Monilinia fructicola, causal agent of brown rot, is one of the most important fungal pathogens of stone fruit. In the summer of 2011, Japanese plum fruit of ‘Larry Ann’ (Prunus salicina Lindl) showing symptoms of fruit rot disease were detected and collected from trees in an experimental field at Alcalá del Río (Seville), southwestern Spain. Fruit rot lesions were brown, sunken, and covered with grayish brown tufts or pustules. The majority of infected fruit became dry and mummified on the trees after 30 days. Symptoms were similar to those caused by three Monilinia species, M. laxa, M. fructigena, and M. fructicola (2). Pieces of infected tissue, previously disinfested in 0.6% NaOCl, were placed on potato dextrose agar (PDA) amended with 50 μg of streptomycin per liter and incubated at 22°C with a 12-h photoperiod for 15 days. The isolates produced abundant, grayish white mycelium, which after sporulation became hazel in color, and colonies displayed concentric rings. Colonies produced scarce conidia, which were arranged in branched, monilioid chains. Conidia were one celled, hyaline, ellipsoid to lemon shaped, and measured 15.42 ± 1.91 × 8.02 ± 0.9 μm. The morphological data and growth rates match the description of M. fructicola (Winter) Honey (2–4). Fungal identification was confirmed by PCR using genomic DNA extracted from the mycelia of pure cultures. The DNA was amplified with a common reverse primer and three specific forward primers obtained from a sequence-characterized, amplified region that distinguishes between M. fructicola, M. fructigena, and M. laxa. The size of the amplified fragment (a product of 535 bp) fit with the one described for M. fructicola (2). To confirm the pathogenicity of the isolate, mature ‘Larry Ann’ and ‘Sungold’ plum fruits (six fruits per cultivar) were inoculated by placing an agar plug from the edge of an actively growing colony on PDA directly on the fruit surface. After 5 days of incubation, typical brown rot symptoms developed on inoculated fruit and the fungus was successfully reisolated, thus fulfilling Koch's postulates. No symptoms appeared on control fruit. To our knowledge, this is the first report of M. fructicola on plums in southwestern Spain. M. fructicola is a quarantined pathogen in Europe and has been reported on imported apricot and nectarine (1) and peach in several European countries (3,4).
References: (1) E. Bosshard et al. Plant Dis. 90:1554, 2006. (2) M. J. Côté. Plant Dis. 88:1219, 2004. (3) A. De Cal and I. Gell. Plant Dis. 93:763, 2009. (4). C. Pellegrino et al. Plant Dis. 93:668, 2009.