The Identification of New Causal Agents of Brown Rot of Pomelo (Citrus grandis) Indicates Vietnam Is a Hot-spot of Phytophthora Diversity

Two distinct Phytophthora taxa were found to be associated to brown rot of pomelo (Citrus grandis), a new disease of this ancestral Citrus species, in the Vinh Long province, Mekong River Delta area, southern Vietnam. On the basis of morphological characters and using the ITS1-5.8S-ITS2 region of the rDNA and the cytochrome oxidase subunit 1 (COI) as barcode genes, one of the two taxa was identified as P. insolita from Phytophthora major Clade 9 while the other one, being closely related to but distinct from the Clade 2 species P. meadii, was informally designated as P. aff. meadii. Isolates of P. insolita and P. aff. meadii were also obtained from necrotic fibrous roots of Volkamer lemon (C. volkameriana) rootstocks grafted with 'King' mandarin (Citrus nobilis) and from trees of pomelo, respectively, in other provinces of the Mekong River Delta, indicating widespread occurrence of both Phytophthora species in this citrus-growing area. Koch's postulates were fulfilled via pathogenicity tests on fruits of various Citrus species, including pomelo, grapefruit (Citrus x paradisi), sweet orange (Citrus x sinensis) and bergamot (Citrus x bergamia) as well as on 2year-old trees of pomelo and sweet orange on 'Carrizo' citrange (C. sinensis 'Washington Navel' x Poncirus trifoliata) rootstock. This is the first report of a Phytophthora species from Clade 2 other than P. citricola and P. citrophthora as causal agent of fruit brown rot of Citrus worldwide and the first report of P. insolita in Vietnam. Results indicate Vietnam is still an unexplored reservoir of Phytophthora diversity.