POTATO INVESTIGATIONS LABORATORY
Mimeo Report 59-2 Revised September 25, 1959
CONTROL OF LATE BLIGHT AND EARLY BLIGHT OF POTATOES
The two most important foliage diseases of potatoes in the Hastings area are
late blight caused by Phytophthora infestans(Mont.) d By. and early blight caused by
Alternaria solani(Ell & G. Martin) Sor, Late blight usually appears earlier in the
season and is much more prevalent and destructive than early blight. Consequently
fungicides are used primarily for control of late blight in the Hastings area. For-
tunately fungicides which are effective against late blight will also control early
blight. Ones recommended for control of both diseases are listed below.
Amounts of Each Ingredient in Water
Fungicide to make following Quantities of Spray
S100 gals. 200 gals, 300 gals. 400 gals. 500 gals.
Nabam 19% 2 qts. 4 qts. 6 qts. 8 qts. 10 qts.
ZnSO4 36% 3/4 lb. 1 Ibs. 2* lbs. 3 Ibs. 3-3/4: lbs.
Zineb 65% 2 Ibs. 4 Ibs. 6 lbs. 8 Ibs. 10 lbs.
Maneb 70% 1- lbs. 3 lbs. 4 lbs. 6 lbs. 71 lbs.
The number and frequency of applications of spray required to control late
blight in the Hastings area vary according to the date of appearance of the disease
and the weather. Weather conditions favorable for its development consist of 8 con-
secutive days when the 7-day average daily temperature ranges from 500 to 770 F. and
the 10-day total rainfall is 1 inch or more. When these weather conditions are met
and the disease is known to be present in the area, it is advisable to start spray-
ing potato plants immediately. During the past 29 potato-growing seasons the number
of times late blight was first reported in the Hastings area by months was as fol-
lows: January, 2; February, 3; March, 11; April, 9; May, 2; and none 2 times. A
maximum of five applications of spray made at 5- to 10-day intervals should afford
adequate protection against the disease during dry growing seasons when weather
conditions favorable for the disease do not occur frequently and are of short dura-
tion. In wet seasons when favorable periods are numerous and of long duration it
may be necessary to spray the plants 10 times or more at 4- to 7-day intervals to
control the disease. The number of spray applications required to control late
blight in the Hastings area can be held to a minimum and money thereby saved if the
grower will keep fully informed on the weather and occurrence and development of the
disease. Information on the weather and late blight development can be obtained
from the Potato Investigations Laboratory, KYrtle 2-5921.
Nabam-zinc sulfate, zineb and maneb are compatible with the insecticides com-
monly used for control of insect pests of potatoes. (See Mimeo Report 60-3).
Directions for Making Nabam-zinc Sulfate Spray
1. Start running water in tank and operate agitator while mixing spray. FgEi
2. Pour in 2 quarts nabam(Dithane D14 or Parzate Liquid) for each 100 gallo -
after agitator is covered with water. Allow at least one minute for t abam
to mix with the water.
3. Slowly pour in 3/4 pound of finely granulated or completely dissolved cOC1 16
sulfate(36 percent metallic zinc equivalent) for each 100 gallons. A at
least two minutes for the zinc sulfate to react with.the nabam.
4. Add insecticides if needed and keep agitator going until spray is applied .