CORVALLIS - Do you have Douglas-firs that look brushy and stunted? Those trees may have Swiss needle cast disease.
Caused by the fungus Phaeocryptopus gaeumannii, Swiss needle cast is a disease of Douglas-fir needles. Symptoms include chlorotic (yellow) needles, premature needle drop, and reduced tree height and diameter growth. Swiss needle cast rarely kills trees, but badly affected trees may have lost most or nearly all of their old needles, and struggle to retain even current year needles. This loss may lead to a tree looking bushy and stunted.
The Swiss needle cast fungus is native to the Pacific Northwest and has been considered by scientists to be a minor pathogen in most circumstances, explained Bryan Capitano, who is studying the disease for his doctorate in plant pathology at Oregon State University.
The disease, he said, can be found on trees in the Cascades, the Willamette Valley, and the coastal mountains. It occurs in forested areas and on landscape trees. In most situations the fungus is not present in large enough numbers to be damaging.
"On the undersurface of Douglas-fir needles are small openings called stomata, which is where water and gas exchange occurs," explained Capitano. "Spores of the Swiss needle cast fungus infect the needle by growing through the stomata. In winter, after the fungus has gained entry and grown within the needle, it produces small black fruiting bodies (reproductive structures) called pseudothecia. These structures grow up through the stomata and look like sooty bands on the undersurface of the needle. They can be seen with a small magnifying lens."
The presence of these fruiting bodies in the stomata blocks the movement of water and gas, effectively "choking" the needle and leading to yellowing and death of the needle over time. In May and June, the fruiting bodies ripen and begin releasing spores into the air. Spores land on nearby needles and begin another infection cycle. Swiss needle cast is a single-cycle disease which means that needles are infected by spores only once every year.
The relationship between tree stress and disease caused by this fungus is poorly understood. Capitano says there is no evidence to suggest that Swiss needle cast fungus spores preferentially infect stressed trees over non-stressed trees (stress does not necessarily cause greater spore infections). However, trees that are already suffering from high levels of Swiss needle cast infection may have reduced roots and fewer needles, making it more difficult to cope with other root disturbances, additional diseases, or other stresses.
While Swiss needle cast is broadly distributed throughout the Pacific Northwest, it is more serious in the coastal mountains, especially near Tillamook, said Capitano. Following the Tillamook burns of the 1930's and 40's much of the forest had been replanted with Douglas-fir gathered from throughout Oregon and Washington. Much of the original Sitka spruce and western hemlock forests have also been logged and replaced largely with Douglas-fir.
One explanation for the increased disease severity near Tillamook is that a large percentage of Douglas-firs are from non-local areas that are more susceptible to disease, he said. In addition, these new populations of trees are growing in areas that receive higher moisture levels than what many Douglas-firs are adapted to. Rainfall, overcast skies, fog, and dew formation keep needles moist for longer periods of time. The coastal region also has fewer temperature extremes. Both moderate temperatures and ample needle moisture provide ideal conditions for needle diseases such as Swiss needle cast.
Relatively few options exist for controlling Swiss needle cast, he said. The most effective method of control is to choose planting sites which do not receive continual year-round moisture and with low surrounding disease pressures. At present, areas of high risk for disease are considered to be all areas west of the coastal mountain crest and between Cloverdale and Nehalem, Oregon. In addition to choosing low risk planting sites, using local seed sources will also reduce risks of disease.
For more information on control, refer to the PNW Plant Disease Control Handbook, on file at your local county office of the OSU Extension Service.