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CORESTA Congress, Berlin, 2016, Agronomy/Phytopathology Groups, AP 05

Working to improve black shank control in flue-cured tobacco

BERTRAND P.F.(1); MOORE J.M.(1); SHEW D.(2); LEWIS R.S.2); VON WALDNER M.(1)
(1) University of Georgia, Tifton, GA, U.S.A.; (2) North Carolina State University, Raleigh, NC, U.S.A.

Black shank is a concern for farmers growing tobacco on land infested with Phytophthora nicotianae. The first superior Php gene variety (NC-71) was introduced in 1995. The Php gene confers complete resistance to the wild type of P. nicotianae (race 0), whereas traditional FL-301 resistance confers only partial resistance. By 2004, black shank was increasing where the Php gene was used as the sole means of control. This was due to a new pathogen race, designated race 1, selected in apparently mixed race populations as race 0 was eliminated by the Php gene. Black shank losses increased sharply in 2013, confirming that Php resistance would no longer control black shank in Georgia. A series of trials was begun in 2013 to improve black shank control. A 2013 trial of varieties and breeding lines conducted in a black shank site showed disease incidence of >85% in popular varieties (K-326, NC-71, and NC-196). Eleven breeding lines containing Wz genetics derived from Nicotiana rustica developed <10% disease. Very heavy rain during and just after transplanting in 2014, resulted in the highest black shank losses in 20+ years. In several cases where losses were high, mefenoxam had been used. Sensitivity tests performed on 30 isolates from such fields found all to be sensitive to mefenoxam. Mefenoxam is known to be prone to leaching with heavy rain. Trials conducted in 2014 found the varieties used for black shank control (NC-196 and GF-318) had only moderate to low levels of FL-301 resistance, providing 36.3% and 21.6% control respectively compared to K-326, the standard of low FL-301 resistance. These same trials identified three varieties that showed relatively good control compared to K-326 (NC-925, 81.4%; CC-143, 70.0%; GL-395, 63.5%). Black shank losses continued to be high in 2015. Variety trials conducted in 2015 confirmed the 2014 results. Flupicolide, a new chemical control option, reportedly less prone to leaching than mefenoxam, was tested in 2015. Control with a layby application of flupicolide (140 grams/ha) versus mefenoxam (560 grams/ha) was equal (p=0.05). Varieties with superior FL-301 resistance coupled with less leaching prone chemical options should improve black shank control. Resistance based on Wz genes offers a tool for further variety improvement.