The effectiveness of crop rotation on management of Verticillium wilt over time

The objective of this research was to determine the effectiveness of using grain crops in rotation with cotton in managing Verticillium wilt of cotton over a 10-year period. The circular shaped field (52 ha) was divided into six equal sized wedges and cotton or grain crops (sorghum, corn, wheat) were rotated from 2001 to 2018, with some wedges having cotton more frequently and others rotated with grain crops more frequently. Verticillium wilt, which is caused by Verticillium dahliae, was first seen in the field in 2007. Verticillium wilt incidence increased as long-term cotton (host) frequency (HF) increased during 2008–2010 and 2012–2014, but wilt incidence was unaffected by HF in 2016–2018. Grain crops planted in rotation with cotton in one of the two previous years reduced wilt incidence in rotated cotton relative to continuous cotton in 2008–2010, but either had no affect or increased wilt incidence in rotated cotton in 2012–2014 and 2016–2018, relative to continuous cotton. Verticillium dahliae microsclerotia density (MS/cm3 soil)1/2 increased with increasing HF for all three time periods. Three consecutive years of cotton increased MS1/2 relative to rotated cotton in 2008–2010 and 2012–2014, but either did not affect MS1/2 or negatively affected MS1/2 density by 2016–2018 (relative to rotated cotton). Yield, adjusted each year to a 0 to 1 scale, decreased with increasing HF in all three time periods. Yield decreased with three consecutive years of cotton relative to rotated cotton in 2008–2010 and was unaffected by rotated cotton in 2012–2014 and 2016–2018. Crop rotation with nonhosts for management of Verticillium wilt, initially (first four years) was very effective at reducing MS, wilt incidence, and increasing yield. However, 9–11 years after the initial Verticillium wilt symptoms occurred, short-term crop rotation no longer reduced Verticillium wilt incidence, reduced microsclerotia density, or increased cotton yield.