POTATO INVESTIGATIONS LABORATORY MIMEO REPORT 55-1
Control of Downy Mildew of Cabbage
A. H. Eddins /
Downy mildew of cabbage, cauliflower, broccoli and closely related crucifers is
caused by the fungus Peronospora parasitica. 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
and may make them unsalable,
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 fol-
lowing 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
400 F. or above 82 F.
Downy mildew can be controlled satisfactorily with several different fungicides.
Ones recommended for its control at Hastings are chloranil, nabam and zineb.
Plant Beds and Seeded Fields.- Forty-eight or 50 percent chloranil (4 lbs. -
100 gals. 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 qts. 1 lb. ZnSO4 100 gals, water) or
zineb (1 lbs. 100 gals, water), as there is less danger of injuring small plants
by overtreatment with these fungicides than with chloranil.
Begin treating the plants with a fungicide 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 00 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 treat-
ing until the plants are thinned to a stand.
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 ex-
ceed those listed in Table 1. Plants less than 4 inches in height are more suscep-
tible to injury from overtreatment 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. Overtreatment with
chloranil also causes malformation of the plant; the petioles may be elongated, the
midribs enlarged and the blades undersize 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 qts. 1 lb. ZnSO4 100 gals, water), 48 or 50 per-
cent chloranil (2 Ibs. 100 gals, 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 and toxaphene which may be used in cabbage
plant beds and fields to control various insects.
Table 1.- Quantities of Spray and Dust to Use at Each
Downy Nildew on Small Plants.
Application for Control of
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
S. 100 Sq. Yds. 100 Sq. Yds.
Less than 2 inches 80 to 100 15 to 20 to 5 3/4 to 1
2 to 4 inches 100 to 120 20 to 25 5 to 6 1 to i
4 to 8 inches 120 to 150 25 to 35 6 to 7 1I to 2