Involvement of Phytophthora citrophthora and Phytophthora nicotianae in severe decline and gummosis of grapefruit (Citrus x paradisi Macfad.) in southern Trkiye

Since the summer of 2020, severe decline and gummosis were observed on 20- to 30-year-old grapefruit trees cv. Star Ruby in Antalya province in the Western Mediterranean region of Turkiye. Grapefruit trees were examined in affected commercial orchards in Aksu, Manavgat, Muratpasa and Serik counties, located near the coast of the Mediterranean Sea. Symptomatic trees exhibited leaf discoloration, defoliation and severe dieback, as well as dark, water-soaked lesions with gum exudates, characteristic of Phytophthora infections, formed on the bark of the main stem and major limbs. No Phytophthora species could be isolated from these bark tissues, while two were obtained from soil samples taken from around symptomatic trees. Based on morphological characteristics, isolates were identified as P. citrophthora and P. nicotianae. Identifications were confirmed by sequencing of the internal transcribed spacer (ITS) and the cytochrome oxidase I (cox1) gene regions. Pathogenicity tests, carried out by inoculations of the main stems and branches of grapefruit seedlings cv. Star Ruby indicated that both species were pathogenic. All P. citrophthora and P. nicotianae isolates obtained in this work were sensitive to mefenoxam. EC50 values of P. citrophthora isolates were between 0.284 and 0.285 mu g mL(-1), whereas those of P. nicotianae were between 0.288 and 0.291 mu g mL(-1). Phytophthora citrophthora was previously known to cause disease on citrus in Turkiye, but this is the first report of P. nicotianae causing citrus gummosis in the country. Because P. nicotianae is more widespread than P. citrophthora, it should be considered a threat to citrus cultivation in this region of Turkiye.