EFFECT OF DIFENOCONAZOLE ON THE FORMATION OF OOSPORES BY PHYTOPHTHORA INFESTANS (MONT) DE BARY S.N. Elansky 1,2 , E.D. Mita 1 , E.S. Skolotneva 3 , M.A. Pobedinskaya 1 and L.Yu. Kokaeva 1 1 Lomonosov Moscow State University, Leninskie Gory, 1, 119991, Russia, Moscow 2 All-Russian A.G. Lorkh Research Institute of Potato Farming, Kraskovo-1, Lorkh str., 140051, Russia, Moscow region 3 National Research Center of Hematology, Novozykovsky pr. 4a, 125167, Russia, Moscow SUMMARY Fungicide difenoconazole is applied for field treatment to protect potato and tomato against early blight, caused by the Alternaria fungi. It is practically important to esti- mate the impact of difenoconazole application against late blight pathogen Phytophthora infestans (Mont.) de Bary be- cause of early and late blight often come together on pota- to. Effect of fungicide difenoconazole on the radial growth of colony of P. infestans and the formation of oospores in agar media and in detached leaves was investigated. There was no significant influence of difenoconazole on the radial growth of P. infestans colony, small inhibi- tion occurred only at high concentration of fungicide (100 mg/l). Difenoconazole inhibited the oospore formation in the medium with positive dependence on concentrations of fungicide and temperature increasing. Tested in leaves difenoconazol inhibited the oosporogenesis with positive dependence on concentrations of fungicide also. The tiers of the plant were considered as source of oo- spores with different potential. In laboratory conditions (test on detached leaves) the higher production of oospores was detected in leaves from middle and low tiers rather than from top tiers. Keywords: Phytophthora infestans , oosporogenesis, late blight of potato, resistance to fungicides, difenoconazole INTRODUCTION The oomycete Phytophthora infestans (Mont.) de Bary causes the serious potato and tomato disease known as late blight. The sexual dormitory stage of P. infestans is a thick walled oospore which serves as the primary source of inoculum. Hybrid oospores from different genotypes increase the genotypic diversity of pathogenic population providing the quick adaptation to new releasing cultivars of potato and tomato as well as to fungicides. Production of oospores by P. infestans in field was reported in Russia (Smirnov et al., 1999), Norway (Hermansen et al., 2002), Sweden (Strömberg et al., 2001), the Netherlands (Kessel et al., 2002) and in other places. Oospores are viable for at least two years keeping in soil (Bødker et al ., 2006) and able to infect the plants after overwintering (Ulanova et al ., 2010; Lehtinen et al ., 2002). The oospore formation is inhibited with chemicals used for late blight protection of potato and tomato. The ef- fect of fluazinam, dimethomorph, cymoxanil, metalaxyl, maneb, propamocarb was shown (Kessel et al., 2002). In our previous experiments in vitro we revealed the same effect for fungicide pencycuron, insecticide imidacloprid, and herbicide metribusin. They did not prevent the growth of P. infestans colonies but reduced the oospore formation (Mita et al., 2014). However number of other chemicals applied on potato do not prevent the growth of P. infes- tans mycelium directly and their effect on the formation of oospores is unknown. The object of this study was to investigate the suppression traits of systemic triazole fun- gicide difenoconazole that controls a broad spectrum of foliar, seed and soil-borne diseases caused by Ascomycetes, Basidiomycetes and Deuteromycetes (Dahmen and Staub, 1992; Kopacki and Wagner, 2006; Pobedinskaya et al., 2012). Difenoconazole is not recommended against late blight on potato or tomato for plants or seeds treatment and we have not found in literature any studies concerning activity of difenoconazole against Phytophthora infestans in vitro or in planta. Difenoconazole is widely used for treatment of potato and tomato plants against early blight (alternariosis). Because of early and late blight often come together on potato it is practically important to estimate the impact of difenoconazole application against late blight pathogen also. It was already investigated that leaves of the same plant were variable in their resistance to P. infestans: apical leaves more resistant then basal leaves of the same plant at the same time (Visker et al ., 2003). Basing on this concept we can suppose the differentiation of the tiers of the plant regarding the oospore production. Therefore, the study of the formation of oospores in leaves of different tiers of potato plant was performed. Journal of Plant Pathology (2016), 98 (1), 123-127 123 Edizioni ETS Pisa, 2015 Corresponding author: S. Elansky E-mail: snelansky@gmail.com