Since Monday, June 21, numerous fields in several counties in Arkansas and Mississippi have been determined to have detectable levels of southern corn rust. While damage from this disease can be significant, plant pathologists urge producers to scout closely before they spray to determine crop growth stage, the type of disease present and the severity of the disease.

Figure 1

Based on the recent past, this is the earliest with regards to calendar date and crop growth stage that southern corn rust has been identified.

In the majority of the cases, southern rust was present on low levels on the leaves. In a few rare cases, entire plants within the middle of a field were observed to have southern rust. One field in particular was observed to have southern rust on leaves and stalks of infected plants in Mississippi.

We have included a few photos in this update in hopes of providing good comparisons between common and southern rust (see Figure 1).

Keep in mind the best way to positively identify whether or not common or southern rust is present is for a trained professional to remove spores from a sporulating pustule and compare the size, shape and morphology of the spores at magnifications that cannot be achieved with a hand lens.

Distinct differences between the two spores are the only method to detect southern rust in the event that there is a light infection.

Figure 2

The optimum temperature for southern rust to develop is within the range of 80 degrees to 90 degrees F. Clayton Hollier from LSU did his doctoral thesis on southern rust and he states that temperatures of 104 degrees F do not reduce levels of infection or sporulation of the fungus.

Currently, common rust appears to still be developing in some fields on the lowest leaves that remain on the corn plant. Although common rust pustules are larger than southern rust, pustules on lower leaves are smaller and can resemble southern rust. (See Figure 2). Southern rust will typically develop in the middle to upper canopy, but this is not to say that lesions can’t be found on the lowest leaves.

One thing to note: looking up into the corn canopy and seeing “flecking” that appears on corn leaves is not always indicative of a rust infection or impending epidemic. More often than not this is caused by pollen granules that have landed on the leaf, resulting in a round, yellowish lesion. Using a leaf of this type to suggest that a southern rust infection has already occurred is not necessarily the case and to us suggests that a field should simply be monitored.

The major characteristics of the two diseases are:

Figure 3

Common rust characteristics:

• Prefers cooler temperatures (61-77 degrees F) and high relative humidity (95 percent).

• Common rust pustules typically form in a pattern along leaves, normally in a line or band where the expanding leaf was infected while it was still in the whorl (see Figure 2).

• Sporulating pustules of common rust are typically darker in color, and appear more cinnamon (see Figure 3).

• Common rust pustules appear to be larger in size and the leaf tissue looks more “shredded” and in some cases there are tan to brown lesions around the infected leaf tissue and pustule (see right side of Figures 1 and 3).

• Common rust typically forms lesions on the upper surface of leaves

Southern rust characteristics:

• Prefers warmer temperatures (80-90 degrees F) and high relative humidity (95 percent).

Figure 4

• Southern rust pustules do not occur in a pattern but can normally be found covering a larger area on each infected leaf (see Figures 1 and 4).

• Sporulating pustules of southern rust are lighter in color, typically appearing orange, and sporulating pustules have the appearance that they could be dusted off the surface of a leaf (see Figure 4).

• Southern rust pustules appear to be smaller in size than common rust pustules (compare Figure 1, both sides).

• Pustules can be found on leaves, stalks, ears, and the tassel, as well as on both surfaces of the leaf (see Figure 5).

The fungi responsible for causing common and southern rust cannot survive the winter in Arkansas and Mississippi nor can the causal fungus survive on stubble left in a corn field between seasons. Each year the fungi are blown into the region from southern overwintering sites (i.e. Mexico and the Caribbean).

2010 is the first year that southern corn rust has been tracked on the Internet on a national basis. If you would like to stay abreast of the situation as southern rust develops, a Web site (IPM: Southern corn rust) has been created much the same as we have tracked soybean rust.

Figure 5

With all of this information in mind, we are NOT suggesting a widespread, blanket fungicide application. The decision as to whether or not to spray a fungicide will be solely up to the producer and the particular field situation. Consultants and other agricultural professionals and producers should scout corn fields diligently and keep several key points in mind:

1. Growth stage of the corn (corn as late as full dent will likely not see an economic benefit/return from a foliar fungicide application)

2. Expected yield.

3. Type of irrigation (center pivot versus furrow irrigation).

4. Presence of the disease based on spore confirmation.

5. Location of the disease within the corn canopy.

6.Extent of the infection and severity within a particular field.

7. ALL hybrids in Arkansas and Mississippi are considered to be susceptible to southern rust.