Identifying plant diseases is often very difficult and requires practice and experience. Questions always come in about what rust is this affecting corn and whether to utilize a fungicide.
Common rust can be found in almost every corn field. It is often found on the upper and lower leaf surfaces. Pustules appear brown to brownish red in color, elongate in shape, and are scattered on the leaf (Figure 1). Common rust prefers cooler weather (61-77°F), and infection usually occurs in early summer.
Southern rust prefers warmer weather (77-82°F), and is only found on the upper leaf surface. Pustules are round in shape, orange to light brown in color, and are found densely packed on the leaf (Figure 2). If southern rust occurs in South Dakota it is often observed in late August to early September. This rust was found at very low incidence and severity in Brookings County last week (Figure 3).
When deciding on whether or not a fungicide application is warranted, the disease needs to cover at least 5% or more of the ear leaf. If all of the lesions you are seeing on the leaf fit into an area less than 5% than a fungicide is not warranted. If a fungicide application is necessary, make sure to apply it to fully tasseled corn in order to avoid arrested ear syndrome.
The good news for South Dakota corn growers is that each year, rust spores must be blown up from the Southern states as they cannot survive our South Dakota winters. If you had rust this year, it does not mean you will have rust again next year. If weather conditions are right and the spores make their way up here, then we will see infection take place. Southern rust made its way to South Dakota quite late this season and with majority of corn fields at or past dent growth stage. Southern rust may not cause significant yield loss and therefore fungicide application may not be needed.