The plants with leaf spots that I have been seeing the most are hydrangeas. The most common diseases on the leaves of hydrangeas are cercospora leaf spot and anthracnose.
The symptoms of cercospora leaf spot are scattered, small, circular brown or purple spots on the leaves. On the bigleaf hydrangea, the centers of these spots eventually turns tan to light gray in color and are surrounded by a brown or purple halo. The spots are usually about e to 1/4-inch diameter.
In contrast, the spots on oakleaf hydrangea appear angular in shape and are dark brown to purple in color. Quite often, heavily spotted leaves turn yellow-green and fall to the ground.
Typically, the spotting begins on the leaves at the base of the plant and then gradually spreads upward through the canopy. Spotting of the leaves, which usually starts in midsummer, is most noticeable by early fall.
Fallen diseased leaves are the primary source of spores of the causal fungus. These spores are spread to the healthy lower leaves by splashing water. Once cercospora is introduced into a planting of hydrangeas, you normally have it every year to some extent. Frequent late summer rains will not only greatly increase the rate of disease spread, but also intensify the level of leaf spotting and defoliation. Extended periods of drought, like we are going through now, will usually suppress disease development and spread.
To help prevent, or at least cut down on the severity of the disease, apply just enough nitrogen to maintain a moderate growth rate, and direct your watering at the base of the plant to help slow the development and spread of cercospora leaf spot. Since the appearance of symptoms is usually delayed until late summer to early fall, it is basically too late to apply protective fungicides.
For effective control with a fungicide, you must begin applications when spotting of the leaves is first seen and continue applying that treatment as needed. Fungicides registered for the control of cercospora leaf spot are Daconil 2787 (chlorothalonil) and Immunox (miclobutanil).
The other disease I mentioned earlier, anthracnose, can attack the leaves and the blooms of the hydrangea. Hot, wet weather appears to favor disease development -- and we sure had plenty of that this summer. Heavily fertilized hydrangeas may be the most sensitive to attack by anthracnose.
At first, brown spots are circular or slightly irregular in shape and somewhat sunken on fleshy leaves of hydrangea. The center of these spots may reach 1 inch or more in diameter and turn light brown to tan. Alternating dark and slightly lighter rings of dead tissue often give the spots a bull's-eye or target-spot appearance. When larger spots border the midvein or other major veins in the leaf, they become distinctly more angular in shape.
Under ideal conditions for disease development, large, dark brown, irregular blotches may spear across the leaves and flower petals. Unlike cercospora leaf spot, symptoms of anthracnose may appear almost simultaneously on leaves and blooms in the lower and upper region of the plant canopy.
There are few options available for controlling anthracnose. Collecting fallen leaves and removing blighted blooms are one way to help. You can also use protective fungicides at the first sign of infection at 10- to 14-day intervals during the summer. Daconil 2787 is the best fungicide for this disease. But like cercospora leaf spot, it is too late in the season to spray now.
Keep in mind that while these diseases rarely kill a plant, spotting on the leaves and premature leaf shed are unsightly, and can reduce plant vigor and ultimately flower bud set.
Reach Sid Mullis, the director of the University of Georgia Extension Service Office for Richmond County at (706) 821-2349 or firstname.lastname@example.org.