Downy mildew

Downy mildew first appears as small, pale green to yellow, angular spots delimited by leaf veins that give the foliage a mottled appearance. Eventually the spots coalesce and the leaf will turn brown. During moist weather, the lower surface of the leaf may be covered with a white to purple growth. Older leaves become infected first.

• Reference: http://www.ipm.ucdavis.edu/PMG/r116101611.html