APR 20 1981
* AGRICULTURAL RESEARCH CENTER
SHastings, Florida I.FAS.- Univ. of Floridc
Hastings Research Report PR1981-4 April 9, 1981
MANAGING FOLIAGE DISEASES OF POTATOES
D. P. Weingartner, Assoc. Plant Pathologist
Reliable forecasting of potato late blight in northeast Florida (NEF) is
intended to provide more effective disease control by improving the timing of
necessary fungicide applications and to eliminate needless applications. The
objectives of disease forecast research at Hastings have been: (i) to deter-
mine the most effective late blight forecast system for NEF; and (ii) to de-
termine the effect of other diseases on potato production when few or no fun-
gicide applications are needed to control late blight. A summary of the
forecast systems being evaluated and observations made through 1976 were
published in Hastings Research Report PR77-5 (6). This report summarizes
observations made since 1976.
B Eight weather stations have been maintained each year at various loca-
tions in the NEF production area. Data acquisition has included total rain-
fall, maximum and minimum temperatures and continuous recording of relative
humidity and temperature. Data have been collected and summarized weekly or
more often if needed. Weather data collected at the Agricultural Research
Center Main Office are used to determine the frequency of data collection at
other locations. Spray advisories have been made as directed by modification
of the original Penn State Blitecast (= Florida Blitecast). Florida Blite-
cast is essentially the same as Penn State Blitecast except that rainfall is
weighted more heavily in the Florida system. Eddins' rainfavorable day de-
veloped in NEF is substituted in Florida Blitecast for the Hyre rainfavorable
day in Blitecast. In addition, if the first late blight warning is attained
via rainfall, Florida Blitecast issues a 7-day advisory whereas Penn State
Blitecast issues a warning only. Spray advisories are recorded on a self
answering tape (i.e. Code-a-phone) thereby providing potato growers at each
location with up-to-date advisories on a 24-hour basis.
The progress of late blight and early blight has been studied in four
potato cultivars (Sebago, Atlantic, Red La Soda, and Green Mountain) which
have been subjected to four different spray programs (7-day, Florida Blite-
cast, Ridomil, and no spray). The varieties and treatment combinations were
chosen to provide different levels and types of resistance to late blight
and different degrees of foliage disease control. The 7-day schedule has
been the maximum program and the highest rate of Bravo (chlorothalonil) is
applied weekly. Ridomil (metalaxyl) is a systemic fungicide which effec-
tively controls late blight, but not early blight and other foliage diseases.
The Ridomil schedule therefore provides a measure of the yield loss due to
diseases other than late blight. The experiments have been performed in
large plots (25 x 32 feet) so as to avoid interplot interference. Data taken
in the experiment have included progress of early blight and late blight, in-
cidence of Verticillium and other diseases, tuber yields, and specific gra-
vity. Depending upon the season, late blight epidemics have been established
naturally or by placing infected foliage in the experimental area to assure
some late blight pressure each season. All early blight epidemics have been
Pest Management Series
The 7-day, F-Blitecast, and Ridomil programs also have been studied as
part of a total pest control system in potatoes. These experiments have been
large factorial design tests with different insect, foliage disease, nematode,
and weed control methods being tested in Atlantic and Sebago cultivars.
Advisories made by Florida Blitecast have been 100% reliable (i.e. first
spray advisory issued before first occurrence of late blight in the area)
providing that accurate green row dates are known, data acquisition has been
accurate and that a 7-day spray advisory is issued in lieu of a warning when
the first late blight warning.is attained via rainfall. Advisories for the
NEF area tend to be similar during seasons with regular rainfall patterns;
during dry seasons irrigation practices alter moisture conditions and spray
advisories in individual fields.
