Leaf blight is a foliar disease of sorghum that infrequently occurs in Arkansas was observed earlier this year. A low level of leaf blight (1-2 lesions per plant on <1% of plants in a field) was detected in a few grain sorghum fields across central Arkansas. Leaf blight was observed in the lower to mid-canopy on grain sorghum at anthesis (50% of the panicle flowering) thus, infection occurred earlier this year and was not progressing into the upper canopy. Leaf blight was also reported in Louisiana and Mississippi so this phenomenon is relatively widespread and it will likely be detected in other fields across the state. There is no recommendation to treat at these low levels of disease—continue to scout fields for leaf blight and other foliar diseases of sorghum.
Leaf blight of grain sorghum is caused by the fungus Exserohilum turcicum, which also causes northern corn leaf blight, a common foliar disease of corn in Arkansas. This pathogen is not new, and this year conditions were favorable for leaf blight of sorghum. Leaf blight can also occur on Johnsongrass, sudangrasses, and gamagrass (Tripsacum dacyloides). Leaf blight lesions are large (1 to 6 in.), elliptical shaped with tan to gray lesions with a reddish margin (Fig. 1 – 3). Lesion morphology does vary among grain sorghum hybrids; for example, Fig. 1 was from a different field than Fig. 2 and Fig. 3. The pathogen persists in crop residue, and disease development is favored by moderate temperatures (64 to 81°F) and light rain or heavy dew. When weather conditions are favorable, the pathogen produces conidia that are dispersed onto nearby plants or disseminated by wind to another field.
Although resistance is available, based on the lesion size these hybrids appear susceptible to leaf blight. Typically, fungicides are not economically practical in commercial fields, especially at these low levels of disease pressure—continue to monitor these fields for changes in leaf blight and other sorghum diseases before considering a fungicide application. Since corn is a host to this pathogen, rotation with corn would sustain inoculum for the subsequent sorghum crop. Proper tillage practices would be beneficial to minimize inoculum carryover in crop stubble.