The suppression of pycnidial production on wheat seedlings following sequential inoculation by isolates of Septoria tritici

Cross-inoculation of two wheat cultivars with two isolates of Septoria tritici was studied. Inoculation of seedlings of the wheat cultivar Seri 82 with the avirulent S. tritici isolate ISR398 followed at 2, 5, or 10 days later by inoculation with the virulent isolate ISR8036 resulted in marked reductions in pycnidial coverage. Significant reductions were also recorded on 'Shafir', which is susceptible to both isolates. Inconsistent reductions resulted from inoculating 'Shafir' with the virulent isolates first, followed by the avirulent isolate. No reductions were observed when the culture filtrates were used instead of conidia of the first isolate in the inoculation series. A sevenfold increase of conidia of the avirulent isolate (ISR398) resulted in a marked suppression of pycnidial coverage compared with a 1:1 ratio between the isolates. Subisolates produced by reisolation from pycnidia of 'Seri 82', which was inoculated first with ISR398 and then with ISR8036 (ISR398(I)/ISR8036(II)), were ISR8036-like as verified by virulence on 'Seri 82' and by probing with the S. tritici minisatellite DNA probe ST398-3.7A. The majority of the subisolates resulting from the reversed order of inoculation (ISR8036(I)/ISR398(II)) on 'Seri 82' were ISR8036-like. The induced seedling resistance of 'Seri 82' to the virulent isolate may be associated with mechanism(s) triggering pycnidial production. The suppression of pycnidial production in the susceptible cultivar Shafir can be explained in part by endogenous competition between the two isolates during colonization of wheat leaf tissue.