Plant Disease Management Handbook

Cause Pythium/Globisporangium spp. and Rhizoctonia solani are soil-infesting microbes that can build up rapidly and attack young germinating seedlings when soil is moist.

Symptoms Stems are attacked as young plants emerge. Plants wilt, fall over, and die. Germinating seed also can be attacked, rotting plants before they reach the soil surface.

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Peppermint (Mentha spp.)

Cause Two fungi, Phakopsora pachyrhizi and P. meibomiae, cause rust on soybean, snap bean, dry bean, cowpea, and numerous other legume species. P. pachyrhizi is considered the more aggressive strain on soybean and also infects kudzu (Pueraria lobata). Disease increases on leaf surfaces during prolonged periods of wet weather and temperatures range from 59°F to 86°F.

Cause The fungus Erysiphe diffusa (formerly Microsphaera diffusa) has been reported from Oregon, Washington and Idaho while Podosphaera clandestine has been reported from Washington and Idaho.

Symptoms White to gray powdery growth on the surface of leaves. Leaves may be deformed if infected when not fully expanded.

Cause Many fungi are capable of causing necrotic leaf spots on snowberry. Of these, Phyllosticta symphoricarpi, Ascochyta tenerrima, Septoria symphoricarpi, and Sphaceloma symphoricarpi (anthracnose) are reported as widespread throughout Northwestern States.

Cause Puccinia antirrhini, a fungus carried in diseased cuttings or spread by wind-blown rain, splashing water or clothes. Spores can also be spread by wind over long distances. Two races of the fungus have been identified. It is not carried in seed but may contaminate seed or seed pods. Plants must be wet 6 to 8 hours for infection to occur. The disease is favored by 55°F to 70°F temperatures but inhibited by temperatures above 80°F. Earwigs eat pustules and spores from their feces are not viable.

Cause The fungus Golovinomyces orontii (formerly Erysiphe cichoracearum) has been reported from Washington. Conidia are released in response to abrupt decreases in relative humidity. Wind blows conidia to healthy foliage where they start new mildew colonies. Too much water, such as flowing water or rain, destroys spores by causing them to burst. The fungus does not grow in a leaf but rather across the surface.

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Greenhouse Plants, Ornamental - Impatiens Necrotic Spot

Cause The fungus-like organism Peronospora antirrhini (and or P. ceperoae) causes a systemic infection of young snapdragons. The disease is favored by cool, wet conditions and/or high humidity. Spores are produced between 44°F and 72°F, though 50°F is optimum. Sporangia are spread by wind, air currents, or splashing water. When they land on a healthy plant they germinate by means of a germ tube. Conditions that keep leaves wet for 6 to 10 hours favors this disease.

Cause Both Pythium spp. a fungus-like microorganism and Rhizoctonia solani, a fungus, can cause pre- and post-emergence damping-off. Pythium irregulare, the species found frequently in the Northeastern United States, is favored by cool and overly moist soil conditions. Pythium spp.