Oomycetous Pathogens of Chestnut Saplings Grown at Some Nurseries in Western Black Sea Region of Türkiye and Use of Phosphorous Acid and Potassium Phosphonate for Their Control

ABSTRACT The occurrence of Oomycetous pathogens and their pathogenicities on chestnut saplings, grown at four nurseries were investigated in this study. Also, the effectiveness of foliage applications of phosphorous acid (H 3 PO 3 ) and Potassium phosphonate (K 3 PO 4 ), against Phytophthora cinnamomi which is the most common root rot pathogen of chestnuts in Türkiye was evaluated. Forty‐three Oomycetous isolates were obtained using selective media, from sixty plant and twenty soil samples collected from the nurseries. Oomycetous pathogens were identified by molecular tools, using ITS, cox1 for Phytophthora spp. and ITS and coxII primers for Phytopythium spp. Phytophthora cinnamomi was the most frequently isolated species, recovered from two nurseries, Gökçebey and Pınar, while Phytophthora inundata was from only the Pınar nursery. Two Phytopythium species, Phytopythium litorale and Phytopythium vexans were also frequently isolated, the former from Pınar and Hendek nurseries and the latter from Gölköy, Hendek and Gökçebey nurseries. Phytopythium spp. did not produced necrosis as large as P. cinnamomi when inoculated on to young saplings. When H 3 PO 3 and K 3 PO 4 were applied on to foliage one or 2 weeks before the pathogen inoculation, they reduced necrosis lengths significantly compared to the simultaneous application of H 3 PO 3 and K 3 PO 4 with the pathogen inoculations. Necrosis lengths obtained by the K 3 PO 4 treatment were significantly lower than H 3 PO 3 treatment.