First Report of Powdery Mildew of European Plum Caused by Podosphaera tridactyla in Mexico

European plum (Prunus domestica L.) is one of the most important stone fruit trees planted in temperate areas worldwide. Powdery mildew, caused by Podosphaera spp., is one of the most common diseases of European plums; however, the specific identity of the pathogen on European plum trees in Mexico has not been determined. During the fall of 2016 and 2017, severe symptoms of powdery mildew were observed on 38 European plum trees in backyards in Texcoco, state of Mexico, Mexico. Disease incidence was 100%. On young and mature fruit, circular spots of white fungal growth were present, and necrotic areas and scabby appearance occurred 10 to 15 days after first symptoms. Symptoms and signs were not often observed on leaves. A voucher specimen was deposited in the herbarium of the Department of Agricultural Parasitology at the Chapingo Autonomous University, Texcoco, Mexico (UACH H191). For morphological characterization, fungal structures were mounted in a drop of lactic acid on a glass slide. Microscopic examination showed nipple-shaped hyphal appressoria. Conidiophores (n = 20) were hyaline, cylindrical, erect, and measuring 90.5 to 154.2 μm long. Foot cells were cylindrical and straight, measuring 45.9 to 81.2 × 7.3 to 9.4 μm, followed by one to three shorter cells, and forming catenescent conidia. Conidia (n = 100) were hyaline, ellipsoid to ovoid, measuring 19.4 to 29.1 × 12.2 to 17.2 μm, and with fibrosin bodies. Germ tubes were simple and more or less terminal to lateral. Chasmothecia were absent during the time of observation. Based on morphological characteristics of the asexual stage, the fungus was identified as Podosphaera tridactyla (Braun and Cook 2012). To confirm the identification, genomic DNA was extracted, and the internal transcribed spacer (ITS) region and part of the 28S gene were amplified by polymerase chain reaction (PCR) and sequenced. The ITS region of rDNA was amplified using the primers ITS5/ITS4 (White et al. 1990). For amplification of the 28S rRNA partial gene, a nested PCR was performed using the primer sets PM3 (Takamatsu and Kano 2001)/TW14 (Mori et al. 2000) and NL1/TW14 (Mori et al. 2000) for the first and second reactions, respectively. The ITS and 28S sequences were deposited in GenBank under accession numbers MH460821 and MH469269, respectively. Phylogenetic analyses using maximum parsimony and maximum likelihood methods and including published ITS and 28S sequence datasets for P. tridactyla and other Podosphaera species were performed. Phylogenetic analyses revealed that ITS and 28S sequences from a Mexican isolate were grouped into a clade with P. tridactyla. Pathogenicity was confirmed by gently dusting conidia from infected leaves onto 20 healthy leaves of 6-month-old European plum trees. Ten leaves sprayed with sterile distilled water served as controls. All plants were maintained in a greenhouse at temperatures ranging from 20 to 30°C. All inoculated plants developed similar symptoms to the original observation after 9 days, whereas the control plants remained symptomless. The fungus present on the inoculated plants was identical morphologically to that originally observed on diseased plants, fulfilling Koch’s postulates. P. tridactyla has been widely reported infecting plums in Europe, Japan, Australia, and New Zealand (Farr and Rossman 2018). However, to our knowledge, this is the first report of powdery mildew caused by P. tridactyla on European plum in Mexico.