High temperature, darkness, and drought predispose black spruce seedlings to gray mold

Gray mold caused by Botrytis cinerea Pers.:Fr. developed in seedlings of black spruce (Picea mariana BSP) that were subjected to high temperatures (30–45 °C) in darkness or to drought conditions immediately before inoculation with the pathogen (10 6 conidia/mL). Incidence and density of spore production of B. cinerea in needles of the treated seedlings increased, and chlorophyll content decreased, with duration of the preinoculation treatments. Gray mold did not develop in seedlings subjected to preinoculation high temperature in the light, and also failed to develop in seedlings that were kept at 1–20 °C in light or darkness with adequate water, inoculated with 10 3 –10 7 conidia of B. cinerea/mL, then subjected to a range of postinoculation humid periods (0–48 h) at various constant temperatures (12, 20, and 28 °C) in light or darkness. Regression models were developed to describe the incidence and density of sporulation of B. cinerea and chlorophyll content in seedling needles as functions of the level and period of preinoculation high temperature plus darkness, preinoculation drought period, and seedling age (R 2 = 0.45–0.95; p ≤ 0.01). It was concluded that high temperature in combination with darkness, and drought stress, predisposed the seedlings to gray mold.