Issue 2, May 1, 2009
Rhizosphaera and Swiss Needle Casts
Rhizosphaera needle cast is primarily a problem on landscape spruce trees, primarily Colorado blue spruce. Swiss needle cast is a common problem on Douglas-fir trees in plantations and forests, especially in wet regions. Trees planted outside their natural range, stressed trees, and wet conditions all promote more infection.
Both of these needle cast diseases are now visible. They cause discoloration (yellow then brown) of one-year-old needles and eventual defoliation (casting) of older needles. Rhizosphaera needle cast is commonly found on Illinois spruce trees and occasionally on Douglas-fir. Swiss needle cast looks almost identical but occurs commonly on Douglas-fir.
Rhizosphaera needle cast is diagnosed by the pin-head sized fruiting bodies of the Rhizosphaera fungus. These protrude through the stomates on the under side of the needles, appearing in neat rows. The first image shows Rhizosphaera needle cast on spruce. Pathologists also look further to confirm the disease. The spores within these pycnidia are unicellular and oval.
The Swiss needle cast fungus, Phaeocryptopus gaeumannii, produces similarly sized fruiting bodies in rows on the underside of needles, but on Douglas-fir. These fruiting bodies are actually pseudothecia that contain ascospores that look much different than the spores of Rhizosphaera. Without a compound microscope, the diseases cannot be differentiated. The second image shows pseudothecia of Swiss needle cast on Douglas-fir needles.
Spores are released from infected tissue starting in April and continuing with wet weather. As with Rhizosphaera on spruce, you can protect new growth of the Douglas-fir trees from infection by the Swiss needle cast fungus. Sprays are initiated at bud break for Douglas-fir and as soon as bud caps fall off for spruce. Chemicals are repeated at label intervals until new growth is fully elongated. Products labeled for both hosts and fungi include Camelot, Chlorostar, Daconil, Echo, Kocide, Manicure, Protect, Spectro, or TwoSome. The first seven products have protective/contact mobility. Spectro and TwoSome are the only products with upwardly systemic activity. Home growers can use chlorothalonil products. Consult the Illinois Home, Yard, and Garden Pest Guide.
Links to information on Rhizosphaera needle cast are abundant. Two links to more details and images on Swiss needle cast follow:
--Nancy R. Pataky