Induced systemic resistance and the role of binucleate Rhizoctonia and Trichoderma hamatum 382 in biocontrol of Botrytis blight in geranium

Three root-colonizing fungi, binucleate Rhizoctonia (BNR) isolates BNR621 and P9023 and Trichoderma hamatum isolate 382 (T382), were studied for suppression of Botrytis blight in geranium by induction of host systemic resistance. Resistance to Botrytis blight was observed in geraniums transplanted into potting mix amended with formulations of P9023 and T382 2 weeks prior to inoculation with Botrytis cinerea when grown under environments either highly or less conducive to disease development. In the less conducive environment, P9023 and T382 provided protection equal to (P < 0.0001) the fungicide control. Specific stages in infection by B. cinerea were tested by counting germination of conidia in leaf extracts or by assessing lesion size in detached leaves taken from BNR- and T382-treated geraniums. No differences in conidial germination were observed. Lesion area results depended on time between application of inducing agents and detachment of leaves for inoculation. In geranium leaves detached and inoculated 7 days after topdressing with a Pesta formulation of BNR621 and P9023, AUDPC calculated from lesion area was smaller (P < 0.0001) than T382 and the inoculated control. Whereas, leaves detached and inoculated 14 days after topdressing with a formulation of T382 had a smaller (P < 0.0001) AUDPC from lesion area than plants treated with BNR621. Restriction of lesion development may play a role in the suppression of Botrytis blight in geranium. Our results may be the first to demonstrate induced systemic resistance by BNR fungi to a foliar pathogen and support additional research into use of T382 in an integrated management program for B. cinerea.