BIOTROPHIC MYCOPARASITISM BY VERTICILLIUM-BIGUTTATUM ON RHIZOCTONIA-SOLANI

Verticillium biguttatum cannot utilise cellulose or nitrate-nitrogen a nd it requires biotin for growth, yet it grew and sporulated abundantl y on Rhizoctonia solani on cellulose, obtaining at least organic carbo n, nitrogen and biotin from R. solani. Videomicroscopy of inter-hyphal interactions on films of water agar showed that V. biguttatum behaved as a biotrophic mycoparasite. From germinating spores, it penetrated the hyphae of R. solani and formed haustorium-like branches without ki lling the host cells, and the haustoria supported an external mycelial network of the mycoparasite. Later the mycoparasite sporulated, and t he infected host cells died. On cellulosic substrata V. biguttatum did not reduce the growth of R. solani, and often enhanced the rate of ce llulose degradation. However, V. biguttatum drastically reduced the pr oduction of sclerotia by R. solani, often completely suppressing scler otium production when the mycoparasite infected only a localized regio n of the host colony. This is ascribed to the creation of a nutrient s ink by the parasite, consistent with biotrophy. On plates of cellulose agar the suppression of sclerotia was not confined to parasitized col onies but extended to adjacent colonies of R. solani that had successf ully anastomosed with the parasitized colony. There was no effect on a djacent vegetatively incompatible colonies, where attempted anastomose s caused cytoplasmic death. In comparable experiments the necrotrophic mycoparasite Gliocladium roseum had no long-distance effect on sclero tium production by R. solani. Suppression of sclerotium production may explain the reported success of V. biguttatum in biocontrol of black scurf of potato in experimental field conditions.