Phylloplane Pseudomonas spp. Enhance Disease Caused by Colletotrichum coccodes on Velvetleaf

The effects of phylloplane bacteria on the conidial germination, germ tube growth, appressoria formation, and disease severity of the mycoherbicide agent Colletotrichum coccodes were investigated. Seven isolates of fluorescent Pseudomonas spp. and two other bacterial isolates were recovered from the mature leaves of velvetleaf (Abutilon theophrasti). Co-inoculation of C. coccodes with each of three isolates of Pseudomonas spp. significantly increased the number of lesions and disease severity caused by C. coccodes on velvetleaf seedlings in two growth chamber experiments, and also accelerated the appearance of symptoms. All bacterial isolates reduced conidial germination and germ tube branching of C. coccodes and three isolates increased the frequency of germ tubes with appressoria on cellophane membranes overlaid on leaf disks of velvetleaf. All isolates reduced conidial germination 30 h after co-inoculation with C. coccodes on detached leaves, but increased the relative frequency of dark-colored appressoria without germ tubes and decreased the total germ tube length. These results suggest that certain phylloplane bacteria may enhance the efficacy of mycoherbicides by stimulating the formation of appressoria and reducing the saprophytic preinfection mycelial growth of the pathogen on the phylloplane. This acceleration of infection may decrease the critical period of moisture required for infection in the field.