POTATO INVESTIGATIONS LABORATORY
Mimeo Report 58-1* October 1, 1957
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 parasitica (Pers.) ex Fr. The fungus appears
on the leaves as a white mold which can be seen easily when the plants are wet.
Downy mildew is a leaf spotting disease. It 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 begins to appear on cabbage at Hastings, Florida, in September
or October and is present from that time 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 nocturnal temperatures range between 500 and
600 F. for several nights in succession. Little of the disease develops at
temperatures below o40 F. or above 820 F.
Downy mildew can be controlled satisfactorily with the fungicides, chloranil,
nabam-zinc sulfate and zineb.**
Plant Beds and Seeded Fields.- Fifty percent chloranil wettable, 4 pounds
in 100 gallons water and 5 percent stabilized chloranil dust are recommended for
control of downy mildew in plant beds. Chloranil 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
ZnSO4 in 100 gallons water, or zineb, 1- 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 seed 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 noc-
turnal temperatures drop to 400 F. or lower or heavy rains interrupt the schedule.
Use 80 to 150 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 usuable
plants are drawn. Where the crop is started from seed sown in the field, continue
treating until the plants are thinned to a stand.
*Revision of Mimeo Report 56-4.
*Spergon is a trade name for chloranil.( C
Parzate liquid and Dithane D1I are trade names for nabam. C
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 one Yard Wide
Beds 40 inches Apart
Plant Height Spray Dust Spray Dust
Gals. per Acre Lbs. per Acre Gals. per Lbs. per
______ ______ 100 Sq. Yds. 100 Sq. Yds.
Less than 2 inches 80 to 100 15 to 20 4 to 5 3/4 to 1
2 to 4 inches 100 to 120 20 to 25 5 to 6 1 to 1
4 to 8 inches 120 to 150 25 to 35 6 to 7 1- to 2
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 my 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 and shape.
Heading Cabbage,- Nabam, 2 quarts + 3/4 to 1 pound ZnSO4 in 100 gallons water,
50 percent chloranil, 2 pounds in 100 gallons water, 5 percent stabilized chloranil
dust and 6.5 percent zineb dust are recommended for control of downy mildew on
heading cabbage. Treatment of plants should begin 1 to 3 weeks before harvest,
depending upon severity of the disease. Continue the treatment at intervals of 6 to
7 days until all marketable heads are cut. Use 100 to 150 gallons of spray or 30 to
35 pounds of dust per acre at each application, depending upon size of the plants.
Use a good commercial spreader-sticker with the spray as recommended on the manu-
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.