Water stress and Sphaeropsis sapinea as a latent pathogen of red pine seedlings.
The role of water stress in the initiation of collar rot by S. sapinea [Diplodia pinea] in asymptomatically colonized 2-year-old 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% in repeatedly watered seedlings to 50% in 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. S. 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.