Characterization of Phytophthora spp. Strains Affecting Citrus Rootstocks in Cuba

摘要

Phytophthora comprises different species of plant pathogens, many responsible for some of the most serious plant diseases. Citrus are sensitive to several Phytophthora species, mainly P. nicotianae and P. citrophthora. In Cuba, where citrus are the most important fruits crop this pathogen is troublesome. Identification of Phytophthora species has traditionally been based on morphological and growth characters; however variations exist leading to difficulties in accurate identification. In the present work soil samples were collected from two locations in Cuba where citrus are commercially exploited. Fungal samples were isolated using the soil dilution plating method and by baiting with selective hosts. Morphological characterization were done based on traditional traits, including sporangial morphology (shape, size and length-breadth ratio); presence of papilla, absence of caducity, branching of sporangiophores, presence or absence of chlamydospores, the hyphal swellings and colony types in different culture media. The morphological characterization indicated that two putative Phytophthora species could be considered for classifying the five fungus isolated, P. nicotianae or P. citrophthora. The physiological characterization based on maximum temperature of growing, showed that all the isolates could be classified as P. nicotianae strains. ITS 5/4 primer combination confirmed the identification of the samples as Phytophthora spp. isolates whereas the Pn5B/Pn6 primer pairs used showed an accurate identification as P. nicotianae. Pathogenicity tests were developed using in vitro plants of citrus rootstocks inoculated with zoospores suspensions and by the inoculation of Mexican limes fruits with mycelia discs. Discoloration of stems and symptoms of blight were observed in the in vitro plants treated, a brown necrosis was also observed in the fruits inoculated. The strains inoculated were re-isolated from both roots collected from rootstocks and peels from fruits.