EFFECTS OF MODERATE WATER-STRESS ON DISEASE DEVELOPMENT BY SPHAEROPSIS-SAPINEA ON RED PINE

The aggressiveness of Sphaeropsis sapinea isolates was compared on wat er-stressed and nonstressed 3-year-old red pines (Pinus resinosa) in g reenhouse and growth chamber experiments. Water was withheld from stre ssed seedlings to achieve mean predawn needle water potentials (psi(PD )) above -1.9 MPa. The lowest mean psi(PD) of well-watered seedlings w as maintained at or above -0.8 MPa. Young shoots were inoculated by pl acing colonized agar plugs on wounds made by removing a needle fascicl e. Two isolates of each recognized morphotype (A and B) were used in t he greenhouse experiment and two isolates of morphotype A were used in the growth chamber experiment. After 4 weeks, isolates of morphotype A caused more severe symptoms and could be recovered farther from the inoculation site on water-stressed than on nonstressed trees in both e xperiments. In the greenhouse experiment, isolates of morphotype A als o caused more severe symptoms and could be recovered farther from the inoculation site than isolates of morphotype B, regardless of watering regime. These results indicate that water stress at levels observed t ypically in the field can result in increased disease development by i solates of S. sapinea morphotype A on red pine. The reduction of water stress of red pines in the field may reduce losses due to Sphaeropsis shoot blight.