Thursday, May 12, 2016

Corn Replanting - Disease Risks

Replanting and delayed planting of corn puts the crop at higher risks for certain diseases. The following is updated excerpt from AGR-195 Replanting Options for Corn.

Diseases and Replanted Corn
Delayed planting or late replanting, could result in increased outbreaks of several diseases.
The "virus complex": Infections of Maize dwarf mosaic virus and Maize chlorotic dwarf virus, viruses which survive in johnsongrass rhizomes and are spread (vectored) by aphids and leafhoppers, respectively, cause the virus complex. Compared to corn planted on time, late-planted corn is at an earlier stage of crop development during periods of peak vector activity, and earlier growth stage infection usually results in more severe disease symptoms.

Gray leaf spot.
Fungal diseases of foliage: Several leaf diseases, especially gray leaf spot, may be more severe when corn is planted late. This risk is especially high for fields in continuous no-till corn. Late-planted crops will be at a comparatively earlier stage of development during periods of high spore activity if weather is conducive for these diseases. Leaf infection early in plant development will reduce yields by decreasing photosynthetic capability and will increase susceptibility to stalk rots.

Fungal ear rots: Growers who plant full-season hybrids for grain production (120-day relative maturity, or greater) after June 1 increase the risk of fungal ear rots, because ears may not have sufficient time to dry adequately before harvest. (Note: Planting 120-day hybrids after June 1 is not recommended in Kentucky.) Planting shorter-season hybrids after June 1 helps assure maturity before frost and lower grain moisture contents at harvest. In addition, fall armyworm can be more severe on late-planted corn. Ear feeding by this insect can increase the incidence of ear rot diseases by providing wounds allowing fungal invasion.

Fungal stalk rots: Increased stalk rot diseases could result from delayed planting. The shorter daylength and drier weather late in the season both may favor stalk rots, as these can result in decreased carbohydrate levels in the stalk and more plant stress, which can favor infection.

Growers who still want to plant corn late should use hybrids with resistance to these diseases. Disease resistance is not as common in mid-season and short-season hybrids as in full-season hybrids. Therefore, growers are advised to be sure mid- and short-season hybrids have resistance to the above diseases, particularly since these could become more severe than normal.

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