Fungi associated with cotton decline in Angola

An investigation into the causes of cotton decline on plantations in Angola was carried our. After other possible biotic and abiotic causes of the decline had been eliminated, the state of the leaves in particular was examined. Eighteen fungi were isolated, of which eight were found with frequencies ranging from 10 to 70%. Alternaria alternata, A. gossypina, Colletotrichum gossypii, Cerospora gossypina, Curvularia spicifera, Phomopsis sp. I, Phyllosticta sp. and Ramularia gossypii. The pathogenicity of the isolated fungi was then tested on callus cultures in vitro and in vivo on 21-day-old plantlets. Four of the isolated fungi were found to be pathogens, causing severe necrosis: A. alternata, A. gossypina, C. spicifera, and R. gossypii. These fungi were reinoculated in vivo on the plant leaves in suspensions of 250,000 conidia/ml and again produced varying degrees of leaf spotting and necrosis, confirming their pathogenicity in vivo. There were interesting associations between these four pathogenic fungi and with the other fungi, which had been isolates with a frequency of less than 10 %: Acremonium spp., Botrytis cinerea, Gliocladium roseum, Gliocladium spp., Phomopsis sp. II, Phoma sp., Rhizoctonia solani, Trichoderma harzianum, T. viride, and ulocladium sp. Of the Four pathogenic fungi, two were already known in Angola: Alternaria gossypina and Ramularia gossypii. The isolated fungi are able to survive on vegetable debris and infected plants. The findings of the study and the above observations, as well as the total lack of preventive measures in Angola (especially the failure to dispose of plant debris and weeds) and the fact that the infections were recurrent, all suggested that these fungi were a significant factor in cotton decline in the Angolan plantations examined.