Everglades Station Mimeo Report EES69-13 May, 1969
Epidemiology of Early Blight of Celery and
Helminthosporium Leaf Blight of Sweet Corn
R. D. Berger 1
Introduction. Cercospora blight of celery and Northern leaf blight of
sweet corn are considered the most important diseases on these two crops.
Investigation of the factors influencing the fungal sporulation and the spread
of the fungal spores was undertaken to try to maintain adequate disease control
but with reduced spray costs. The ultimate aim is a reliable forecasting
method for both diseases.
Instrumentation. 1) A portable, continuously sampling spore trap
(designed by T. W. Casselman, EES); 2) hygrothermograph (records temperature
and humidity on weekly charts); 3) Recording anemometer (records wind speed
and direction); 4) Rain gauge.
Results and Discussion. Spores of both fungi (Cercospora apii and
Helminthosporium turcicum) were formed under periods of high humidity (gener-
ally at night) and were released into the air as the humidity decreased
rapidly (usually early to mid-morning). Trapped spores of both fungi
reached a peak concentration around noon or shortly thereafter. The numbers
of spores trapped tapered off toward late afternoon and very few spores
(about 3% of total) were trapped at night (8 P.M. 8 A.M.).
Influence of temperature. Since 100% relative humidities are experienced
in the Glades almost every night, the percent humidity was not a limiting
factor in the sporulation of these blight fungi. What was limiting was the
duration of the high humidity period and the concurrent temperature. Both
fungi required at least eight hours of continuous high humidity to achieve
significant sporulation. Rains, morning fog and heavy overcast extended the
high humidity period. Practically no sporulation of either fungus occurred
when temperatures were below 560F (measured four feet above the ground), even
though humidities were 100%. Sporulation was slow at temperatures between 560
and 650F (i.e., humidity periods must be of considerable duration for signifi-
cant sporulation to occur at these temperatures).
With several successive days of weather favorable for sporulation, the
numbers of spores trapped increased rapidly with each passing day. With the
onset of cold weather (below 560F) the sporulation dropped to nil regardless
of the numbers of spores trapped the preceding day. Several successive days
of warm humid weather were again necessary for the fungi to resume significant
1/ Assistant Plant Pathologist, Everglades Experiment Station, Belle Glade,
The numbers of hours favorable for sporulation was perfectly correlated
With the actual number of spores trapped. Both "favorable hours" and spores
were perfectly correlated with the disease which subsequently developed.
Wind. The role oftwind ih the actual detachment of spores from leaf
lesions as not been fully invtetigated. The detachment of spores appears to
be affected more strikingly by changes in relative humidity. Wind, of course,
plays a very important part in the distant spread of both diseases.
Summary. When less than eight hours of high humidity at temperatures
above 560F are experienced daily, spread of both diseases is negligible and
consequent savings in fungicide can be made.