Among the pathogens that cause damage to sorghum (Sorghum bicolor), Colletotrichum sublineolum, the causal agent of anthracnose, is one of the major problems. In trying to find alternative ways to control the disease, the effect of two cyanobacteria, Synechococcus leopoliensis and Nostoc sp., was tested on fungal development in vitro and on the protection of sorghum plants kept under greenhouse conditions. Filtrates from both cyanobacteria improved conidium germination in assays carried out on glass slides covered with polystyrene. The in vitro mycelial growth of the pathogen was stimulated by S. leopoliensis too. The cyanobacteria did not induce the accumulation of phytoalexins in etiolated sorghum mesocotyls. The protection of a susceptible sorghum cultivar, with six true leaves, was low and only observed when the cyanobacterium treatments were applied twice before the inoculation of the plants with the pathogen. The stimulation effect of the cyanobacteria on C. sublineolum was likely due to nutrients present in the cyanobacteria culture filtrates. The low level of antracnose control may be explained by the low cyanobacterial capacity to produce fungal inhibitory compounds or to induce an important resistance response in the plants, such as, the phytoalexin accumulation. Therefore, based upon this work, S. leopoliensis and Nostoc sp. did not exhibit potential as biological control agents of C. sublineolum in sorghum.
