PERSISTENCE OF INDUCED SYSTEMIC RESISTANCE IN CUCUMBER IN RELATION TOROOT COLONIZATION BY PLANT-GROWTH PROMOTING FUNGAL ISOLATES

Systemic resistance against Colletotrichum orbiculare was induced in c ucumber plants by amending potting medium with the barley grain inocul a of plant growth promoting fungal (PGPF) isolates obtained from zoysi agrass rhizosphere. Out of 16 isolates tested, 10 protected plants aga inst a spore concentration of 10(4) spores ml(-1) of C. orbiculare. Ho wever, as the spore concentration of the pathogen was increased, the p rotection afforded by PGPF isolates decreased. Among those isolates, f ive protected plants even at a high pathogen spore concentration of 10 (6) spores ml(-1). Three, among these five isolates, colonized roots; GS8-3 colonized roots to the maximum extent, while GS8-1 and GS8-2 wer e moderate and poor colonizers, respectively. These three root coloniz ers, along with one root non-colonizer (GU21-2), were tested for their induction ability over a period of 9 weeks. All four isolates protect ed plants but their ability to initiate resistance varied. Root coloni zation by GS8-1 and GS8-3 correlated with the protection offered. Thei r failure to induce resistance when provided as autoclaved barley grai n inocula suggests that the initiation of resistance was attributed to invasion of roots. The other two isolates, on the other hand, induced systemic resistance to some extent when provided as autoclaved inocul a suggesting that thermostable factors might be responsible. The four select isolates enhanced growth by increasing plant height and biomass significantly, which was quite evident after 5 weeks of growth. The p lants associated with GS8-1 and GS8-2 were induced to produce a signif icant increase in the number of fruits. The dual nature of these soil- borne sterile fungi as inducers of resistance and as growth promoters over prolonged durations are discussed.