Efficacy of different fungicides against the leaf spot of lettuce caused by Allophoma tropica

Abstract

The efficacy of different fungicides, belonging to different chemical groups, was in vitro and in vivo tested in glasshouse trials against Allophoma tropica, the causal agent of a new leaf spot on lettuce. The in vitro trials showed that only a few fungicides were effective in inhibiting the mycelial growth of the four tested strains of A. tropica: the most effective fungicide, with the lowest ED₅₀ and MIC values, was penconazole. Prochloraz, propiconazole, boscalid and fludioxonil showed low ED₅₀ values, but high MIC values. The results from the in vivo trials showed that all the tested fungicides significantly reduced Phoma leaf spot incidence and severity, although differences were observed in the achieved disease pressure in the four trials. Generally, in the presence of an average disease severity in the untreated control, the best disease severity reduction (up to 90%) was provided 9 days after the application of fludioxonil, mancozeb, pyraclostrobin + boscalid, propiconazole and iprodione. Fludioxonil and iprodione showed a persistent effect (up to 90%) still being evident 12 days after the application. Generally, in the presence of a high disease severity in the untreated control, 9 days after the application of fludioxonil, mancozeb, iprodione and pyraclostrobin + boscalid provided a disease severity reduction up to 80%. Moreover, copper provided a disease severity reduction from 84 to 89%, applied as preventative treatment, resulting of special interest, particularly for crops grown under organic farming rules.