Effect of density and age of lesions on sporulation capacity and infection efficiency in wheat leaf rust (Puccinia recondita f.sp. tritici )

The effects of lesion ageing and crowding on spore production and infection efficiency of an isolate of wheat leaf rust (caused by Puccinia recondita f.sp. tritici  ) were studied under controlled conditions. Spore production per lesion per day decreased exponentially with increasing lesion density for lesions older than 13 days after inoculation. Spore production of younger lesions was not affected by crowding. A synthetic formula relating the accumulated spore production to lesion age and density was derived, allowing the evaluation of spore production per lesion on a daily basis. Infection efficiency was comparatively much less affected by lesion ageing and crowding than was spore production. The most efficient spores were produced by 'mature' lesions (14 to 21 days old). At high density, there was no compensation for reduced spore production by an increase in infection efficiency, except for 14-day-old lesions. The Daily Multiplication Factor (the product of spore production per lesion and infection efficiency) was higher at low lesion density and may contribute to a rapid increase of populations of sporulating rust lesions in wheat fields. The detrimental effect of lesion crowding, occurring later in the course of the epidemics, may have negligible effects on disease progress.