Study of infection process of five species of Colletotrichum comparing symptoms of glomerella leaf spot and bitter rot in two apple cultivars

Abstract

Bitter rot (BR) and glomerella leaf spot (GLS) occur in all the Brazilian South orchards with different species frequency per region. The main species found in Brazil belong to the Colletotrichum acutatum complex (Cac) and to the C. gloeosporioides complex (Cgc). The objectives of this study were to evaluate the favorable conditions for the germination and growth of five Colletotrichum species and, to compare pathogenicity and aggressiveness among species and between two apple cultivars in fruit, causing BR, and in leaves, causing GLS. The highest germination of conidia (above 90%) was achieved between 15 and 30 °C for Cgc species while for Cac species the highest germination (mean of 71%) was at 25 °C. The optimal temperature for mycelial growth did not differ between complexes and ranged from 23 to 27 °C. Althought, Cgc had a higher estimated maximum temperature then Cac. C. siamense cause BR symptoms regardless of cultivar or type of inoculation. C. nymphaeae and C. melonis caused symptoms in wounded and unwounded fruit but for C. paranaense and C. fructicola the symptoms were observed only in wounded fruit. Leaf symptoms of GLS was more severe in stressed leaves inoculated on the abaxial side compared to non-stressed leaves inoculated on the adaxial side. C. fructicola and C. nymphaeae were able to cause GLS symptoms in leaves with severity varying from 17.5 to 25% (cv. Gala). C. fructicola was the most severe causing GLS symptoms in cv. Gala (severity around 20.5%) and, in cv. Eva (severity around 11%). We discuss the roles that each species plays as a pathogen to better understand BR and GLS in different orchards conditions.