POTATO INVESTIGATIONS LABORATORY
Mimeo Report 59-1 Revised September 25, 1959
CONTROL OF DOWNY MILDEW AND ALTERNARIA LEAF SPOT OF CABBAGE AND OTHER CRUCIFERS
Downy Mildew of cabbage, cauliflower, broccoli and closely related crucifers is
caused by the fungus Penonospora parasitica(Pers.) ex Fr. The fungus appears on the
leaves as a white mold which can be seen easily when the plants are wet. The
disease stunts or kills young seedlings and may retard the growth of older plants.
Spots on the head leaves of cabbage mar the appearance of the heads. Badly spotted
heads are not salable.
Downy mildew appears on cabbage at Hastings, Florida, in September or October
and is present thereafter until the end of the growing season the following spring.
Outbreaks of the disease occur when heavy dews or rains keep plants wet for long
periods and night-time temperatures range between 500 and 60o F. for several nights
in succession. Little of the disease develops at temperatures below 400 F. or above
Alternaria Leaf Spot of crucifers is caused principally by the fungus, Alter-
naria oleraceae Milbrath and to a lesser extent by Alternaria brassicae(Berk.) Sacc.
The disease is particularly troublesome during warm, moist periods in the fall and
spring. The disease causes damping-off of seedlings and stunting of young plants.
It produces brown cankers on the stems and spots on the leaves. Numerous Alternaria
spots on cabbage heads and cauliflower curds ruin their appearance and salability.
Fungicides which have given good control of downy mildew and Alternaria leaf
spot in experimental plots and in growers' fields in the Hastings area are as
SPRAY(Amounts in 100 gals. water) DUST
Plant Bed Only Plant Bed Only
Chloranil 50%, 4 Ibs. Chloranil 5%
Plant Bed and Field Field Only
Nabam 19%, 2 qts. + 3/4 lb. Zinc Sulfate 36% Zineb 6.5%
Zineb 65%, 2 lbs.
Maneb 70%, 1 lbs.
Chloranil(trade name spergon) gives better control of downy mildew in plant
beds than the other fungicides, but it is too costly to use in the field. Nabam-
zinc sulfate, zineb and maneb sprays give good control of downy mildew and Alter-
naria leaf spot in the plant bed and are recommended for field use. Zineb dust also
is recommended for field application, when a grower is not equipped to apply fungi-
cides in spray form.
Plant Beds and Seeded Fields.- Begin spraying or dusting the plants a week to
10 days after the seedbed is planted or before then if downy mildew is present.
Continue the treatment 3 times each week with a one- to two-day interval between
applications except when nocturnal temperatures drop to 400 F. or lower or hea
rains interrupt the schedule, Use 80 to 150 gallons of spray or 15 to 35 p
dust per acre at each application, depending upon size of the plants. Usa ( quan-
tities listed in Table 1. Treat plant beds until all useable plants are fn.
Where the crop is started from seed sown in the field, continue treating 6ff
plants are thinned to a stand,
Table 1,- Quantities of Spray and Dust to Use at Each Application for
Control of Downy Mildew and Alternaria Leaf Spot on Small Plants.
Plant Height Narrow 2- to 3-dri Row Beds 40 Inches Apart
SSpra., Gals. per Acre Dust, Lbs, per Acre
Less than 2 inches 80 to 100 15 to 20
2 to 4 inches 100 to 120 20 to 25
4 to 8 inches 120 to 150 25 to 35
Sprayer and auxiliary tanks should be calibrated and the correct amount of
fungicide and water used to make the required quantity of spray at the strength
recommended. Young cabbage plants may be injured or killed if treated with chlora-
nil spray which is stronger than recommended and if the amounts of chloranil spray
or dust used at each application exceed those listed in Table 1. Plants less than
4 inches in height are more susceptible to injury from over-treatment with chloranil
than larger ones. The stem of a severely-damaged seedling appears water-soaked at
first and later turns white; the seedling falls over antd dies as if affected with
damping-off. Over-treatment with chloranil also causes malformatior of the plant;
the petioles may be elongated, the midribs enlarged and the blades undersized and
rolled slightly upward and inward. Leaves of affected plants often are shaped like
rabbit ears. A chloranil-injured, malformed plant will recover and grow normally
after the treatment is discontinued if it has not been severely stunted and burned;
after "rabbit eared" plants are set in the field, new leaves formed are normal in
size and shape.
Heading Cabbage and Cauliflower.- Treatment of maturing cabbage and cauliflower
for control of downy mildew and Alternaria leaf spot is recommended at six- to seven-
day intervals when the disease is severe. Use 100 to 150 gallons of spray per acre
at each application. Zineb dust should be applied at 25 to 30 pounds per acre, A
good commercial spreader-sticker should be used in the spray as recommended on the
manufacturer's label. Residue tolerance for Nabam-ZnS04, Zineb and Maneb is 7 ppm,
and 7 days is the minimum time between the last application and harvest.
Compatibility with Insecticides.- Chloranil, nabam-ZnS04, zineb and maneb are
compatible with chlordane, DDT, parathion, TEPP, toxaphene, endrin and phosdrin
which may be used in cabbage plant beds and fields to control various insects.
Eddins, A.H. Diseases, deficiencies and injuries of cabbage and other crucifers in
Florida. Fla. Agr. Exp. Sta. Bul. 492. 1952.
Control of downy mildew of cabbage with fungicides. Fla, Agr. Exp.
Sta. Bul. 543. 1954,