Southern rust and tar spot of corn are on the move

In Southeast Farm Press

by Carl Bradley, University of Kentucky Extension Plant Pathologist

Two corn diseases are already making some news this season. Southern rust and tar spot have been detected in southern states and could potentially make their way to Kentucky this season. So, keeping a lookout for these two diseases is a good idea.

Southern rust

Southern rust (caused by Puccinia polysora) made an appearance in Kentucky last summer, and turned many combines orange during harvest because of the large number of orange-colored pustules present on the leaves.

Although the fungus that causes southern rust is not able to over-winter in Kentucky, it has already been found in corn fields in Texas, Louisiana, and Georgia this year. This pathogen will move north during the season, but timing is everything. If the disease is found in Kentucky corn fields prior to the milk stage (R3 growth stage), then yield losses may occur.

Wet spring weather did delay corn planting in some areas this year, so the table could be set for this disease to be a problem, especially in late-planted corn.

Tracking the whereabouts of southern rust in the U.S. will help determine when it may move into Kentucky, and this will continue to be a topic addressed in Kentucky Pest News— which can be found here — this season. Many fungicides are able to effectively protect against infection by the southern rust pathogen. Extension plant pathologists from corn-growing states have put together a fungicide efficacy table for corn (Fungicide Efficacy for Control of Corn Diseases, BP-160-W), which is published by Purdue

Tar spot

Tar spot (caused by Phyllachora maydis) was found in the U.S. for the first time late in the 2015 growing season in the northern portions of the neighboring states, Illinois and Indiana. Although this disease appeared too late in the growing season to cause yield losses, it was observed across several counties in both of those states.

On June 10, 2016, tar spot was detected on corn growing in Palm County, Fla. Although little is known about the ability of this disease to cause losses to corn grown in the U.S., it is important to be on the lookout for the disease.

Since this disease has never been reported in Kentucky, it is critically important to work with your local county Extension agent to submit any suspect samples to the University of Kentucky Plant Disease Diagnostic Lab.

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