August 25, 1995
Have you noticed that many lilac leaves at this time of year are more white than green? The whitish appearance is caused by the powdery mildew fungus. The white "powder" is composed of fungal structures (mycelium and spores).
White spots on leaves usually start to develop in mid-summer and enlarge as the summer progresses. By late summer or fall entire leaves may appear white. Also in the fall, tiny specs (cleistothecia) appear on leaves. These fungal structures are especially evident on lower leaf surfaces.
It is that time of year when Phytophthora root rot starts showing up in rhododendron and azalea plantings. The fungus is favored under extremely wet conditions and in heavy, poorly drained soils. The wet spring may have aggravated the condition more this year.
Phytophthora root rot of rhododendrons is caused by several species of Phytophthora. The pathogens, primarily P. connamoni, P. citriocola and P. cactorum, are soilborne and invade roots under wet conditions. Most cultivars of rhododendrons are highly susceptible to attack by Phytophthora.
From mid-August through the end of the summer is when the fall webworm will be noticed on walnut and other hardwood trees. This insect is usually identified by the loose, gray, silk tent spun by a cluster of caterpillars feeding on the leaves at the end of the branch. The caterpillars are tan to yellow in color, hairy and up to 1 inch long.
For myself, gardening is enjoyable all year long. Others, however, are not so fortunate. Seasonal allergies can stop some people in their tracks with sneezing attacks, watery eyes and noses or an inability to breathe. Spring begins the allergy season with the pollen of ash, birch, elm, hickory, and other trees. Late spring and summer brings problems with grasses. Summer and fall continues the agony with numerous weeds such as dock, ragweed and amaranth.
The fungus Pythium, which is commonly found in the soil, can infect geranium root and stem tissue and cause plant death. Wet, poorly drained soil favors disease development.
Infected plants are usually stunted. Leaves turn a yellow or brown color, wilt, shrivel and eventually die. Lower leaves often show symptoms first. In early stages of the disease, plants may show recovery at night, but wilt again the following day.
The Hortline will resume normal hours on September 1. Hortline hours will be 10-12 and 1-4:30, Monday through Friday. The student interns that helped answer the Hortline during the summer months have returned to classes. They did a terrific job and will be missed.
This article originally appeared in the August 25, 1995 issue, p. 129.