Water stress and Sphaeropsissapinea as a latent pathogen of red pine seedlings.

•  The role of water stress in the initiation of collar rot by Sphaeropsis sapinea in asymptomatically colonized Pinus resinosa seedlings is reported. • Mortality and frequency of identification of the pathogen was quantified for seedlings subjected to different water regimes or watering regime-fungicide (benomyl) combinations in glasshouse experiments. •  In experiment 1, seedling mortality ranged from 8% of repeatedly watered seedlings to 50% of those in the driest regime; data analysis indicated a high probability that mortality was not independent of watering regime. Seedlings developed symptoms resembling those of Sphaeropsis collar rot, and S. sapinea was identified from living (42%) and dead (92%) seedlings. In experiment 2, mortality of repeatedly watered seedlings was low, irrespective of fungicide application. For nonwatered seedlings, however, mortality was greater among seedlings not treated with fungicide (61%) than among benomyl-treated seedlings (37%); data analysis indicated a high probability that mortality was not independent of fungicide treatment. • Sphaeropsis sapinea can act as a latent pathogen; physiological alteration, through water stress, can effect release from the quiescent condition to result in rapid disease development.