Plant Disease Management Handbook

Cause Hyaloperonospora parasitica (formerly Peronospora parasitica), a fungus-like organism. Downy mildew was first found in 1990 on Alyssum saxatilis in Oregon and sporadically since then. No written report exists for this disease in North America. P. galligena has been reported in Europe; however, the Oregon isolate agreed with the description for P.

Cause Both Pythium spp. and Rhizoctonia solani can cause pre- and post-emergence damping-off. Neither has been reported in the Pacific Northwest but Pythium damping-off has been found by the OSU Plant Clinic. Pythium is favored by cool and overly moist soil conditions.

See:

Greenhouse Plants, Ornamental - Gray Mold

See:

Pansy (Viola spp.) - Thielaviopsis Root Rot (Black Root Rot)

Cause Freezing, desiccation by exposure from soil heaving, and ice formation within living root and crown tissue.

Symptoms Thin stands following severe winters with no snow cover. Crowns and upper part of taproot are dead and discolored on the outside; damage extends part way into the woody tissue. Winter injury is usually complicated and made more severe by the establishment of parasitic bacteria and fungi in the damaged area.

Cause Verticillium alfalfae (formerly V. albo-atrum) or V. dahliae, soilborne fungi, may infect numerous plant species without causing foliar symptoms. The disease is in eastern Washington, the Yakima Valley, the Willamette Valley, the Columbia Basin, and irrigated areas in northern, southwestern, and south-central Idaho.

Cause A fungus, Phoma medicaginis, which persists in crop refuse, infected crowns, and taproots. The disease may also spread through infected seed. Disease is most common in wet springs when it reduces yield and quality of first cuttings.

Cause Sclerotinia trifoliorum or S. sclerotiorum, which are soilborne fungi that can survive indefinitely as sclerotia in soil. They can attack stems and crowns in wet, cool springs. Damage is most severe on stems in solidly planted mature alfalfa fields. Early-growing varieties such as DuPuits are particularly susceptible. In the Pacific Northwest, sclerotia germinate in spring, producing small (0.25 to 0.5 inch in diameter), light brown, stalked discs (apothecia).

Cause Phytophthora megasperma f. sp. medicaginis (= P. medicaginis), a soilborne fungus-like microorganism favored by excessive soil moisture. The microorganism survives as hardy spores in soil or infected plant debris. Irrigation water can transport the microorganism.

Cause Ditylenchus dipsaci is a nematode that lives principally in the stem portions of plants. It can persist in soil without a susceptible crop for at least 2 years. Flooding of affected areas helps to disseminate nematodes and increase damage. Most damage is done by nematode feeding in winter.