Bulletin 208, Cucumber Diseases in Florida


outline and are delimited by the veins of the leaf blade. (Fig. 1,
cover.)
 The spots may be few or many on a single leaf, depending upon
 the number and distribution of spores causing the different in-
 fections. A single spot seldom develops to more than a centimeter
 in diameter. However, if the spots are numerous enough on a
 leaf they will coalesce and in a day or two kill the whole leaf,
 which will dry out and turn brown. The dead leaves remain
 erect and the edges of the blade curl inward very characteristical-
 ly. (Fig. 3.) A few days after the yellow spots become visible on
















 Fig. 3. Downy mildew killing the leaves of cucumber
 plants, last stage.

the leaves a careful examination of one of these spots on the lower
surface will reveal a fine, white downy growth. Under certain
conditions the growth is seen with difficulty, even with the aid of
a good hand lens. The spores of the fungus are borne on the
tips of this downy growth. (Fig. 4.) These spores are usually
dark colored and give the areas of the leaf which they occupy a
grayish color. Upon closer examination, the spots appear to be
covered with very fine earth particles which are sticking to the
surface among the hairs of the leaf. These small particles are the
spores. They are barely distinguishable without the aid of a
lens. These spores are easily detached and because of their light
weight and small size are carried considerable distances by the
wind. They may cause additional infections on the same leaf or
adjacent healthy plants in the next 24 hours, should temperature
and moisture conditions be favorable for their germination.