Black Stain Root Disease in Ozone-Stressed Ponderosa Pine. M. E. Fenn, Research Plant Pathologist, Forest Fire Laboratory, Pacific Southwest Forest and Range Experiment Station, USDA Forest Service, Riverside, CA 92507. P. H. Dunn, and R. Wilborn. former Supervisory Microbiologist, and former Ecologist, Forest Fire Laboratory, Pacific Southwest Forest and Range Experiment Station, USDA Forest Service, Riverside, CA 92507. Plant Dis. 74:426-430. Accepted for publication 5 December 1989. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1990. DOI: 10.1094/PD-74-0426.

In order to determine the effects of ozone exposure on black stain root disease, caused by Leptographium wageneri var. ponderosum, 2-yr-old ponderosa pine seedlings (Pinus ponderosa) were fumigated with ozone in open-top chambers during daylight hours for 11 wk. The ozone exposure profile was based on ozone exposures measured during August 1987 at Giant Forest in Sequoia National Park, California. The ozone concentration gradually increased during the morning and early afternoon, reached a peak in the afternoon, and then decreased until sundown. The peak concentrations were 0.1, 0.2, and 0.3 ppm in three ozone treatments. The cumulative ozone exposures in charcoal-filtered air and in the three ozone treatments were 0.26, 0.95, 1.90, and 2.84 ppm-hr/day. Injury on the previous year’s needles increased (R² = 0.94) and stem growth decreased (R² = 0.61) in inoculated and noninoculated seedlings as the ozone concentration increased. Needle injury was greater and stem growth was less in seedlings inoculated with L. wageneri var. ponderosum than in noninoculated seedlings. The length of the black stain in roots of inoculated seedlings increased as the ozone exposure increased (R² = 0.40); 43% of the seedlings exposed to charcoal-filtered air (with an ozone concentration of 0.26 ppm-hr/day) had visible black staining, whereas 79% of those exposed to ozone at 2.84 ppm-hr/day were stained. From these studies, we suggest that in areas where black stain root disease occurs on ponderosa pine, exposure to elevated levels of ozone is likely to result in increased losses due to the disease.