POTATO INVESTIGATIONS LABORATORY
Mimeo Report 59-1 October 1, 1958
CONTROL OF DOWNY MILDEW OF CABBAGE
A. H. Eddins
Downy mildew of cabbage, cauliflower, broccoli and closely related crucifers
is caused by the fungus Penonospora arasitidca (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 50 and 600 F. for several nights
in succession. Little of the disease develops at temperatures below 400 F. or
above 820 F.
Downy mildew can be controlled satisfactorily with the fungicides, chloranil,
nabam-zinc sulfate and zineb.**
PlantBeds and Seeded Fields.- Fifty percent chloranil wettable, 4 pounds
in 100 gallons water, and percent stabilized chloranil dust are recommended for
control of downy mildew in plant beds. Chlorenil usually has given better control
of the disease than other fungicides. However, if chloranil cannot be applied at
the rates recommended, spray the plants with nabam, 2 quarts + 3/4 to 1 pound
ZnSO ir 100 gallons water, or zineb, 1I pounds in 100 gallons water, as there is
less danger of injuring small plants by over-treatment with these fungicides than
Begin spraying or dusting the plants a week to 10 days after the seedbed is
planted or before then if the disease is present. Continue the treatment 3 times
each week with a one- to two-day interval between applications except when noctur-
nal temperatures drop to 40o F. or lower or heavy rains interrupt the schedule.
Use 80 to 153 gallons of the spray or 15 to 35 pounds of the dust per acre at each
application, depending upon size of the plants; quantities used successfully in
narrow plant beds are listed in Table 1. Treat plant beds until all useable plants
are drawn. Where the crop is started from seed sown in the field, continue treat-
ing until the plants are thinned to a stand.
*Revision of Mimeo Report 58-1.
*Spergon is a trade name for chloranil.
Parzate liquid and Dithane D14 are trade names for nabam.
Table 1.- Quantities of Spray and Dust to Use at Each Application
for Control of Downy Mildew on Small Plants.
Narrow 2- to 3-drill Row Beds
Plant Height 40 inches Apart
Gals. per Acre 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 less than 8 inches in height may be injured or
killed if treated with chloranil spray which is stronger than recommended and if
the amounts of chloranil spray or dust used on small plants at each application
exceed those listed in Table 1. Plants less than 4 inches in height are more sus-
ceptible 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 and dies as if affected with damping-off. Over-treatment
with chloranil also causes malformation 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
Heading Cabbage.- Treatment of maturing cabbage for control of downy mildew
is recommended at six- to seven-day intervals when the disease is severe. Use
Nabam, 2 quarts + 3/4 to 1 pound zinc sulfate in 100 gallons water, or 11 pounds
zineb in 100 gallons water. Use 100 to 150 gallons of spray per acre at each
application. A good commercial spreader-sticker should be used in the spray as
recommended on the manufacturer's label. Residue tolerance for the above fungi-
cides is 7 ppm. and 7 days is the minimum time between the last application and
Compatibility with Insecticides.- Chloranil, nabam and zineb 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. Control of downy mildew of cabbage with fungicides. Fla. Agr. Exp.
Sta. Bul. 543. 1954.