Late blight was epidemic only during 1979 and 1980 (Table 1). There
were no significant differences in severity of late blight among the three
fungicide programs in either year. All three programs effectively controlled
In contrast to late blight, early blight developed in the test area each
season (Tables 2 and 5). The disease resulted in nearly complete defoliation
in 1977 and 1980. The crop was harvested before defoliation was complete in
1978 and heavy rains forced early termination of the experiment in 1979.
The most complete data were obtained in 1980 (Table 5). A significant
reduction in yield was associated with severity of early blight (r = -.68)
with a difference of 39 cwt/acre observed between the Ridomil and seven day
spray schedules. Interestingly, there were no significant differences in
specific gravity associated with levels of disease control. During the
course of the 1977-1980 studies an average of 7.3 and 4.8 fungicide applica-
tions were made in the seven day and Florida Blitecast programs, respectively
Pest Management Series
Late blight did not occur in this series of experiments. Terminal early
blight or defoliation ratings were significantly greater in the Ridomil plots
each year (Table 6). In 1978, there was a significant increase in severity
of early blight in the Florida Blitecast plots when compared to the seven day
spray schedule. Yields were significantly lower in Ridomil plots only in
1978 and 1979 (Table 7).
Researchers have varied in opinion concerning the importance of early
blight in Florida potatoes. Sherbakoff (4) and Weber (5) attributed substan-
tial yield losses to early blight. Eddins (1,2,3), however, believed the
disease to be important only in the southern part of the state. Data pre-
sented in this report show that early blight can reduce yields in NEF, how-
ever, with one exception, the disease was controlled by both the seven day
and Florida Blitecast fungicide schedules. Although Ridomil is totally in-
effective against early blight, three applications of Bravo made to Ridomil
plots in the 1980 Pest Management Experiment (Table 8), when weather condi-
tions favored development of the disease, prevented losses in yield (Table 7).
1. Fungicides scheduled according to Florida Blitecast have effectively
controlled both early and late blight diseases.
2. Left uncontrolled, early blight can significantly reduce potato
yields in NEF. The degree of loss depends upon the length of the growing
season and earliness of attack by the disease.
1. Eddins, A. II. 1938. Losses caused by potato diseases in the
Hastings section, Florida. Plant Disease Reporter 22:272-274.
2. Eddins, A. H. 1943. Potato diseases in the Hastings section of
Florida this season. Plant Disease Reporter 27:240-241.
3. Eddins, A. HI. 1948. Incidence of potato diseases at Hastings,
Florida in 1948. Plant Disease Reporter 32:302-303.
4. Sherbakoff, C. D. 1917. Some important diseases of truck crops in
Florida. Fla. Agri. Exp. Sta. Bull. 139. pp. 193-277.
5. Weber, G. F. 1923. Potato diseases and insects. Fla. Agri. Exp.
Sta. Bull. 169. pp. 102-164.
6. Weingartner, D. P. 1977. Potato late blight forecasting and spray
advisories. Hastings Research Report PR77-5. pp. 1-11.
Table 1. Terminal late blight ratings in 1979 and 1980
Fungicide Terminal late blight rating/
schedule 1979 1980 Average
Seven day 1.OA 1.OA l.OA
Florida Blitecast l.OA 1.OA 1.OA
Ridomil 1.OA l.OA 1.OA
No spray 2.3B 3.4B 2.9B
1/ Late blight rated on a scale of 1-10 (1 = 0 blight
and.10 = all plants dead) in 1979 and on the 1-12
Horsfall Barratt system in 1980. Values followed by
the same letter do not vary significantly via Duncan's
Multiple Range Test (P = .05).
Table 2. Terminal early blight
ratings as affected by
Fungicide Terminal early blight rating /
schedule 1977 1978 1979 1980 Average
Seven day 3.1 1.9A 2.2 8.0B 3.8
Florida Blitecast 5.9 1.8A 2.0 7.0A 4.2
Ridomil 6.7 5.8B 2.4 10.9C 6.5
No spray 7.2 4.9B 2.4 10.9C 6.4
1/ Early blight rated on a scale of 1-10 (1 = 0 disease
and 10 = all plants dead) during 1977-1979. The 1-
12 HIorsfall Barratt system was used in 1980. Values
followed by the same letter do not vary significant-
ly via Duncan's Multiple Range Test (P = .05). Data
from 1977 were nonreplicated.
