Plant Disease Management Handbook

Cause Physiologic: any sudden change in environment or even a gradual change to below-optimum growing conditions. Cold drafts, cold water, dark location, time of year, or simply moving it to a different location in the house can cause leaves to drop. This plant needs plenty of indirect light and 60°F to 70°F temperatures.

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California Wax-myrtle (Morella californica)

Cause A virus, zucchini yellow mosaic virus (ZYMV), that is found worldwide and can cause devastating losses. Several aphid species non-persistently spread this virus; it can be spread by mechanical means. Little is known about its overwintering.

Symptoms A yellow mosaic and severe malformations, necrosis, severe stunting of size and leaf lamina, and fruit cracking. Seeds may be deformed.

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 microorganism, Pythium spp., and the fungus, Rhizoctonia solani, are reported on watermelon. Fusarium spp. also are common damping-off fungal organisms. These pathogens are likely to be in any soil. Damping-off frequently attacks almost all kinds of vegetable seedlings just before or after they emerge from the soil.

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.