Corn Disease Update

Bob Mulrooney, Extension Plant Pathologist;

Northern Corn Leaf Blight and Gray Leaf Spot
Northern corn leaf blight and gray leaf spot have been pretty common this season on many hybrids. The cooler season and increased rainfall in many areas has resulted in more of both diseases this year. Fungicide applications to corn should pay dividends this season.

rectangular lesions of gray leafspotNote the rectangular lesions of gray leaf spot.

Northern corn leafblightNorthern corn leaf blight lesions are wide with bluntly rounded ends. Note the gray leaf spot lesion above it for comparison.

Stalk Rot and Ear Rot in Corn
Be on the lookout for stalk rots and ear rots as harvest approaches. Diplodia ear rot has been diagnosed in two sweet corn fields last week and Fusarium and Gibberella ear rot will probably be seen as well. The following article from Kentucky is on the Kent County Ag Blog and is a good treatment of the rots were are likely to see this fall.

The common late-season stalk rots are caused by fungi and include: Gibberella stalk rot (Gibberella zeae = Fusarium graminearum), anthracnose (Colletotrichum graminicola), Fusarium stalk rot (Fusarium moniliforme), charcoal rot (Macrophomina phaseolina), and Diplodia stalk rot (Diplodia maydis). It is common for more than one stalk rot organism to attack a plant at the same time.

Stalk rots caused by Gibberella, Fusarium and Diplodia fungi are not usually apparent until several weeks after pollination. Diseased plants may die suddenly in various areas within the field, with leaves first turning a dull, grayish-green similar to the color caused by frost or drought damage. Death of the entire plant follows within 7 to 10 days in susceptible hybrids. The lower internodes turn from green to tan, straw-colored, or dark brown and are spongy and easily crushed. When the stalks are split lengthwise, only the vascular strands are intact and the pith tissue is decayed.

Stalks infected with the Gibberella fungus have a characteristic pink to reddish discoloration of the pith and vascular strands. The breakdown of pith tissues starts at the nodes soon after pollination and becomes more severe as the plant matures. Rotting also commonly affects the roots and crown as well as the lower internodes. An additional identifying feature is the presence of small, round, bluish-black perithecia (fungal-fruiting bodies) which form on the surface of Gibberella-infected stalks in the fall or the following spring. These fruiting bodies are easily scraped off with a thumbnail. Fusarium stalk rot looks similar to Gibberella, except that the discoloration of infected tissues commonly varies from whitish-pink to salmon.

Diplodia stalk rot can be distinguished from other stalk rot diseases by the numerous, small, black dots (pycnidia) which the fungus produces at or near the lower nodes of infected stalks. Unlike the perithecia formed by the Gibberella fungus (which may also be clustered near the lower nodes), the pycnidia of Diplodia are embedded in the rind and cannot be scraped off with a fingernail. However, individual stalks may have fruiting bodies of both fungi if a double infection has occurred.

Corn anthracnose has become much more prevalent in Kentucky since the early 1970s. In addition to rotting the lower stalk, the anthracnose fungus is capable of attacking the stalk above the ears, causing dieback and breakage of the plant tops (borer injury in the top of the plant may cause similar symptoms). The fungus also commonly causes a leaf blight. Although the lower stalk rot phase of anthracnose may cause very susceptible hybrids to be killed before pollination, most hybrids are killed only a week or two before normal maturity. A shiny black or dark brown discoloration of the rind late in the season is a typical symptom of anthracnose on the stalk. This black discoloration usually extends up the stalk for several internodes and may uniformly discolor the rind or give it a blotchy or speckled appearance. The pith tissue beneath these lesions becomes brown or black, especially around the nodes. When lodging occurs, it is usually higher on the plant than with other stalk rot diseases.

Here are two pictures of Diplodia ear rot on sweet corn which would also be applicable to field corn:

Diolodia enters the ear at the leaf sheathDiplodia enters the ear at the leaf sheath and progresses up the ear shank causing the rot. It usually is not found on the tips of the ears initially.

Diplodia ear rot -- look for white fungal growth and small black pycnidia 

Look for the white fungal growth and the small black reproductive structures of the fungus on the husks.