Certain varieties of Kentucky bluegrass are susceptible as
well, particularly when they are grown in shade.
The powdery white substance on these Dahlia leaves is a
fungal disease called powdery mildew
usually think of powdery mildew as a dry, humid weather problem
(water on the foliage actually inhibits the germination of the
fungal spores), the high humidity we have had creates a very
favorable environment for its development. Powdery mildew is favored
by warm days with low relative humidity and cool nights with high
relative humidity. Although it not usually life threatening for most
plants, it can make them unsightly. Infected leaves may yellow and
drop prematurely, and some herbaceous plants turn black. It can kill
vegetable crops such as cucumbers and zucchini.
The best defense against powdery mildew is to grow resistant
varieties when they are available. Although they can get the disease
when conditions are extremely favorable, they are less likely to
become infected than varieties that are extremely susceptible. I was
not able to find any dahlia varieties listed as resistant, although
there are cultivars and species of other plants that are.
Cultural controls include spacing the plants carefully, pruning and
thinning, and controlling weeds to allow good air circulation and
sun penetration. Plants grown in full sun are not affected as
severely as those grown in shade. Avoid overhead irrigation late in
the day or at night because it elevates the humidity level around
susceptible plants. It is best to water early in the morning.
Removing infected leaves where practical helps reduce the amount of
fungal spores present to continue the infection. Fungicide
applications can be made preventatively to keep powdery mildew from
becoming established on susceptible plants. Dithane (mancozeb),
Spectracide Immunox Multi-Purpose Fungicide (myclobutanil), Bayer
Advanced Disease Control for Roses, Flowers and Shrubs (tebuconazole),
and Ortho Funginex (triforine). Organic gardeners can use sulfur,
but not when temperatures are above 85 degrees.
Blossom End Rot