Plant Disease Management Handbook

Cause Heterodera humuli. Cyst nematodes are sedentary endoparasites. Surveys in 1961-1962 determined the presence of this nematode in hop yards near Independence and Salem, OR. Movement of hop roots from yard to yard and area to area, combined with annual flooding, probably has widely distributed the pest.

Symptoms White to brown protuberances (cysts) on roots.

Cause The fungus, Fusarium sambucinum (teleomorph: Gibberella pulicaris), survives in soil and diseased plants and has been found in OR, WA, and ID. Confirmed by OSU and the USDA-ARS in 'Chinook', 'Citra', 'Fuggles', 'Galena', 'Glacier', 'Mt. Hood', 'Nugget', 'Sterling', 'Willamette', and 'Zeus' varieties in the Pacific Northwest. The incidence of hop canker in the field is sporadic, not every bine on a hill is affected.

By D. H. Gent and C. M. Ocamb

Cause The fungus-like microorganism, Pseudoperonospora humuli, persists from year to year in infected hop crowns or plant debris in soil. It is an obligate parasite specific to hop. Disease is promoted by wet or foggy weather.

Cause Two fungal species, Fusarium avenaceum and F. sambucinum, cause cone tip blight and the disease has been found in OR, WA, and ID. These two fungi can survive in soil or plant debris. Field observations suggest that the onset of disease appears to be more severe in sites with more humid conditions during cone development, especially with overhead irrigation.

Cause A fungus-like microorganism, Phytophthora citricola, that survives in soil by long-lived oospores. It also may survive in infected plant parts but not in dead tissue except as oospores. The disease requires abnormally wet soils and is most often observed in areas of fields with soil or irrigation conditions that cause water to pool.

By C. M. Ocamb and D. H. Gent

Cause Honeysuckle yellow vein mosaic virus (HYVMV) and honeysuckle yellow vein virus (HYVV) have been isolated from Japanese honeysuckle showing yellow vein symptoms. Both viruses have a monopartite DNA genome, and are transmitted in a persistent manner by whiteflies. These viruses have been reported infecting tomato in Korea and Japan. Japanese honeysuckle cultivars such as Gold Net and Yellow Net show yellow vein symptoms and are assumed infected with one or both viruses.

See:

Rhododendron - Phytophthora Root Rot

Notes Phytophthora has been found by the OSU Plant Clinic a few times in both nursery and landscape situations. There is little information on this problem and it is suggested to see other Phytophthora root rot diseases for possible management tactics.

Cause The fungus Erysiphe caprifoliacearum var. flexuosa (formerly Microsphaera caprifoliacearum var. flexuosa) has been reported from Idaho and Washington. The disease has been diagnosed twice at the OSU Plant Clinic. This fungus is favored by conditions that produce high humidity but dry leaves. It is a highly specialized pathogen that forms a close association with the host.

Cause Erysiphe ravenelii (formerly Microsphaera ravenelii), a fungus endemic to North America. Succulent new growth late in the season is particularly susceptible. 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.