Phylogenetic characterization, infectious process and alternative control of Colletotrichum in avocado

Abstract

The anthracnose caused by species of the genus Colletotrichum, is among the main limiting factors of the world avocado production. However, available information on the etiological agent, together with the knowledge of the infectious process and the influence of starch films on the inhibition of the disease, could help in the management strategies and reduction in fruit loss. Therefore, the objective of this study was to evaluate the diversity of Colletotrichum species associated with avocado anthracnose, study the infectious process of Colletotrichum bredae sp. nov. and evaluate the effect of different starch-based films concentrations on avocado anthracnose control. In the first study Colletotrichum isolates were characterized by morphological markers, phylogenetic analyzes of the genes gapdh, act, tub2 and ApMat and pathogenicity tests. Three species previously described C. karstii, C. dianesei, C. communis were identified, and a new specie described and illustrated as C. bredae sp. nov. Morphological characters were not informative enough to delimit the species. All species incited anthracnose symptoms in the avocado fruits. In the second study, samples of inoculated avocados were collected for periods of 3, 4, 6, 12, 24, 36, 48, 72, 96, 120, 144, 168 and 240 hours after inoculation (hai). The conidia germination occurred between 6 and 12 hai. Penetration occurred by wounding during inoculation, 48 hai. The colonization strategy of C. bredae sp. nov. was intracellular hemibiotrophic at 144-240 hai. Sporulation occurred 240 hai through the emergence of acervuli and formation of conidiophores that produced conidial masses. In the third study, avocado fruits were treated with cassava starch and corn starch films at individual concentrations of 1, 2, 3 and 4% and combined in the curative and preventive methods. Mean area injured was assessed; the area under the disease progress curve (AUDPC) and the fruit fresh weight were evaluated, as well as to verify the action of the films on the fruits using Scanning Electron Microscopy (SEM). In the curative method, there was an inhibition of the mean area injured and AUDPC in the fruits treated with 1% and 4% cassava starch films, 1% corn starch, and with the cassava starch + corn starch combinations at 2, 3 and 4%. In the preventive method, the fruits treated with cassava starch and 2% individual corn starch and with cassava starch + corn starch combinations 2 and 3% provided lower AUDPC and mean area injured. In these treatments were also verified smaller weight losses in the fruits. SEM images revealed inhibition of conidia germination of C.bredae sp. nov. and the formation of a protective layer on the fruit surface, which delayed the ripening of the fruits, prevented the penetration of the pathogen and led to deformation of hyphae.