by Thomas Isakeit, Professor and Extension Plant Pathologist; t-isakeit@tamu.edu ,979-862-1340
Southern rust of corn (Puccinia polysora) (Figure 1) is present in fields in several Upper Coast counties. This disease has the potential to cause yield loss in susceptible hybrids and growers should be scouting for it. I have a bulletin that gives guidance for scouting, action thresholds for spraying, and a list of fungicides, at this address:
http://aglifesciences.tamu.edu/plantpathology/wp-content/uploads/sites/19/2011/05/Southern-Rust-of-Corn_2014.pdf
I have frequently encountered common rust (Puccinia sorghi) this season. This is not a disease of concern in Texas. Northern leaf blight is present in many areas as well, but this is also not a problem except for very susceptible hybrids.
Figure 1. Southern rust of corn. Note the “shotgun pattern”, i.e. a lot of lesions that are closely concentrated. This feature is useful for field scouting. Additionally, southern rust lesions tend to be only on the upper part of the leaf.
In several fields in the Blacklands, I have seen leaf spots on lower leaves of cotton (Figure 2). Although the symptoms resemble “Ascochyta blight” (also known as “wet weather blight”), by examining the fungi in the lesions with a microscope, I have only found a Phoma species.
Figure 2. Symptoms of a leaf spot of cotton associated with a Phoma species.
In addition to Phoma leaf spot, in one field, I found a few plants with petiole and stem lesions (Figure 3). The anthracnose fungus (Colletotrichum gossypii) was associated with these lesions, as confirmed with a microscopic examination. These lesions are brittle and break easily.
Figure 3. Stem lesion of cotton associated with the anthracnose fungus.
The appearance of these cotton diseases have been associated with frequent, rainy weather. It appears that with warm, dry weather, the cotton plant outgrows the fungus. In one field, I did a fungicide application (two active ingredients) on some rows and after four days, I did not see any difference between the treated and non-treated rows. In this time, there were some clear, warm days and new and upper leaves of all plants looked good. There may be a problem with stand in some fields where anthracnose is present, but the Phoma leaf spot should not be a long-term problem. If you suspect either of these problems, feel free to contact me for a diagnosis.
Thomas Isakeit, Ph.D.
Professor and Extension Plant Pathologist
College Station, TX
t-isakeit@tamu.edu