First Report of Glomerella Leaf Spot of Apple Caused by Colletotrichum fructicola in Uruguay

Glomerella leaf spot (GLS) is a relatively recent and destructive disease of apple (Malus domestica Borkh.). GLS is present in regions with humid and subtropical climate such as southern Brazil (Velho et al. 2015), southeastern U.S.A. (Taylor 1971), and more recently eastern China (Wang et al. 2012). This disease is favored by rainfall and temperatures between 23 and 28°C, frequently resulting in severe defoliation. In summer 2014, necrotic spots were observed on apple leaves (cvs. Cripps pink and Gala) in Kiyu, San José, Uruguay (34°38′58″S, 56°45′19″W) and in summer of 2015 became a severe disease. On leaves, small (1 to 3 mm in diameter) purple to brown spots initially appeared and after a few days developed into larger (5 to 20 mm in diameter) necrotic lesions. The larger necrotic lesions were irregular in shape, but expansion was often limited by leaf veins. In some instances, the whole leaf or large sections of the leaf became light brown to gray in color with a dark margin and acervuli and perithecia inside. Lesions were observed on both sides of leaf, but acervuli and/or perithecium appeared only on the upper leaf surface. Necrotic lesions, especially on younger leaves, often coalesced and caused generalized chlorosis followed by premature abscission and defoliation of the affected tree. On fruit, circular, slightly sunken lesions of 0.5 to 3 mm in diameter and 0.5 to 2 mm in depth appeared and were usually surrounded by a red border. Pure cultures were obtained by monosporic isolation from leaf lesions and incubation on PDA at 25°C and a 12-h photoperiod under fluorescent lighting. Monosporic isolates (C45, C46, and C47) were obtained and subject to morphological and molecular characterization. Colony color varied from pale gray to gray, and conidia were predominantly oblong and all of them were hyaline and straight. The length and width of conidia ranged from 16.5 to 22.9 µm (avg. 18.8) and from 4.9 to 7.8 µm (avg. 6.5), respectively. The actin (ACT, KX859090 to KX859092), glyceraldehyde-3-phosphate dehydrogenase (GAPDH, KX859087 to KX859089), and beta-tubulin (TUB2, KX859093 to KX859095) genes were sequenced and deposited in GenBank. The ACT, GAPDH, and TUB2 sequences for three isolates were 100% identical to Colletotrichum fructicola (JX009436, JX009923, JX010400). To confirm pathogenicity, susceptible apple seedlings (cv. Gala × cv. Fuji) were inoculated with a suspension of 1 × 106 conidia ml–1 by spraying the leaves and incubating trees in a moist chamber at 25°C and 100% RH for 48 h. The initial symptoms appeared 4 days after inoculation and were similar to those observed in the field. Noninoculated trees remained symptomless. The pathogens were reisolated from lesions confirming Koch’s postulates. This species was previously reported to cause GLS in southern Brazil (Velho et al. 2015) and eastern China (Wang et al. 2015). In Uruguay, it was reported causing apple bitter rot (Alaniz et al. 2015). This is the first report of GLS caused by C. fructicola on apple in Uruguay. This discovery will alert apple researchers and producers in Uruguay due the potential threat this disease poses. © 2017, American Phytopathological Society. All rights reserved.