Plant Disease Management Handbook

Cause The fungus Peronospora digitalidis has been found once by the OSU Plant Clinic and reported once from a commercial nursery sample. The disease is common throughout coastal production in California. It can be found in the PNW nursery industry and in the landscape in Washington. Cool wet weather favors this disease. Conidia of this fungus are dispersed by wind or rain-splashed.

Cause The fungus Colletotrichum fuscum has been reported from Oregon and is favored by warm wet weather. It overwinters in plant debris.

Symptoms Small, light, or purplish brown spots develop on leaves. Spots remain small, about 0.12 inch, with a purple margin and can be circular or angular. Numerous fruiting bodies (acervuli) of the fungus may be seen at the spots' centers. If seedlings are attacked, they may wilt and die.

Cause There are 9 different viruses reported on Forsythia. Tobacco ringspot virus (TobRV) and arabis mosaic virus appear to have been found in Washington. These are nematode-transmitted viruses.

Cause The OSU Plant Clinic has isolated the bacterium Pseudomonas savastanoi from galled forsythia tissue from several locations. This bacterium is also involved with a disease of olive called olive knot where similar galls are found on twigs and branches. It is suspected that it is a new pathovar but that remains to be confirmed. Infection may occur in the fall through leaf scars as in the olive knot pathogen system.

Cause Pseudomonas syringae pv. syringae, a bacterium. Environmental factors play an important role with diseases caused by this bacterium. Rainy weather, winter or frost injury, and poor growing conditions favor disease development. Although the bacteria survive on the outside of the plant they must get inside and multiply in the space between plant cells (apoplast) to cause disease.

See:

Greenhouse Plants, Ornamental - Powdery Mildew

Cause A fungus, Fusarium oxysporum f. sp. lini, which survives in soil and plant debris as thick-walled, dormant chlamydospores. These spores germinate in response to exudates from nearby plant roots. Hyphae then penetrate the roots, colonize the cortex and move into the xylem tissue. Small spores (microconidia) are produced and carried up into the plant. The fungus sporulates on decaying plant tissue, and these spores can be moved by wind or rain to spread disease.

Cause Melampsora lini, a fungus that survives year to year in flax stubble and moves by spores from field to field. Spores also can be carried on small pieces of plant debris mixed with seed. The fungus has many different races, each of which infects a different set of Linum spp. Weakens plants and reduces quality and yield of linseed.

See:

Fir, True (Abies spp.) - Root Rot, Phytophthora

See:

Fuchsia - Rust

Bracken Fern (Pteridium aquilinum) - Rust

Willow (Salix spp.) - Rust