Table 3. Yields of US Size A tubers as affected by
Fungicide Yield Size A tubers/
schedule 1977 1978 1979 1980
cwt cwt cwt cwt
Seven day 161 81 156 252A
Florida Blitecast 125 88 150 246A
Ridomil 157 137 158 213B
No spray 122 96 158 203B
V/ Values followed by the same letter do not vary sig-
nificantly via Duncan's Multiple Range Test (P = .05).
Data from 1977 were not replicated. The large dif-
ferences observed during 1978 were due to drainage
patterns in the field.
Table 4. Number of fungicide applications made in
Fungicide No. fungicide applications
schedule 1977 1978 1979 1980 Average
Seven day 7 7 7 8 7.3
Florida Blitecast 1 4 4 10 4.8
Ridomil 1 3 3 3 2.5
No spray 0 0 0 0 0.0
Summary of terminal foliage disease ratings, yields of US Size A
tubers, and specific gravities as affected by fungicide schedules.
1980 Epidemiology .Experiment.1/,2/
Terminal Terminal Yield No.
Fungicide early late US Specific applica-
schedule blight blight Defoliation Size A3/ gravity tions
Seven day 8.0B 1.OA 9.9A 252B 1.062 8
F-Blitecast 7.0A 1.OA 9.8A 246B 1.062 10
Ridomil 2E 10.9C 1.OA 11.3B 213A 1.062 3
Control 10.8C 3.4B 11.5B 203A 1.062 0
1/ Early blight, late blight, and defoliation were rated on a scale of 1-
12 according to the Horsfall Barratt system.
2/ Values followed by the same letter within a column do not vary signi-
ficantly (P = .05) via Duncan's Multiple Range Test. Lack of a letter
3/ Yields in cwt/acre.
Table 6. Summary of terminal early blight severity
ratings made in 1977-1980 Pest Management
Fungicide Early blight rating-1/,2/
schedule 1977 1978 1979 1980"3
Seven day 3.8a 2.0a 2.4a 9.9
Florida Blitecast 4.0a 2.3b 2.2a 9.4
Ridomil4/ 6.9b 5.8c 7.5b 10.3
1/ Early blight rated on a scale of 1-10 during 1977, 78,
and 1979 with 1 = 0 disease and 10 = 100% all foliage
lesioned; Horsfall Barratt (1-12) system used in 1980.
2/ Values followed by .the same letter do not vary signi-
ficantly (P = .05) via Duncan's Multiple Range Test.
3/ 1980 data defol.
SDuring 1980 Ridomil was applied for controlling late
blight and three applications of Bravo 500 were made
according to FAST for early blight control.
Table 7. Summary of yields of US Size A tubers as in-
fluenced by fungicide schedules in Pest Manage-
ment Experiments 1977-1980.
Fungicide Potato yields (cwt/A)1/
schedule 1977 1978 1979 1980 Mean
Seven day 270 260a 217a 310 264
Florida Blitecast 271 255a 214a 313 263
Ridomil 271 245b 207b 314 259
1/ Values followed by the same letter do not vary sig-
nificantly (P .05) via Duncan's Multiple Range
Table 8. A summary of the number of fungicide applica-
tions made in Pest Management Experiments
during the 1977-1980 seasons.
schedule 1977 1978 1979 1980 Average
Seven day 7 6 6 8 6.75
Florida Blitecast 5 3 5 10 5.75
Ridomil 5 3 3 4+31/ 3.75
1/ During 1980 Bravo applications were made over the
Ridomil sprays whenever weather conditions, as indi-
cated by FAST, favor development of early blight.