Selected Pre‐emergent Herbicides and Soil pH Effect on Seedling Blight of Grain Sorghum

Establishment of grain sorghum [Sorghum bicolor (L.) Moench] can be a problem in central and south central Kansas. Studies were conducted in 1989 and 1990 to determine the influence of soil pH, pre‐emergent herbicides, and Fusarium spp. on seedling blight, plant population, and yield of grain sorghum in south central Kansas. A split‐plot experiment with soil pH as main plots and herbicide treatments as subplots was established in both a wheat (Triticum aestivum L.)‐grain sorghum (WGS) rotation and a grain sorghum‐grain sorghum rotation (CSGS). There were no interactions between soil pH and herbicide for any of the variables. Fusarium graminearum was the primary pathogen isolated from the roots of seedling grain sorghum plants. The rate of infection at the lowest pH decreased over time in 1989, but increased over time in 1990. Fusarium graminearum isolation frequencies from the roots of seedling grain sorghum plants were significantly higher at the lowest soil pH levels regardless of year or rotation. Secondary root development was signficantly less at the lowest soil pH level in both 1989 rotations and the 1990 GSGS rotation and did not change over time. In the 1990 WGS rotation, the rate of secondary root development had a greater increase at pH 6.3 than at pH 5.9 or 4.7. Secondary root development was significantly higher at pH 5.9 than at pH 6.3 at 14 d but there were no differences among the treatments by 25 d after planting. No differences in plant population or yield were detected due to soil pH in either year or either rotation except plant population was significantly ess at the lowest soil pH in the 1990 WGS rotation. Fusarium graminearum isolation frequencies from the roots were significantly higher when alachlor plus atrazine, metolachlor lus atrazine or atrazine alone was used compared with a watertreated control regardless of year or rotation. In addition, F. graminearum isolation frequencies were significantly higher from the alachlor plus atrazine treatment than in the two other herbicide treatments. Except for the 1989 WGS rotation this response decreased over time following planting. Secondary root development paralleled root infection with the least root development occurring in the alachlor plus atrazine treatment at the earliest sampling dates regardless of year or rotation. This response decreased over time following planting. There were no significant differences in plant population or yield due to soil pH except there was a significantly lower plant population at pH 4.7 in the 1990 WGS rotation. Alachlor plus atrazine significantly reduced stands in both rotations in both years. There was no difference in yield among herbicide treatments. Research Question Seedling blights cause Kansas grain sorghum producers to replant ≈2% of the grain sorghum acreage each year. Most seedling blight problems occur in central and southcentral Kansas where soil pH is generally low (often <5.0) and sorghum is grown either continuously or in rotation with wheat. Sorghum is planted in early to mid‐June and the most common herbicide programs are pre‐emergent applications of atrazine, alachlor plus atrazine, or metolachlor plus atrazine. Fusarium graminearum is the primary pathogen isolated from diseased roots. The objective of this research was to determine the role of soil pH and pre‐emergent herbicides in seedling blight development on grain sorghum caused by F. graminearum. Literature Summary The establishment of acceptable grain sorghum stands has been a problem since the 1940s. Fusarium spp. damage to the root systems of some sorghum cultivars grown in low pH (4.7) soil in southcentral Kansas has been reported. Acidic soils are also associated with increased levels of F. graminearum in the roots of corn and wheat. Numerous reviews have been published on the ability of some herbicides to increase plant disease. A South African study reported that atrazine, propachlor, and propachlor plus propazine all increased seedling blight severity on grain sorghum, but the pathogen(s) involved were never identified. Study Description Field studies were conducted in 1989 and 1990. Soil: Lady Smith silty clay loam Previous crop: wheat or sorghum Main plots: Soil pH: 1989 (wheat‐grain sorghum [WGS] and grain sorghum‐grain sorghum [GSGS] rotations), 5.1, 5.7, 6.1; 1990 (WGS), 4.7, 5.9, 6.3; 1990 (GSGS), 4.8, 6.0, 6.5 Subplots: water control, atrazine, atrazine + alachlor + flurazole, atrazine + metolachlor + CGA‐92194 Measurements: Emergence, percentage root infection, secondary root development, yield Applied Questions Are grain sorghum plants more susceptible to Fusarium seedling blight under acid soil conditions? The highest percentage root infection and poorest secondary root development occurred at the lowest soil pH in both years. By the final sampling date (5–6 wk after planting), significant differences in root infection and secondary root development remained only at the lowest pH level. This effect was independent of rotation. The only significant decrease in emergence occurred in the 1990 WGS rotation at pH 4.7. There were no differences in yield in either year. Can pre‐emergent herbicides increase the susceptibility of grain sorghum to seedling blight caused by F. graminearum? The percentage of Fusarium root infection was significantly higher for all herbicide treatments compared with the water‐treated control in both rotations and in both years. This effect was still present after 6 wk for all treatments except atrazine in the GSGS rotation in 1989 and for all treatments in both rotations after 4 wk in 1990. Only the alachlor plus atrazine treatment in the WGS rotation still had an increased percentage of root infection after 5 wk in 1990. All herbicide treatments caused significant reductions in secondary root development 2 wk after emergence in both rotations and in both years. By 6 wk after planting in 1989, root development was similar to the no herbicide control for all treatments except the alachlor plus atrazine treatment. Root development was not significantly different among any of the treatments in either rotation by 5 wk after planting in 1990. The alachlor plus atrazine treatment significantly reduced plant population in both rotations in both years. In addition, the metolachlor plus atrazine treatment reduced plant populations in the WGS rotation in 1990. There were no differences in yield among the treatments in either rotation in either year. Recommendations Grain sorghum stand establishment problems in Kansas appear to be caused by several factors that may be additive. Low pH, hybrids sensitive to herbicide and safener stress, and F. graminearum can reduce root function, resulting in slow growth of plants. Under severe conditions, plants die. Rotation, with either wheat or grain sorghum, did not affect stand improvement. Our results suggest those sorghum plants that survive these initial stresses will eventually produce a near normal root system, with little or no effect on final yield. Therefore, much of the current replanting is probably unnecessary. Management recommendations for this problem would include liming soils to a pH of 6.0 or higher, the use of hybrids more tolerant to acid amide herbicides, or the replacement of pre‐emergent herbicides with post‐emergent ones.