By Katie Nichols
Alabama Cooperative Extension System plant pathologists have confirmed the presence of southern corn rust in a corn variety trial at the Gulf Coast Research and Extension Center.
Alabama Extension plant pathologist Dr. Austin Hagan, said the weather patterns over the past week have been favorable for rust intensification.
“The forecast calls for rain showers for the next week, so producers can expect to see continued disease development,” Hagan said.
Southern Corn Rust
Dr. Kassie Conner, an Alabama Extension plant pathologist, found the southern rust on corn at the Gulf Coast Research and Extension Center.
Southern rust (Puccinia polysora) poses a significant threat to corn production in southern Alabama—particularly in Baldwin and Mobile counties.
While destructive southern rust outbreaks do not occur every year in Alabama corn, they typically occur every three to four years. Disease onset usually occurs at tasseling or silking during outbreak years. The likelihood of a negative impact on yield decreases when disease onset occurs between tasseling and the dough stage. Later outbreaks have little impact on yield.
Southern Corn Rust pustules
Common rust is present in Alabama corn as well. Common rust pustules are dark brown, as compared with the orange to orange-brown coloration of southern rust pustules. Southern rust pustules are numerous on the upper leaf surface. In contrast, common rust pustules will be found in equal numbers on both leaf surfaces. Southern rust pustules are also found in clusters of numerous pustules as compared with the more solitary common rust.
Late corn sown after wheat in South Alabama is particularly vulnerable to southern rust as inoculum pressure can be very high in July and August. Rapid disease development and consequential yield losses are more likely in corn with irrigation than dryland corn, regardless of plant date. Frequent showers, coupled with weather systems moving from Mexico or Florida over the Gulf of Mexico can accelerate disease onset and development.
Southern Rust Treatment
Hagan said protective fungicide treatments will be key to slowing disease progress and protecting kernel yield.
“Previous Alabama field trials show yield gains from fungicide inputs are visible when serious damage from either southern rust or northern corn leaf blight develops on ear and ear-1 leaves,” Hagan said. “So, no disease activity translates into no yield gains from fungicide inputs. All corn fields are not candidates for fungicide treatments.”
Name-brand fungicides at $20 to $30 per acre are costly. Producers are likely to see good returns on fungicide investments in irrigated or dryland fields with yield potentials exceeding 150 to 175 bushels per acre. This is true particularly with corn prices hovering around $3.70 per bushel. Ideally, fungicide treatments for rust control should begin based on a scouting report and the corn growth stage.
Conner said producers who are unable to confidently diagnose southern rust or any other plant diseases should submit samples to the Plant Diagnostic Laboratory in Auburn for testing.
“Before sending samples, visit the Plant Diagnostic Laboratory website for specific directions on packaging and mailing plant samples,” she said.
Prices for plant disease diagnosis range from $10-$30 depending on the tests and analyses required.