THE ROLE OF PSEUDOMONAS SPP AND COMPETITION FOR CARBON, NITROGEN AND IRON IN THE ENHANCEMENT OF APPRESSORIUM FORMATION BY COLLETOTRICHUM-COCCODES ON VELVETLEAF

Colletotrichum coccodes is currently being investigated as a mycoherbi cide against the weed velvetleaf (Abutilon theophrasti). Two isolates of Pseudomonas spp. (Ps2 and Ps5) reduced the percentage of germ tubes and increased appressorial formation of C. coccodes on detached leave s of velvetleaf. A study was conducted to see whether this effect coul d be attributed to competition for nutrients or iron between C. coccod es and Pseudomonas spp. Ps2 and Ps5 had no effect on early spore germi nation, but reduced the percentage of germ tubes at 24 and 30 h, compa red to the nontreated control. This reduction was diminished by the ad dition of nutrients but not Fe3+. Ps2 and Ps5 stimulated the formation of dark-coloured appressoria without germ tubes (AWGT), but this stim ulation was diminished by the addition of nutrients or Fe3+. Germ tube branching at 30 h was also inhibited by the bacteria, but was not dim inished by the addition of nutrients or iron. EDTA stimulated conidial germination at 10 h, which was reduced by the addition of Fe3+. Howev er, EDTA did not stimulate the formation of appressoria (AWGT). These results suggest that the reduction in the percentage of germ tubes and the increase in the percentage of appressoria induced by the bacteria may be due to the competition for carbon or nitrogen. Iron competitio n may also be involved in the stimulation of appressorial formation, b ut not in the reduction in germ tube percentage and branching. Phyllop lane bacteria may compete for carbon, nitrogen and iron, limiting the saprophytic phase of the pathogen on the phylloplane and accelerating the development of the parasitic phase. This may enhance the field eff icacy of C. coccodes as a biocontrol agent against velvetleaf.