Expression of induced systemic resistance in poinsettia cuttings against Rhizoctonia stem rot by treatment of stock plants with binucleate Rhizoctonia

Three cultivars of poinsettia (V14 Glory, Freedom Red, and Angelica White) were evaluated for induction of host resistance to Rhizoctonia stem rot in cuttings acquired from stock plants treated with binucleate Rhizoctonia (BNR) in a Pesta formulation. Resistance to stem rot in cuttings of V14 Glory was induced within 7 days of treatment with BNR while Freedom Red and Angelica White did not express resistance until 10 or more days after application of BNR to stock plants. None of the cultivars expressed resistance to stem rot when cuttings were taken from stock plants that were treated with BNR for only 3 or 5 days before propagation. Population dynamics of BNR on poinsettia root tissue were consistent with the expression of induced resistance. A dramatic increase of root colonization by BNR was observed in all cultivars between day 5 and 7. The integrated use of both BNR and Burkholderia cepacia, previously characterized as a stem rot control agent during poinsettia propagation, was evaluated to test for enhanced resistance against stem rot. No additive effect on stem rot control was observed when cuttings taken from stock plants treated with BNR were propagated in rooting strips treated with B. cepacia compared to cuttings propagated in root strips treated with B. cepacia alone. Our results may be the first to demonstrate induced systemic resistance by BNR fungi during vegetation propagation where the biocontrol agent applied to roots of a stock plant was able to protect cuttings from Rhizoctonia solani during propagation.