Build-up of tomato brown root rot caused by Pyrenochaeta lycopersici Schneider and Gerlach

The incidence in unheated glasshouses of tomato brown root rot (BRR) was recorded on five successive crops starting with the first ever grown on the experimental site. In the protected environment the changes followed a pattern that was defined mathematically, the regression linking angular or logit transforms of per cent BRR, observed at different intervals after planting, accounting for c. 72% of the variation. Rates of build-up, seemingly related to initial amounts of inocula, increased in successive seasons, there being more BRR in the fifth season than in the first. Four per cent BRR recorded 5 weeks after planting increased threefold during the next 3 months to 12 %, but in later years, when amounts of early-season infection were doubled to 8%, the incidence of root rot subsequently increased five times. Tomato roots colonized by Pyrenochaeta lycopersici either brown and rot or become corky. The proportion of end-of-season BRR attributable to corkiness increased from the first to fifth seasons. The monthly increments of fruit yield were inversely proportional to the incidence of BRR.