hours and renew their attack.
The fungus passes through this whole life cycle within a few days
when the weather is warm and moist. A little disease in the field
before harvest will furnish spores enough to spot the bulbs very
badly if a few days of moist weather come during harvest or while
the crates arc stacked in the field.

CONTROL OF SMUDGE.

Since the development of the disease is dependent largely on
abundant moisture, special care is needed in handling the white
varieties. Harvest the crop without delay, avoiding any exposure to
rain, if possible. Stack in an open shed and allow the onions to cure
thoroughly. Place white sets in thin layers in shallow orates.

SUMMARY OF CONTROL MEASURES.

Control onion smut by applying formaldehyde solution (1 pint to
16 gallons at the rate of 200 gallons per acre) by means of a drip
attachment on the seeder.

Control blight by avoiding excessive soil moisture, by crop rota-
tion, and by thorough cultivation. If spraying is to bo practiced it
should bo done early and thoroughly.

In general, storage diseases are to be controlled by the practice
of sanitary measures, the sorting out of diseased bulbs at harvest,
protection from rain after harvest, thorough curing, and storage in
a dry, well-ventilated warehouse at 32° to 35° F.

PUBLICATIONS OF THE UNITED STATES DEPARTMENT
OF AGRICULTURE OF INTEREST IN CONNECTION
WITH THIS BULLETIN.

AVAILABLE FOR FREE DISTRIBUTION BY THE DEPARTMENT.

Culture nnd Uses of Okrn, (Farmers' Bulletin 232,)
Rcnns, (Farmers' Bulletin 280.)

Discuses of Cabbage anU Related Crops nnd their Control. (Farmers' Bulle-
tin 488.)

Votiito-Tuber Diseases. (Farmers' Bulletin 544.)

Blackberry Culture. (Farnfers' Bulletin 043.)

The Control of Hoot-Knot. (Farmers' Bulletin 048.)

The Sniiash-Viiie Borer. (Farmers' Bulletin 008.)

Sweot-l'otato Diseases. (Farmers' Bulletin 714.)

The Common Cabbage Worm, (Farmers' Bulletin 7GG.)

Watermelon Diseases. (Fanners' Bulletin 821.)

The Asparagus Beetles ami Their Control. (Farmers' Bulletin 837.)

Control of Diseases and Insect Enemies of the Dome Vegetable Garden.

(Farmers' Bulletin 850.)
How to Increase the Potato Crop by Spraying. (Farmers' Bulletin 808.) .
Control of the Melon Aphis. (Fanners' Bulletin 014.)
Cabbage Diseases, ( Farmers' Bulletin 025.)
Control of tbe Onion Tlulps. (Farmers' Bulletin 1007.)
The Sweet-Potato Weevil and Its Control, (Farmers' Bulletin 1020.)
Fxperlments In the Control of Potato Leak. (Department Bulletin 577.)
Celery, Storage Experiments, (Department Bulletin 570.)
Miscellaneous Truck-Crop Insects in Louisiana, (Department Bulletin 703.)
Anthracnose of Cucurbits, (Department Bulletin 727.)
Potato Wart — A Dangerous New Disease. (Department Circular 32.)
Diseases and Insects of the Home Garden, (Department Circular 35.)
Insect Enemies and Diseases of the Tomato, (Department Circular 40.)
A Spot Disease of Canllllowcr. (Bureau Plant Industry Bulletin 225,)
Wart Disease of tbe Potato, (Bureau Plant Industry Circular 52.)

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24 WASHINGTON ; GOVERNMENT PRINTING OFFICE : 1913