While Mother Nature has given a break to South Texas farmers long enough for most fields to begin drying out after excessive rains in recent months, a few Upper Coastal Bend farmers scouting corn fields are turning up early signs of southern rust.
Bobby McCool, Texas AgriLife Extension Agent in Aransas and San Patricio Counties, says more reports are coming in of Southern rust in the Coastal Bend and recommends increased scouting of corn fields.
Southern rust is a foliar disease that can reduce yield potential. Caused by the fungus Puccinia polysora, Southern rust is the most important foliar disease of corn in the Upper Coast region of Texas. In some wetter years like the current growing season, it may require a fungicide treatment to minimize yield loss. This is especially true when susceptible hybrids are grown, according to Tom Isakeit, professor and Texas AgriLife Extension plant pathologist.
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During a Field Day near College Station last year, Isakeit told growers that an early application of fungicide will help prevent rust from developing on leaves even if the farmer can’t visually see the rust infection.
"Southern rust is recognized by small circular orange pustules and this is in comparison to common rust, which is reddish and more elongated,” Isakeit said. “If you have corn at the tasseling to early grain development stage, check lower leaves. If leaves have 3 to 5 percent pustules, that will be your trigger to spray."
The symptoms of the disease are slightly raised, circular (1 to 2 mm diameter), orange pustules that are mainly on the upper sides of leaves. Common rust caused by the fungus Puccinia sorghi is seen early in the season and never progresses to economic damage in coastal Texas corn.
Initially, Southern rust occurs on the lower foliage and progresses to the upper canopy during the growing season. The severity of disease in the upper canopy is increased by wet conditions. However, in determining whether to spray or not, Isakeit said when the crop is still in the vegetative stage, refrain from spraying. When the crop is near flowering and development of the ear of corn is the most beneficial time to apply fungicide.
While plant pathologists say an early application of fungicides can be a good treatment option, they also warn that the purpose of a fungicide application is to protect the upper leaves of the plant during kernel development. If applied too early (e.g. the vegetative stage), it may not protect the critical leaf tissue and another fungicide application may be necessary during kernel development, so growers should check with their crop advisor or county agent in making decisions on the best timing for treatment.
While rust currently poses the greatest risk in the Upper Coast and Coastal Bend areas, corn growers in the Lower Rio Grande Valley and in Southeast Texas are advised to scout fields carefully for early signs of the disease. While first signs usually occur on lower foliage, Southern rust can also affect stalks and husks.
Severity of disease in the upper canopy is increased by wet conditions, as is the case for much of the Texas corn crop this year. A high severity of rust on upper canopy leaves can lead to premature drying, which may affect yield, primarily by reducing kernel weight.
While Southern rust can be problematic for coastal Texas, it is not a serious problem every year. The most recent epidemics of Southern rust in the Upper Coast region were in 2012 and 2007.
Isakeit warns that little fungicide efficacy data, with side-by-side comparisons and replications, exist in the Upper Coast area. Three classes of fungicides are labeled on corn, with different modes of action. Fungicides in the same class can differ in activity and stability. Based on Upper Coast data, with a limited number of fungicides, it appears that any of the labeled fungicides will do an adequate job if applied in a timely manner.
Fungicides will have activity two to three weeks after application, so with an early-season treatment, another application may be needed if disease pressure continues during the growing season.
As with all plant diseases, scouting fields remains a priority in order to catch the development of Southern rust at the earliest possible stage.