Persistence of induced systemic resistance in cucumber in relation to root colonization by plant growth promoting fungal isolates

Abstract

Systemic resistance against Colletotrichum orbiculare was induced in cucumber plants by amending potting medium with the barley grain inocula of plant growth promoting fungal (PGPF) isolates obtained from zoysiagrass rhizosphere. Out of 16 isolates tested, 10 protected plants against a spore concentration of 10 4 spores ml −1 of C. orbiculare. However, as the spore concentration of the pathogen was increased, the protection afforded by PGPF isolates decreased. Among those isolates, five 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 were moderate and poor colonizers, respectively. These three root colonizers, along with one root non-colonizer (GU21-2), were tested for their induction ability over a period of 9 weeks. All four isolates protected plants but their ability to initiate resistance varied. Root colonization by GS8-1 and GS8-3 correlated with the protection offered. Their failure to induce resistance when provided as autoclaved barley grain 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 inocula 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 plants associated with GS8-1 and GS8-2 were induced to produce a significant 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.