Powdery Mildew

Signs
The ear­liest sign of powdery mildew is the presence of small circular white patches with feathery edges randomly scattered across the lower surfaces of one or more leaves.  Yellow or purple blotches may also appear, particu­larly on the upper leaf surface.

Damage
Under ideal conditions, patches will rapidly increase in size until the surfaces of the leaves and tender shoots are covered with the white, cottony fungal growth.  The foliage of heavily diseased hydrangea is often stunted and off-color. The beauty and size of the flowers of powdery mildew-damaged hydrangeas will also be greatly reduced, as young stems and flower stalks are infected and killed off.

Ideal Conditions
Usually worse in shady sites and when the relative humidity approaches 95 percent, powdery mildew often affects plants that are under stress.  The warm days, cool nights, and light rainfall of the spring and fall favor disease out­breaks. Typically, frequent showers will suppress the devel­opment of powdery mildew.

Prevention and treatment
Regular feeding, mulching and watering will reduce the likelihood of mildew attacks.  Good air circulation around the plant is also helpful.  Ask your local garden center for a fungicide for powdery mildew on hydrangeas.

Cercospora Leaf Spot

Signs
Scattered, small circular brown or purple spots first appear on leaves near the base of the plant.  The centers of these spots eventually turn tan to light gray in color and are sur­rounded by a brown or purple halo. The spots are usually about 1/8 to 1/4 inch in diameter. This combination of a pale center and dark margin is usually called a frogeye leaf-spot pattern.  The spotting of the leaves, which usually starts in midsummer, is most noticeable by early fall.

Damage
Heavy spotting of the leaves and pre­mature leaf shed may reduce plant vigor and flower bud set.

Ideal Conditions
Frequent late summer rain showers will not only greatly increase the rate of disease spread, but also intensify the level of leaf spotting and defoliation.  Extended periods of drought will usually suppress disease development and spread.

Prevention and Treatment
Remove dead diseased leaves, as fallen diseased leaves are the primary source of spores.  Water at root ball rather than on leaves, and avoid watering late in the day.  Ask your local garden center for a fungicide for leaf spot on hydrangeas.