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Things to watch for during August From our experience of samples we have received during August in previous years in the OSU Plant Clinic, this page will highlight some problems that may occur this month. Fungal disorders Samples of peppermint with verticillium wilt caused by the fungus Verticillium dahliae have frequently been received during August from Union, Marion, Linn, Lane, Benton and Baker counties. This fungus lives in soil where it infects roots and enters the plant's vascular system causing the characteristic wilt symptoms of leaf yellowing, browning and bronzing, and stunted, dying plants. click on image for more information Look out for tomato late blight caused by the fungus-like organism Phytophthora infestans. This organism is brought into the garden on transplants, and on seed potatoes on which it is also a problem. Although favored by cool, moist conditions, the disease becomes a problem in hot, dry conditions when foliage is irrigated later in the day and remains moist. Diseased leaves, petioles, and stems have green watersoaked spots which turn purplish-black, and a white mold may appear at the edges of the lesions. Fruits may also be affected. click on image for more information
Watch for powdery mildew on cucurbits (cucumber, squash, pumpkin, melon) which starts with small white spots on leaves and stems. This fungus then spreads to cover many parts of the plants. click on image for more information
Abiotic disorders Sunscald click on images for more information
Chemical injury click on images for more information
Further information can be found at these links: How Herbicides Work - Oregon State University Extension Service Herbicide Injury Symptoms - University of California Cooperative Extension Diagnosing Herbicide Injury on Garden and Landscape Plants - Purdue Extension Preventing Herbicide Drift and Injury to Grapes - Oregon State University Extension Service Bacterial disorders Cucurbits, including squash and cucumbers, often show symptoms of angular leaf spot caused by the bacterium Pseudomonas syringae pv. lachrymans. Irregularly shaped spots bounded by the veins occur on leaves. Stems and fruit may also be affected. This bacterium overwinters in diseased plant material and on seed and is then spread by rain, sprinker irrigation, as well as on hands and clothing. click on images for more information
Viral disorders Hosta Virus X is a virus relatively recently found in Hosta that can be spread by mechanical means from infected to healthy plants. Symptoms include mottling, mosaic and deformation of leaves. Some cultivars do not show symptoms even when infected. click on image for more information
see also "The Plant Clinic Year"
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July 28, 2009
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Oregon State University Plant Clinic 
