ETIOLOGY OF CROWN ROT OF ORGANIC BANANAS

Crown rot is one of the most important postharvest diseases with a great negative impact on banana fruit quality. Bananas are harvested green and many packaging processes are carried out before coming on the market. The infections occur at harvest, but the symptoms appear after overseas transportation. Different pathogens are involved in crown rot, varying according to farming area. Little is known about its etiology and causal agents in organic farming. Dominican Republic is one of the leading exporters of organic bananas, and therefore, in this PhD thesis, five organic farms and their corresponding packing stations located in Valverde were investigated. To the best of our knowledge this is the first study covering Dominican Republic and it focused in particular on: the disease etiology, conditions, infection time and mechanisms that determine its development. Over a period of three years, 558 banana hands were collected and a total of 5000 fungal colonies were obtained from the crown tissues and 1750 representative colonies were purified. The identification of mycoflora associated with crown tissues was carried out with the final aim to search for disease management strategies compatible with organic production. Fungi were found in all the analyzed samples collected from various processing stages: from field to packing houses, and obtained in high rate starting from field from flowers as well as crown parts. The diffusion of the pathogen inoculum occurs principally during the banana processing, especially during the dehanding and in washing tanks. The final crown trimming followed by washing and quality of water used in the application of protective products were the critical points of crown infections. Five hundred and eighteen representative colonies were characterized and identified using morphological and molecular methods. The fungal community was dominated by Fusarium, the most frequent genus (55%) found in more than 80% of all analyzed samples. It was represented by nine species; F. incarnatum 53%, F. verticillioides 12%, F. sacchari 12%, F. proliferatum 7%, and F. solani 6%. Strains belonging to eight less frequent genera were represented by Colletotrichum musae 7% and found in 13% of all samples; Lasiodiplodia theobromae 4% and L. pseudotheobromae 1%, both found in 7% of all samples; Nigrospora sp. 11%, Alternaria spp. 6%, Phoma spp. 2%, Pestalotiopsis sp. 2%, Curvularia spp. 1% and Microdochium sp. 1%. Considering the main genera, the results based on morphological and molecular aspects showed a high variability among strains. By conducting the experimental inoculation trials, C. musae strains resulted from the most virulent among different species, followed by F. sacchari, L. theobromae, L. pseudotheobromae and F. verticillioides. The remaining strains had low pathogenicity, and their role could be ancillary in the crown rot development, or could be considered saprophytic. Summarizing the isolation frequency and pathogenicity tests, F. incarnatum strains played the main role in crown rot disease of organic bananas in the investigated areas.