Plant Disease Management Handbook

Cause Verticillium dahliae, a fungus that survives in soil as microsclerotia. Another species V. isaacii has also been isolated from wilted sunflower plants in the Columbia Basin, Washington. Survival structures germinate and developed into mycelium on root hairs, elongation zones, and caps of lateral roots. Within a week, the mycelia colonize vascular bundles of lateral roots and taproots.

Cause Heat or moisture stress can result in a sunburn on the flower heads.

Symptoms Brown or blackened areas on the side of the plant continuously exposed to sun.

Cultural control

• Maintain adequate irrigation.

Cause The fungus Sclerotinia sclerotiorum overwinters as black sclerotia in soil, among seeds, and in plant debris for 3 to 7 years. Dry, cool soil conditions favor survival of sclerotia. Sclerotia germinate and infect adjacent roots unless they are buried too deeply. The fungus also can spread from plant to plant by root contact. Sclerotia may also germinate to form small mushrooms (apothecia) that produce a sexual spore stage (ascospores), which are blown by wind and can infect flowers or stems.

Cause The fungi, Golovinomyces cichoracearum (formerly Erysiphe cichoracearum), has been reported from Idaho and Washington and G. spadiceus has been reported from Washington. Podosphaera xanthii and G.

By O. Nehers and C. M. Ocamb

By O. Neher and C. M. Ocamb

Cause Ramularia beticola, a fungus. Most likely it is seedborne. It is believed to persist in beet debris at least 1 year. The disease is not considered an economic problem.

Symptoms Leaf spots, which occur on older leaves, are light brown and fairly large (4 to 7 mm). As they mature, they develop a gray to white center (sporulation of the fungus) and a dark to reddish-brown margin.

By O. Neher and C. M. Ocamb

Cause The fungus, Erysiphe betae (syn = E. polygoni, Microsphaera betae), affects plants in the Polygonaceae. The disease has been widespread in several western states since 1974. Fungal spores may blow in from overwintering sources. Overwintered infected seed beets may also be sources of infections. The fungus tolerates a wide range of environmental conditions.

By O. Neher and C. M. Ocamb

Cause The fungus-like soilborne organisms, Phytophthora drechsleri and Pythium aphanidermatum, cause root rot primarily in waterlogged soils at high temperatures. Low areas of the field are where these two diseases are most often found.

By O. Neher and C. M. Ocamb