IDENTIFICATION OF PHYTOPHTHORA SPECIES INFECTING CITRUS IN VIETNAM.

In a survey aimed at identifying Phytophthora species infecting fruit crops in Viet Nam, P. citrophthora and P. nicotianae were the species most commonly recovered from soil and trees in citrus plantations. P. nicotianae was prevalent in citrus groves in the Ben Tre and Tien Giang provinces (southern Viet Nam). Interestingly, both A1 and A2 mating types of the latter species were found, while in other citrus-growing areas of the world, such as the Mediterranean region, South Africa, Australia and the Americas, A1 is the only mating type occurring in citrus groves, suggesting southern-east Asia could be a centre of origin of this Phytophthora species. Mixed infections of three Phytophthora species were detected on the fruits of pomelo (Citrus grandis) with symptoms of brown rot collected in the Mekong Delta region (southern Viet Nam). The three species were identified as P. colocasie, P. insolita and P. meadii on the basis of morphological characters and DNA analysis. The ITS-rDNA sequence of three representative isolates obtained from pomelo fruits showed 99, 99 and 100% similarity with reference ITS-rDNA sequences of P. colocasie, P. insolita and P. meadii from GenBank, respectively. Koch’s postulates were fulfilled by pathogenicity tests on fruits of pomelo and other citrus species. To our knowledge, this is the first report of natural infections of these three Phytophthora species on citrus fruits worldwide and the first report of P. insolita in VietNam.