Plant Disease Management Handbook

Cause Verticillium dahliae, a soilborne fungus that infects a wide range of crops, vegetables, fruits, and ornamentals, and which survives in soil up to 14 years as microsclerotia produced in infected plants. It infects via roots, invading water-conducting tissue, and spreads systemically throughout the plant. Moist soil and temperatures between 70°F and 81°F favor the disease. No resistant or tolerant varieties are available.

Cause Fusarium oxysporum f. sp. niveum, a fungus that may survive several years in soil as durable spores (chlamydospores) or in association with plant debris. Seed can also transmit Fusarium wilt, usually by contamination of the seed coat. The disease is favored by warm, sandy soils like those in watermelon-growing areas east of the Cascade Range. There is specialization within this fungal species towards various plant hosts affected.

Cause The fungus-like microorganisms, Pythium aphanidermatum, Globisporangium irregulare (formerly P. irregulare), and G. ultimum (formerly P. ultimum) as well as the fungus, Rhizoctonia solani, are reported on watermelon. Fusarium spp. also are common damping-off fungal organisms.

Cause Acidovorax avenae subsp. citrulli (syn. = Pseudomonas pseudoalcaligenes subsp. citrulli), a seedborne bacterium that affects watermelon, cantaloupe, and different types of melons. Damage in the greenhouse can be high. Field losses can be severe particularly in humid environments.

Cause Alternaria sp., a fungus. The disease has caused severe losses in Hermiston, OR area fields using overhead irrigation.

Cause A fungal disease (Geosmithia morbida), vectored by the walnut twig beetle (Pityophthorus juglandis), that can cause death of black walnut (Juglans nigra). The disease has been detected in many western states including eastern Washington, Idaho, and throughout Oregon on both sides of the Cascade Range (although insect trapping in the Grand Ronde Valley of Oregon did not detect the beetle).

Cause Many Phytophthora species, fungus-like microorganisms, cause a root and root crown rot of walnuts. The more important ones in California are P. cinnamomi and P. citricola. After river flooding, however, P. pini was a major problem on walnut scions. Cool, wet, or waterlogged soils favor the disease. Prolonged (24 hours or more) and repeated periods of saturation favor infections.

Cause Pratylenchus vulnus is a nematode that lives in soil but is rarely found in Oregon. Root-lesion nematodes are migratory endoparasites where part of the population is in soil and part in the roots at all times. All walnut rootstocks are susceptible but trees on Paradox hybrid are sometimes more vigorous than nearby trees on black walnut rootstock. The rootstock VX211 is tolerant. Can cause serious replant problems so be sure to sample before replanting into an area that has had walnut.

See:

Hazelnut (Corylus avellana) - Mosses and Lichens

Algae, Lichens, and Mosses on Plants

Cause The fungus Microstroma juglandis is widespread in western production areas. The disease is more common in wet years and in orchards with poor air circulation. Hyphae of the fungus emerge from stomata, especially along veins and produce white masses of spores.