Relatively low disease in Tennessee Corn

With no reports of southern rust in Tennessee (only some in GA and LA reported at this time), it is a relatively quiet disease season in corn, with the exception of grey leaf spot and physioderm brown spot in Tennessee.


One can track the reports of confirmed southern rust at https://corn.ipmpipe.org/ (Image 1 – reports as of 7/4/2019).

Confirmed reports of southern rust as of 7-4-2019

While southern corn rust is the one of the most damaging diseases, be sure it is correctly identified and not confused with common rust. The characteristics that differ between the rusts are the location, color, shape, and distribution of pustules (see table and images 2 and 3). When in doubt don’t hesitate to contact your local county agent for advice on disease management and identification.

 

 

Table 1. Comparison of Southern and Common Rust

 Pustule Characteristics: Common Rust Southern Rust
 Location upper and lower leaf  surfaces primarily upper leaf surface
 Color brownish, red orange to light brown
 Shape elongated round
 Distribution scattered densely packed
 Other differences:
 Optimal Temperature 61-77 °F 77-82 °F
 Probability of Effecting  Yield low moderate to high
Image 2. Southern rust on upper side of corn leaf
Image 3. Common rust on corn

While physoderma brown spot is not of extreme concern, on the leaves (reports I have gotten); it is of concern if it gets on the stalk and causes rot. It is easily identified by the dark spots in the mid-rib of corn leafs (image 4).

Image 4. Physoderma brown spot symptoms on leaf and some spotting on stalk

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Grey leaf spot (image 5-6) could still warrant a fungicide application in certain situations, but this on a field by field basis.

Image 5. Beginning lesions of GLS in lower canopy
Image 6. Lesions of GLS elongating on corn leaf

Consider the major factors for disease to develop, have significant effect on yield, and become a significant PEST.
Pathogen presence – influenced by field history
Environment – the right conditions for disease to develop
Susceptible host – a variety that is susceptible to disease
Time – regarding the growth stage of the plant and when the disease develops

Foliar fungicide applications in corn is more likely to provide a response in yield when disease is present or there is significant risk of disease, weather conditions favor disease development, the field is corn after corn, and a disease susceptible hybrid is planted. Research data from University of Tennessee and other universities has continuously found these factors to strongly influence the magnitude and consistency of yield response to a foliar fungicide application in corn. One can find the corn fungicide efficacy table at UTcrops.com (Corn Fungicide Table) to better pair fungicide with particular targeted diseases when fungicide is warranted.

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