With the tremendous upswing in grain sorghum acres this year and early planted sorghum about a week or so away from heading, this may be a good time to revisit basic sorghum growth and disease management.
Grain sorghum produces about 12 leaves prior to heading. The boot stage (head enclosed in flag leaf sheath) occurs about 60 days after planting. Head emergence occurs over about a 2 day window and flowering starts when the head is fully exposed. The grain sorghum head blooms from the top to the bottom. Post-heading fungicides should be applied at about 50% bloom. Midge insecticides can be tank mixed with a post-heading fungicide.
Diseases in sorghum tend to be more prevalent when sorghum is planted no-till following corn or as multi- year sorghum. Stalk rots in corn can infect sorghum. The chances of having damaging levels of foliar disease inoculum increases in no-till as certain diseases spread from old sorghum residue. Gray leaf spot (Cercospora sorghi) and Target leaf spot (Bipolaris sorghicola) are two foliar diseases that can occur on sorghum in Tennessee. Bacterial leaf stripe or streak can also be found in some years. Head blight (Fusarium spp.) may occur in years with prolonged wet weather prior to or during flowering. Grain mold (various causal fungi) is worse when seed develop and ripen in wet weather. In 2014, milo samples were brought into the lab with stalk rots. The University of Missouri has excellent photos and more detailed disease descriptions at http://extension.missouri.edu/p/G4356#Diseases.
Fields should be scouted for foliar diseases around the 8 leaf stage and again at heading. If no foliar disease is found, do not apply a fungicide prior to heading. Most of our sorghum is never sprayed for disease. If conditions at heading/bloom are conducive to disease, yield potential is good and foliar disease is confirmed then a fungicide application after heading may be needed. Dr. Heather Kelly’s work with post-heading fungicides in 2014 showed that a post-heading fungicide increased sorghum yield by up to 4 bushels/acre (not statistically different from unsprayed check) in second-year sorghum infected with gray leaf spot and what appeared to be target leaf spot. With the exception of Quilt Xcel, all fungicides tested reduced the level of disease.
Headline, Aproach, Quilt Xcel, Topguard and Priaxor are labeled on sorghum in Tennessee. Headline, Topguard and Priaxor gave similar levels of foliar disease control in Dr. Kelly’s work. Do not tank mix adjuvants or foliar fertilizers with post-heading sprays unless specifically allowed on the fungicide and insecticide label.