Plant Disease Management Handbook

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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.

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

See:

Fuchsia - Rust

Bracken Fern (Pteridium aquilinum) - Rust

Willow (Salix spp.) - Rust

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Fir, True (Abies spp.) - Shoot Blight

Cause Two fungi, Lirula abietis-concoloris and Virgella robusta, occur infrequently on white, grand, noble, Pacific silver, and California red fir. These fungi have a 2-year life cycle. Newly emerged needles are infected but persist through the first and second growing seasons. During the third growing season, fruiting bodies and spores are produced.

Cause Unknown. It is thought that it is due to the fungus Mycosphaerella sp.; however, several other fungi colonize the affected needles but are found less frequently. They include Phaeocryptopus nudus, Phyllosticta abietina, and Rhizosphaera pini. It is unknown whether one or more of these fungi are the primary cause or simply infect necrotic needles that have died from some other factor(s).

Cause The fungus Grovesiella abieticola can cause a canker and dieback on true fir. The fungus is found growing primarily on lower branches and stems. Young trees are attacked most often, making this a more serious disease in nurseries and Christmas tree plantations than in natural stands. White fir and shasta fir are more susceptible than noble fir or grand fir.

Cause Many conifers are susceptible to drought stress especially noble fir, grand fir, Pacific silver fir, subalpine fir, Alaska yellow cedar, Engelman spruce, Sitka spruce, western red cedar, and western hemlock. Douglas-fir and incense cedar are more tolerant of drought than noble or grand fir but can have similar problems on lower elevation, south-facing or marginal sites. Newly planted trees are most at risk to drought damage. Drought-stressed trees are highly susceptible to secondary attack from insects and canker-causing fungi.