Efficacy of fungicidal protection of newly developing potato leaves against Phytophthora infestans

Three experiments aiming to explore the efficacy of fungicides to protect newly developing potato leaves were carried out between 2000 and 2002. Contact, translaminar and systemic fungicides were applied in field trials at Lelystad in the centre of the Netherlands. Efficacy of the fungicides was established using a bioassay in which detached new grown leaflets (ev Bintje) were inoculated with Phytophthora infestans. The interval between the last fungicide application and picking the leaves to be used in the bioassay varied between 4 and 11 d. Disease incidence was assessed after incubation for approximately a week. Protection of the newly developed leaves depended on the intrinsic property of the fungicides, time since the last fungicide application and growth rate of potato leaves in the field since the last fungicide application. Shorter time intervals between spraying and inoculation resulted in better (re)distribution of (contact) fungicides on newly developed leaves resulting in better protection. Also, there was less dilution of the fungicide due to less leaf expansion during a 4 d compared with 7 d spray interval. A similar effect occurs when crop growth rate decreases. Ridomil Gold MZ, with the systemic active ingredient metalaxyl and Ramnan (cyazofamid) resulted in the best protection of newly developed leaves at the beginning of the growing season when crop growth rate was high. Contact fungicides containing cyazofamid or to a lesser extent, mancozeb, can protect newly developing leaves, due to good redistribution of the compounds.