UNIVERSITY PARK, Pa. -- The good news is that most infectious plant diseases hate drought. The bad news is that a few plant diseases like dry conditions.

"Infectious plant diseases typically are caused by fungi, nematodes, bacteria and viruses," says Gary Moorman, professor of plant pathology in Penn State's College of Agricultural Sciences. "Most of those organisms require prolonged periods of moisture to spread infection to other plants."

Dry weather inhibits many diseases that attack plant leaves and stems, such as fireblight, leaf spot, scab and needlecast of evergreens. Diseases that thrive in drought conditions usually live in the soil or within the vascular tissues of plants.

Moorman describes a few of the more common diseases that thrive during dry weather.

--Verticillium Wilt. Verticillium is a soil fungus that affects trees and shrubs. The fungus enters through roots and wounds, growing in the vascular system and depriving the plant's upper branches of water and nutrients.

"Yellowing and leaf wilt on one or two branches is a common symptom," Moorman explains. "Diseased limbs should be removed and destroyed. If the entire plant dies, replace it with another plant that is not susceptible to the disease."

--Dutch Elm Disease. Also a fungal disease, Dutch Elm disease causes vascular wilt. The fungus is transferred from tree to tree by elm bark beetles, which are attracted to stressed trees. Symptoms are yellowing on one or two branches. "Dutch elm disease usually is fatal," Moorman says. "If your tree is infected, contact a professional for treatment."

--Root Rots. Hot, dry soil conditions favor fungal diseases caused by Fusarium and Rhizoctonia that affect the roots and stems of plants. Moorman says these diseases seem to affect younger plants, particularly in nurseries. Symptoms include off-color foliage and wilting.

"As the disease progresses, upper branches will begin to die," Moorman says. "The disease can be controlled by reducing stress -- in drought conditions, that means watering."

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EDITORS: For more information, contact Gary Moorman at 814-863-7401.

Contacts: John Wall jtw3@psu.edu 814-863-2719 814-865-1068 fax