Plant Disease Management Handbook

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Aster - Yellows

Cause Candidatus (Ca.) Phytoplasma asteris is spread by leafhoppers. The organism overwinters on wild weed hosts. Aster yellows also affects carrot, lettuce, endive, tomato, parsley, onion, salsify, many weeds, and several ornamental flowering plants. The disease is rare in Oregon.

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Cucumber (Cucumis sativus) - Angular Leaf Spot

Cause Alternaria cucumerina, a fungus that overwinters primarily in infected plant debris.

Symptoms First, small brown spots are on the oldest leaves, which turn brown to black as the disease progresses. Spots on the upper leaf surface take on a target-spot or ringed look. Fruit decay is dry and tough on the surface but moist and spongy inside the squash.

Cultural control

Cause The fungus Podosphaera euphorbiae can be found in gardens in western Oregon and Washington. It does not seem to be a debilitating problem as plants continue to grow well despite its presence. Shady locations encourage disease development.

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Cedar (Cedrus spp.) - Needle Blight

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Rhododendron - Rusts

Cause Rhizosphaera kalkhoffii is the most prevalent cause although several other fungi (such as R. pini, Lophodermium sp., Lirula sp.) can cause needle cast diseases on spruce. The fungus overwinters in infected needles attached to the tree or on the ground. Rain-splashed spores spread to current-season needles. Shearing crews also can spread spores if foliage is wet.

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Hemlock (Tsuga spp.) - Phytophthora Root Rot

Cause Unknown. Serbian spruce (Picea omorika) appears to sustain the most damage, but has been observed on a number of different spruce species and cultivars. Examinations of spruce, even before the bud cap has fallen off, have failed to find any signs of fungi, insects (which includes frass or exuvia) or eriophyid mites. Herbicide damage was suspected but the same degree of incidence was observed in treated vs. non-treated plots.

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Fir, True (Abies spp.) - Heterobasidion Root Disease

Note: Heterobasidion root rot is mostly a butt or trunk rot in spruce that becomes important when trees are 60 to 80 years old.