Florida Agricultural Experiment Station


during the year. In addition to cucumbers, the fungus also at-
tacks watermelons, cantaloupes, gourds, pumpkins, squashes and
gherkins, but none of these plants suffers from the disease as
much as cucumbers.
 The fungus attacks severely only the leaves, rapidly killing
them. Spores are produced by the fungus a few days after it is
evident that the leaves are diseased. These spores are scattered
far and wide by the wind. They settle on the cucumber leaves and
germinate in the moisture supplied by the dew and infect the
plant. After three or four days the infection can be detected by















 Fig. 2. Downy mildew on plants in field, early stage.

 the appearance of yellow spots. (Fig. 2.) The disease spreads
 most rapidly during periods of comparatively low temperature,
 heavy dews and rains and windy days.
 Symptoms: Because of the similarity of the several leaf spots
 caused by different organisms on the leaves of cucumbers, it is
 often difficult to determine with certainty the disease at first
 glance. The earliest evidence of infection with downy mildew is
 a slight yellowing of small areas of the leaf blade. This yellow-
 ing appears several days after infection. When the yellow areas
 appear it is possible in the early morning before the dew has
 disappeared to detect them in a water-soaked condition. This is
 the result of water soaking into the plant tissue among the cells
 which are invaded by the fungus. The cells that have not been
 attacked by the fungus will not appear water-soaked. During the
 middle of the day this condition disappears but will again become
 evident with dew or rain. These spots are more or less angular in