Southern rust has been detected in fourteen other counties since it was first reported on July 19 in Crittenden County. These new counties include: Arkansas, Chicot, Cross, Desha, Lee, Lonoke, Lincoln, Jefferson, Jackson, Phillips, Poinsett, Prairie, St. Francis, and White (Fig. 1). Given the wide distribution of rust reports, it is highly probable that rust has developed but gone undetected in all eastern corn-producing counties in Arkansas.
Among these new reports, disease severity was relatively low at <1 to 3% in hot spots on corn ranging from early milk to late dent growth stages. In a few cases, fungicides were applied early before rust was detected or threatened, and now the disease protection provided by the fungicide has expired so it is likely rust severity will increase in these situations. Though there is no threshold for economical use of a fungicide in corn for southern rust, there is no yield benefit from a fungicide applied at two weeks before black layer, ~50% starch line in the kernel. A fungicide application at tasseling or silking (in a crop with good yield potential) when southern rust has been observed or threatens, may be the most beneficial at suppressing disease development.
If a second application or late application is warranted, an effective option is a triazole fungicide like propiconazole or tebuconazol. (See MP154 for numerous trade names and harvest restrictions). Further, a fungicide applied with 5 to 7 gal/acre that reaches mid canopy where southern rust is often first detected provides the greatest effectiveness. Based on yield loss estimates due to defoliation, a 3% yield loss will result from 20% defoliation at milk stage and 3% yield loss will result from 40% defoliation at dent. Consider economical return when applying a fungicide, in addition to the length of time corn will remain in the field, yield potential, other diseases in the field, and the last time a fungicide was applied. See earlier publications on this blog for fungicide efficacy tables and other management considerations.