Evaluation of fungicides and tobacco cultivar resistance to Rhizoctonia solani incited target spot, damping off and sore shin

Abstract

Diseases incited by Rhizoctonia solani Kühn (teleomorph Thanatephorus cucumeris (Frank) Donk.) often cause serious losses to tobacco growers in Georgia. Isolates were collected from target spot lesions and stems of diseased plants across the tobacco growing belt of Georgia and evaluated for sensitivity to several fungicides. Isolates ranged up to 263 times for their ED50 values to a fungicide. ED50s in μg/ml were calculated for iprodione (0.54, 0.52), flutolanil (0.99, 0.12), carboxin (4.19, 0.57), PCNB (13.84, 3.96), chlorpyriofos (38.36, 26.72), and mancozeb (52.43, 70.73) for AG-3 isolates originating from target spots and isolates from tobacco stem lesions, respectively. Triflumizole had an ED50 of 3.17 μg/ml for stem isolates. Iprodione at the rate of 1.12 kg ai/ha reduced incidence and severity of target spot in seed beds. Coker 371-Gold, Speight G-70, K326 and breeding line 1071 grown in a greenhouse appeared to be equally susceptible to stem infection by all isolates tested. Seed bed trials identified Speight G70 and K-149 as more susceptible to lower stem infection than McNair 944, Coker 371-Gold and NC2326 with K-346, K-326, and Speight G-28 intermediate. Flutolanil, iprodione, fluazinam, and tebuconazole were all active in reducing stem infection in seed beds. Rhizoctonia target spot was severe in the field in 1994, and of the 66 tobacco entries evaluated, NC72, NC71, K-326, Coker 371-Gold, Speight G-70, Speight G-168, NC95, NC2326, and K-149 had relatively low levels of disease compared to other cultivars.