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Mimeo Report 62-1*
October 6, 1961
CONTROL OF DOWNY MILDEW AND ALTERNARIA LEAF SPOT OF CABBAGE AND OTHER CRUCIFERS
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. 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, cauliflower and broccoli at Hastings, Florida%
in September or October and is present thereafter until the end of the growing
season the following spring. Severe 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 820 F.
Alternaria Leaf Spot of crucifers is caused principally by the fungus, Alter-
naria oleraceae Milbrath and to a lesser extent by Alternaria brassicae (Berki.
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. Plow under abandoned seedbeds and harvested fields to prevent downy
mildew and Alternaria leaf spot from spreading to new plantings.
Fungicides recommended for control of downy mildew and Alternaria leaf spot in
the Hastings area are as follows:
SPRAY (amounts in 100 gals. water)
Plant Bed Only
Chloranil 50~, 4 lbs.
Plant Bed and Field
Nabam 19%, 2 qts. + 3/4 lb. Zinc Sulfate 36%
Zineb 65%, 2 lbs.
Maneb 80%, 1 Ibs.
Plant Bed Only
Chloranil (trade name spergon) gives better control of downy mildew in plant
beds than the other fungicides, but its use on cabbage, cauliflower and broccoli
plants after they are set in the field is prohibited due to residue tolerance
limitations. Nabam-zinc sulfate, zineb and maneb sprays give good control of downy
mildew and Alternaria 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 fungicides 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/4j present.
Continue the treatment 3 times each week with a one- to two-day in1 y;between
applications except when nocturnal temperatures drop to 400 F. or Jer or he~vy-
rains interrupt the schedule. Use 80 to 150 gallons of spray or l5\t1 35 pounds '-o
*Revision Of Mimeo Report 59-1.
POTATO INVESTIGATIONS LABORATORY
dust per acre at each application, depending upon size of the plants. Use the
quantities listed in Table 1. Treat plant beds until all usable plants are drawn.
Where the crop is started from seed sown in the field, continue treating until the
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-drill Row Beds 40 Inches Apart
Spray, 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 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, Cauliflower and Broccoli.- Treatment of maturing plants 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 tolerances for nabam-ZnS04 and zineb on cabbage,
cauliflower and broccoli is 7 ppm. and 7 days is the minimum time between the last
application and harvest. Residue tolerance for maneb on the three crops is 10 ppm.
Days between last application of maneb and harvest are as follows: cabbage, 7 days;
broccoli, 3 days or trim and wash; and cauliflower, no time limitation. Refer to
Fla. Agr. Exp. Sta. Cir. 193A or revision of same for additional information on
tolerances and time limitations for the above fungicides.
Compatability with Insecticides.- Chloranil, nabam-ZnS04, zineb and nabam are
compatible with insecticides recommended for control of insects of cabbage and other
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.