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C.O.R.N
Newsletter
2008-22
July 14, 2008 -
July 22, 2008
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Detecting and Managing Leaf Diseases of Corn
by
Pierce Paul, Dennis Mills
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The recent warm, wet weather conditions have had some Ohio corn growers concerned about possible foliar disease problems and the need to make preventative disease management decisions. Corn is currently between late vegetative growth stages and early reproductive growth stages (VT and R1) – growth stages at which foliar diseases normally begin to appear and fungicide application decisions have to be made. While it is true that most of the common foliar diseases of corn (gray leaf spot, northern corn leaf blight and rust) develop well under our recent weather conditions (warm temperatures, high relative humidity and wetness), with the exception of a few lesions of anthracnose on the lower leaves of some hybrids, we have not yet seen any major foliar diseases of corn in our plots nor have we had reports of any such problem in the state. Generally, disease problems show up first in no-tilled fields planted with susceptible varieties, and it is in these fields that yield reduction tends to be greatest and fungicide application most profitable.
Before applying fungicides, it is important to scout fields to determine whether the disease of concern is present. Scout no-tilled fields and fields with susceptible hybrids first and use this information as a guide as to whether you are likely to have a disease problem. Gray leaf spot, northern corn leaf blight, and eye spot fungi all survive in corn residue left on the soil surface from the previous crop, and as such, these diseases are likely to show up first in no-till fields. However, the presence of residue is no guarantee that disease will develop and reach economic levels. Fungal spores buildup in crop residue over time, and if disease levels were low last year (as was the case here in Ohio), the contribution of last year’s crop residue to disease development this year will more than likely be low. However, on-farm crop residue is not the only source of spores. Spores may be blow in from other location. For common rust, spores are generally blown in from southern states. So, it is always important to scout fields before making fungicide application decisions.
GRAY LEAF SPOT ( http://ohioline.osu.edu/ac-fact/0038.html ) – Typical lesions appear tan to brown in color, rectangular in shape, and flanked by the veins of the leaf. When fully expanded, individual lesions may be 3 to 4 inches long and 1/16 to 1/8 inch wide, depending on the distance between veins; however, if several infections occur near each other on the same leaf a broader lesion will result.
NORTHERN CORN LEAF BLIGHT ( http://ohioline.osu.edu/ac-fact/0020.html ) – Lesions are typically 1 to 6 inches long, cigar-shaped, gray-green to tan in color and appear first on the lower leaves. As the disease develops, the lesions spread to all leafy structures, including the husks.
EYE SPOT ( http://ohioline.osu.edu/ac-fact/0021.html ) - Affected leaves are covered with numerous small round spots. Spots are about 1/8 inch in diameter, oval to circular, and initially they appear water soaked. The central area of the spot soon dies, leaving a tan to cream-colored center surrounded by a distinct brown to purple border.
COMMON RUST ( http://ohioline.osu.edu/ac-fact/0031.html ) - Dark, reddish-brown pustules scattered over both the upper and lower surfaces of the corn leaves. Pustules appear oval to elongate in shape and are generally small (less than 1/4 inch long).
Foliar diseases of corn cause yield reduction when severe blighting of the ear leaf and the leaves above the ear leaf occur before and during grain fill. Most foliar diseases move up the plant from the lower to the upper leaves, with the lower leaves contributing spores for infection (penetration) of the upper leaves. If fields are scouted now and disease is detected on the lower leaves, growers will still have enough time to apply fungicides to protect the upper leaves of the plants from becoming infected. Depending on the weather and the susceptibility of the hybrid, it can take between 7 and 14 days for a lesion to produce a new crop of spores. Once these spores infect a new leaf, it may take another 7 to 14 days (or longer depending on the weather) for new lesions to develop. It will then require adequate temperatures (cooler temperatures for common rust and eye spot and moderate to high temperatures for gray leaf spot and northern corn leaf blight) and high moisture for lesions to expand and disease to spread to the upper leaves.
Several fungicides are available for use on corn to control foliar diseases. However, the economics of corn production have not permitted extensive used of these products. To achieve the desired effect, fungicides should be applied at the right time. The main goal of fungicide application is to prevent disease from spreading to the ear leaf and the leaves above the ear, since these leaves contribute a significant portion (about 75%) of the carbohydrates for grain fill. Applications should begin when the first few lesions are observed on the leaves below the ear leaf. Fields should be scouted regularly to determine the appropriate time for fungicide application. Research has shown that two applications may be more effective than one at reducing the level of disease; however, more than one fungicide application is generally not economical. Yield increase is often not enough to offset the cost of two applications.
The following should be taken into consideration when making decisions regarding fungicide application:
1- Susceptibility and yield potential of the hybrid
2- The amount of disease in the field
3- The growth stage of the crop
4- Weather conditions
5- Fungicide and application cost
6- Grain price
7- Directions and restrictions on product label
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C.O.R.N. is a summary of crop observations, related
information, and appropriate recommendations for Ohio
Crop Producers and Industry. C.O.R.N. is produced by
the Ohio State University Extension Agronomy Team, State
Specialists at The Ohio State University and Ohio Agricultural
Research and Development Center. C.O.R.N. Questions
are directed to State Specialists, Extension Associates,
and Agents associated with Ohio State University Extension
and the Ohio Agricultural Research and Development Center
at The Ohio State University.
Information presented above and where
trade names are used, they are supplied with the understanding
that no discrimination is intended and no endorsement
by Ohio State University Extension is implied. Although
every attempt is made to produce information that is complete,
timely, and accurate, the pesticide user bears responsibility
of consulting the pesticide label and adhering to those
directions.
All educational programs conducted by
Ohio State University Extension are available to clientele
on a nondiscriminatory basis without regard to race,
color, creed, religion, sexual orientation, national
origin, gender, age, disability or Vietnam-era veteran
status.
Issued in furtherance of Cooperative
Extension work, Acts of May 8 and June 30, 1914, in
cooperation with the U.S. Department of Agriculture,
Keith L. Smith, Director, Ohio State University Extension.
TDD # 1 (800)
589-8292 (Ohio only) or (614) 292-1868
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