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Crop Disease Update

Posted by romeethredge on April 27, 2013

UGA Extension Plant Pathologist, Bob Kemerait, gives us the following plant disease update.

Fungicides and Corn:

My graduate student Suzette Arcibal has spent much of last season assessing the impact southern corn rust, northern corn leaf blight and southern corn leaf blight.  Her graduate studies have been sponsored by BASF and I am grateful for their support.  Based upon her findings, here are my comments for use of fungicides on corn:

1.  For management of foliar diseases on corn planted in March and April:

a.  Watch sentinel plot reports for detection of southern corn rust.

b.  Control of southern corn rust:  In most situations I believe that a fungicide application made at first tassel (VT) will be sufficient to lay a good foundation to rust control and may be all that is needed for the season; however depending on the pressure, growers may be advised to make a second application within 3 weeks of the first.

i.  Triazole and strobiluirn fungicides are effective against rust; the combination of both will have a longer protective window and broader activity against other pathogens.

c.  Control of northern corn leaf blight (NCLB):  Unlike southern corn rust which must be reintroduced into Georgia in 2013, the northern corn rust pathogen is already here surviving in last year’s crop debris.

i.  Northern corn leaf blight will not be a problem in every field, but in fields where it is a problem, timely use of fungicide program can protect yield.

ii.  From Suzette’s data, northern corn leaf blight is most effectively managed by a combination of an early application (for example the 5th true leaf stage) and a follow-up application at late tassel-early silking stage.

iii.  The spores of the NCLB fungal pathogen can be spread between fields; however the earliest infections will likely occur as rain and irrigation splash the spores from the crop debris and soil to the lower leaves of the corn plants.

iv.  I recommend that as the corn crop approaches the 5th-true-leaf stage, growers should scout to determine if northern corn leaf blight is developing.  Again, not all corn fields need to be treated with a fungicide for management of northern corn leaf blight; however it is important note if the disease is likely to be severe.

v.   I believe that northern corn leaf blight is more difficult to control than is southern rust (assuming you are on time with applications) and that a combination of strobilurin and triazole fungicides is an important consideration.

2.  For management of foliar diseases on corn planted after April:

a.  The threat from southern rust becomes more severe.

b.  The threat from southern corn leaf blight increases.

c.  The threat from northern corn leaf blight remains important.

d.  The potential benefit to beginning a fungicide program 

e.  The potential yield benefits from use of a fungicide becomes even greater as a percentage of potential yield for late-planted corn.

f.  The benefit of planting a rust-resistant hybrid becomes more important with late-planted corn.

Two web sites to note: (National site to follow the progress of Asian soybean rust)  We have not found any soybean rust on NEW GROWTH kudzu but did find some on OLD GROWTH kudzu in Miller County.  Those old leaves have been destroyed.  Soybean rust is present in northern Florida, Alabama, and Louisiana.  Sentinel plot site for southern corn rust.  Southern corn rust has not been found in Georgia yet in 2013.

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