First Report of Colletotrichum siamense Causing Apple Bitter Rot in Central Argentina

FERNANDEZ, LAURA; ALANIZ, SANDRA; MONDINO, PEDRO; ROESCHLIN, ROXANA; MAUMARY, ROXANA; GARIGLIO, NORBERTO; FAVARO, M. ALEJANDRA

PLANT DISEASE

AMER PHYTOPATHOLOGICAL SOC

Año: 2017

0191-2917

Apple production in Argentina is mainly located in dry and cold areas of the south. Low-chill apple cultivars allowed the expansion of apple production into warmer areas of northern and central Argentina. Bitter rot is one of the prevalent apple diseases in humid and warm areas worldwide. A variety of Colletotrichum species such as Colletotrichum fructicola, C. theobromicola, C. melonis, C. nymphaeae, C. godetiae, C. fioriniae and C. siamense has been associated with this disease in the last years (Baroncelli et al. 2014, Kou et al. 2013, Munir et al. 2016, Velho et al. 2015). In December 2014, bitter rot symptoms were observed on apple fruits cvs. ?Eva?, ?Caricia? and ?Princesa? in Santa Fe province, central Argentina. The rot began as brown, 1-2 mm circular spots which enlarged rapidly and became sunken and extended toward the fruit core in a V-shaped pattern. Under high humidity conditions concentric rings of pinhead-size salmon acervuli formed in the lesions. Causal agent was isolated by touching acervuli with a sterile needle and monosporic cultures were obtained on PDA at 25°C, with a 12-h light period. Colonies were white to gray on the top and pink on the underside, where concentric rings of salmon acervuli were clearly distinguished. The width and length of one hundred conidia were examined in three isolates, ranging from 3.37 to 5.54 μm (avg. 4.46), and from 11 to 17.85 μm (avg. 14.58), respectively. Conidia were mainly cylindrical, with rounded ends. These morphological characteristics correspond to species belonging to C. gloesporioides complex (Weir et al. 2012). To accurately identify the Colletotrichum species, DNA was extracted from the three isolates and genes corresponding to glyceraldehydee-3-phosphate dehydrogenase and to ß-tubulin were amplified and sequenced. Sequences presented a 100% of identity with C. siamense for glyceraldehydee-3-phosphate dehydrogenase gene. For ß-tubulin gene, sequences of one isolate presented 99% of identity with C. siamense, whereas the other two isolates presented 100% of identity with this specie. The nucleotides sequences were deposited in GenBank (KY656675, KY656676, KY656677, KY656678, KY656679, and KY656680). Pathogenicity test were performed inoculating eight fruits of ?Pink lady? cultivar with conidial suspension from three isolates. Two drops of 10 μl (1x105 conidia per ml) were deposited in wounded and non-wounded areas on fruits. Drops of sterile water were deposited in eight fruits as control. Fruits were kept under high humidity conditions at 25°C for ten days. First symptoms appeared 3 days after inoculation (DAI) in wounded areas and 5 DAI in non-wounded areas. After that, lesions enlarged and acquired the appearance of those previously described. The pathogen was reisolated from lesions, confirming Koch?s postulates. To our knowledge, this is the first report of C. siamense affecting apple in Argentina. C. siamense was reported as the most aggressive specie causing apple bitter rot in the United States (Munir et al. 2016). However, this specie was more sensitive to fungicides than C. fructicola and C. theobromicola (Munir et al. 2016). These results encouraged the development of specie-specific management strategies for this pathogen in central Argentina.