Interactions among arbuscular mycorrhizal fungi, a plant pathogen and cultivars of Coffea arabica (L.) (Rubiaceae)

This thesis comprises three studies of interactions among arbuscular mycorrhizal (AM) fungi, coffee cultivars (Coffea arabica L.) and a foliar pathogen (Cercospora coffeicola Berk. & Cooke). The first part compares the extent of extraradical mycorrhizal hyphae among three coffee cultivars. The second part is a study of the effects of AM colonization on leaf spots caused by C. coffeicola in coffee seedlings. The third part determines under controlled conditions if the mycorrhizal symbiosis increases the development of leaf spots caused by C. coffeicola and the effects of C. coffeicola inoculation on AM root colonization. The results showed that coffee cultivars responded differently to arbuscular mycorrhizal fungi although they were generally associated with the same morphotypes of AM fungi in their rhizospheres. Furthermore, some AM fungi present in roots of different cultivars had identical DNA sequences. Differences in extrametrical hyphae among cultivars may contribute to differences in their responses to fertilization. Comparison of paired diseased and healthy seedlings showed that C. coffeicola and percent AM colonization were significantly positively associated. Furthermore, percent AM colonization and percent leaf area diseased were positively correlated among diseased seedlings, though little variation was explained. These results suggest AM colonization may increase susceptibility of seedlings to Cercospora infection. However, results of the inoculation experiment in Chapter 3 showed that Cercospora infection increased AM colonization, which could have contributed to the positive correlation found in Chapter 2. Inoculation with AM fungi slowed the development of C. coffeicola lesions in older plants. Interactions between arbuscular mycorrhizal fungi and a leaf pathogen in coffee were complex, but should allow for the development of integrated crop management systems incorporating AM fungi with further research. The results presented here provide a basis for Puerto Rican farmers to use arbuscular mycorrhizal management in coffee as an alternative to application of inorganic fertilization and promote sustainable farming. Such an approach may have added benefits such as slowing the development of Cercospora leaf spot. Additional studies would be needed to fully develop integrated crop management systems for coffee incorporating AM fungi because of the complexity of the interactions between pathogens, cultivars, nutrient levels and AM fungi.