Thursday, July 10, 2014

Illinois Corn Disease Scouting Report - Should I spray fungicide?


High Alert Diseases

Gray Leaf Spot -


Northern corn leaf blight -

Gray leaf spot and Northern corn leaf blight are showing up within the Burrus/Hughes footprint.  Gray leaf spot typically shows up first on lower leaves because it overwinters on residue.  Northern corn leaf blight also overwinters in residue, but a lesion can show up wherever a fungal spore infects leaf tissue.  A wound is not needed for the infection of these diseases.  Wet weather and warm temperatures are needed for disease development of gray leaf spot.  Northern corn leaf blight tends to like slightly cooler weather than gray leaf spot.  

Northern corn leaf blight is showing up across Illinois and if you are seeing disease pressure in your field on a susceptible hybrid, I would be a bit quicker to spray fungicides, especially if it is showing up before silk! If you have this disease present, and a susceptible hybrid, lesions of this disease can develop every 7 to 12 days in a favorable environment!  Northern corn leaf blight can spread quicker, when compared the gray leaf spot.

We are seeing higher disease pressure of gray leaf spot in Western and Southwestern Illinois.  There are pockets of higher disease pressure in Central Illinois. 



Gray leaf spot Northern corn leaf blight
(Cercospora zea-maydis) (Setosphaeria turcica)
Fungal spore survival overwinters in leaf debris overwinters in leaf debris
Temperatures 75-85 degrees 64 -80 degrees
humidity/moisture high humidity/wet  prolonged moisture/dew
Plant part affected leaves leaves
occurrence silking to maturity silking or after silking
lesion development 1  1/2 to 2 weeks 7 to 12 days
management resistance/tillage/rotation fungicides resistance/fungicides

 If trying to decide if fungicides are needed, here are some questions you need to answer:

1.)    What growth stage is the plant:  Field should be scouted for  disease at pollination (2 weeks before and after tassel).  If your corn is within this growth stage range.......
2.)    Have you had issues with leaf disease in the past or is there a high amount of residue in the field (corn on corn/no- till) that could be harboring disease?  If so, you may be more at risk for gray leaf spot or Northern corn leaf blight........
3.)    Are there disease symptoms near the ear of the plant (1 to 3 leaves below the ear)?......
4.)    How susceptible is your hybrid?  Every corn hybrid has a disease rating.  Check the disease rating for your hybrid and if it is a low number, you may be more likely to use a fungicide if disease pressure is high.
5.)    What is the future weather outlook?  If wet weather is in the forecast, this may be another indicator that fungicides may be needed if your hybrid is susceptible and has high disease pressure appearing on at least half of the field.


For more information on Fungicide Efficiency of for Control of Corn Diseases, check out this chart from Purdue:  https://www.extension.purdue.edu/extmedia/BP/BP-160-W.pdf

 For example, I have observed the spread of gray leaf spot to the next leaf up on the plant within 3 days this week on a:
susceptible hybrid (5 leaves from the bottom of the plant and near the ear) - may need fungicides
moderate hybrid (4 leaves from the bottom of the plant and near the ear) - may need fungicides 
resistant hybrid (3 leaves from the bottom of the plant and not near the ear) - may not need fungicide


Low Alert Diseases

Common Rust -

Common rust has started to show up in some fields due to spore movement from the Southern US combined with wet and cooler weather. As temperatures get warmer, this disease will come to a halt and generally, fungicides are not needed.


Physoderma brown spot -

The previous hot and wet weather were perfect conditions for Physoderma brown spot to show up if the spores of this disease were splashed into the whorls of susceptible hybrids.  This disease should not be a concern and there will be no need for a fungicide application.  This disease is less likely to be found in Northern Illinois because of the cooler weather.


Common smut -


Common smut can show up when spores infect within a wound of any actively growing tissue of the corn plant.  It may be a bit more prevalent this year due to storms causing wounds via hail or blowing particles.  We have also seen smut infect corn due to insect or equipment injury. Common smut is not usually considered to be a major disease of field corn and fungicides are not warranted.
­­­­­­­­­­­­­­Non-Disease

Purple leaf sheath -

Purple leaf sheath is appearing at pollen shed on the sheath and stalks of corn plants.  This is not considered to be a disease pathogen and does not infect a plant.  It is a harmless discoloration caused saprophytic fungi and bacteria developing on pollen and particulate matter lodged between the stalk and leaf sheath. Fungicides are not needed!
 
Disease Lesion Mimic Mutants of Maize -

Lesion Mimics are still not completely understood.  Symptoms of dead cells (lesions) on leaves were first thought to occur due to a defect of plant defense genes or pathways that spring into action during infection of a disease, but these or similar symptoms have also been found to occur because of disruptions of biological processes or loss of stable equilibrium within cells, which can also cause cells to die in a shape of a “lesion” on corn leaves.  These mutant corn plants can be an indicator of plant stress caused externally by the environment or internally due to issues within the plant.  Again, this not considered a disease; therefore a fungicide is not needed. 

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