Fungicidal effects of azoxystrobin and epoxiconazole on phyllosphere fungi, senescence and yield of winter wheat

The effects of the fungicides azoxystrobin (a strobilurin) and epoxiconazole (a sterol biosynthesis inhibitor) on phyllosphere fungi, senescence and yield were studied in winter wheat in field trials free of visible disease and under controlled environmental conditions. In two field trials, treatments with each of the two fungicides prolonged green leaf area retention and increased yield compared with untreated control plots. Azoxystrobin maintained green leaf area for longer than epoxiconazole and, in one trial, treatments with azoxystrobin gave a greater yield response than epoxiconazole, Mycelial growth on leaf surfaces, mainly originating from saprophytic fungi, was reduced by each of the fungicides. Papilla formation and hypersensitive reactions, almost exclusively against infection attempts by Mycosphaerella spp. (most probably M. graminicola), occurred with high frequency in the leaves. These defence reactions presumably incurred a significant energy cost, accelerating plant senescence. Fewer defence reactions were recorded in azoxystrobin-treated leaves than in epoxiconazole-treated and untreated leaves. Inoculation in a glasshouse experiment with the saprophytic fungi Alternaria alternata and Cladosporium macrocarpum accelerated wheat senescence. Control of the saprophytes by azoxystrobin or epoxiconazole treatments caused a delay in the accelerated senescence, but without significant increase in above-ground biomass and yield. Neither fungicide influenced senescence, above-ground biomass or yield in noninoculated wheat plants. In growth chamber experiments azoxystrobin inhibited spore germination and mycelial growth of A. alternata and C. macrocarpum, Epoxiconazole had little inhibitory effect on spore germination, but strongly inhibited mycelial growth of both saprophytes. Both fungicides reduced A. alternata-induced papilla formation in wheat leaves, with epoxiconazole being more effective. Inoculation with either of the two saprophytes did not significantly increase wheat leaf respiration, in contrast to inoculation with the nonhost pathogen Erysiphe graminis f.sp. bordei. Treatment with azoxystrobin did not affect this latter increase in respiration whereas it was reduced by epoxiconazole treatment. It is proposed that the greater inhibition of infection attempts from Mycosphaerella spp. by azoxystrobin, compared with epoxiconazole, may account for the greater yield given by azoxystrobin in field plots.