Persistence of Sphaeropsis sapinea on or in asymptomatic shoots of red and jack pines.
To investigate the possibility that the conifer shoot blight and canker pathogen S. sapinea [Diplodia pinea] persists on or in asymptomatic red pine (Pinus resinosa) and jack pine (P. banksiana), dormant shoots were collected from nursery seedlings and forest trees in Wisconsin, USA. One-hundred asymptomatic red pine seedlings were collected in November 1994 in each of 2 nurseries and 100 asymptomatic jack pine seedlings were collected at 1 of the nurseries. Fifty shoots of each species were collected at each of 3 forest sites. Each shoot was divided into 4 subsamples: 5 needle pairs from the current year's growth, 5 needle pairs from the previous year's growth, a stem segment from the current year's growth and a stem segment from the previous year's growth. Subsamples were surface-disinfested, placed into water agar slants and incubated for approximately 3 months. Identification of D. pinea was based on examination of spores from pycnidia that formed on the plant tissue. The fungus was associated with 27.5% of the red pine nursery seedlings but with none of the jack pine seedlings. With only 1 exception, the fungus was associated with stem segments of the previous year's growth. On samples collected in the forest, the pathogen was associated with both years' stem segments and with previous year's needles of each host. Characterization of isolates by means of RAPD marker analysis indicated presence of both A and B morphotypes of D. pinea which were confirmed as virulent by inoculation of red and jack pines, respectively. The ability of virulent strains to persist on or in asymptomatic hosts may help explain pathogen survival and rapid disease development under conditions that induce host stress.