Detection of Phytophthora cactorum in the irrigation water in commercial orchards of 'Bartlett' pear in Villa Regina, Rio Negro, Argentina

The collar and root rot caused by Phytophthora cactorum on apples and pears is one of the most economically significant diseases worldwide. P. cactorum has the ability to disperse through the water, either rain or irrigation, which could facilitate the transmission of the pathogen from diseased plants to healthy plants. The aim of this study was to detect by serological methods the presence of P. cactorum in the irrigation water system in commercial orchards of ‘Bartlett’ pear. Fifteen water samples were taken from two orchards at two different timings in the year: December (early summer) and March (early fall). Samples were taken on the diseased plants and the healthy plants located just before and after the infected one along the lines of irrigation (n=5 in each case). Water samples were passed through a 0,45-µm nitrocellulose filter using a vacuum pump. The filters were re-suspended in extraction buffer (GEB2), homogenized, and aliquots of 100 µl were placed in 96-well polystyrene plates Nunc to detect Phytophthora spp. through DAS-ELISA (double-antibody sandwich enzyme-linked-immunosorbent assay) by a polyclonal antiserum kit (Agdia). The data indicated the presence of Phytophthora spp. in water used for irrigation from the two selected plantations in both times of the year. During December the pathogen was detected on 66% of positive water samples from infected plants, and 34% from samples taken after infected plants (orchard 1). In March, an increase of 270% was observed in detection levels in relation to the total positive samples detected in December in the same orchard. The pathogen was detected on 25% of water samples from diseased plants, 25% from previous plants and 50% from subsequent plants. Posterior isolations in Corn Meal Agar indicated that the species present in water samples was Phytophthora cactorum in all cases. The results would infer that the irrigation water carries propagules of this pathogen from diseased plants to healthy ones and indicate that the spread is most evident with the arrival of autumn.