Several Ear Rots of Corn

Arnold J. Ullstrup.

Diplodia ear rot, or dry rot, is a prevalent and destructive disease of corn ears. Bleaching of the husks is one of its earliest symptoms. Early infection results in complete rotting of the ear, which becomes grayish brown and shrunken by harvest. The growth of the fungus makes the husks adhere tightly to the ear. Such ears remain upright for a long time. Ears that become infected later show a grayish-white mold on and between the kernels at harvest. When infection occurs late in the season, the ears may show no external evidence of the disease; only when such ears are broken, or kernels removed, is infection apparent. Mold will be seen only on the cob and at the discolored tips of the kernels. Diplodia ear rot begins usually at the butt of the ear and involves all the kernels as it progresses toward the tip.

Diplodia zeae is the fungus that causes diplodia ear rot. Its spores are olive brown, long oval, straight to slightly curved, and two-celled. A second type of spore, often found, is tiny, colorless, and threadlike. Both types are borne in black, globose, flask-shaped structures, called pycnidia, which are formed partly embedded in the stalks, husks, shanks, and cobs. Mature spores are released from the pycnidia in wet weather. The spores are carried by wind and lodge between the husks where, in the presence of moisture, they germinate and penetrate the host tissue.

Corn is the only known host of the fungus. Besides ear rot, the fungus causes a stalk rot of corn. A similar species, Diplodia macrospora, causes an ear rot of corn in warm, humid areas. Its spores are about twice as long as those of D. zeae. No sexual stage of either species is known.

The disease is most prevalent in years when June and July are dry and August and September are wet. Hybrids with loose husks that permit exposure of the ear tip or that hold their ears upright for a relatively long time show a high incidence of ear rot. Upright ears tend to retain water between the husks and make conditions ideal for germination of the spores that lodge there. Early-drying hybrids usually show less disease than do those that dry slowly. Ears are most susceptible to infection within 2 to 3 weeks after silking. The inbred lines 540, Ill. R4, and Ill. 90 transmit resistance; the lines Ind. Tr, I11. Hy, and M14 tend to transmit susceptibility.

GIBBERELLA EAR ROT, or red ear rot, is somewhat more prevalent in the northern and western parts of the Corn Belt. It is seldom of major importance over more than small areas. Infected corn is especially poisonous to hogs.

It is distinguished by a pink to brick-red fungus growth, which discolors the ears, husks, and kernels. Infection usually begins at the tip of the ear and progresses toward the base. The causal fungus, Gibberella zeae, produces two types of spores. The colorless asexual spores tend to be crescent-shaped, with sharp, pointed ends, and have three to five cross-walls. The sexual spores, the ascospores, are colorless and long oval in shape and have one to three cross-walls. Ascospores are borne in saclike structures, the asci, each of which usually contains eight ascospores. The asci are produced in black, globose, flask-shaped structures, the perithecia. Perithecia develop on cornstalks and then mature the following spring and early summer. Mature ascospores ooze from the apex of the perithecia in wet weather. Wind carries the spores to susceptible hosts where, in wet weather, they germinate and penetrate the tissues.

G. zeae causes also a stalk rot of corn, a seedling blight of corn, and scab or head blight of small grains. Barley and wheat sometimes are severely damaged by scab.

Wet weather at silking time favors infection of corn ears. Hybrids with loose, open husks that expose the ear tips frequently show more of the disease than do those offering greater husk protection.

Rotation and clean plowing tend to reduce the spore population of the fungus, but little benefit from such measures can be expected unless they are practiced over a wide area.

FUSARIUM KERNEL ROT, or pink kernel rot, is widely distributed. Damage to individual ears usually is less severe than with diplodia or gibberella ear rot. The disease is frequently prevalent in the drier parts of the Corn Belt and in some of the Western States.

Infected kernels, often distributed at random over the ear, are recognized by pale-pink to lavender discoloration on the kernel caps resulting from growth of the fungus. The fungus often gains entrance to the ear through channels made by earworms and corn borers.

Fusarium kernel rot is caused by Gibberella fujikuroi. The asexual spores are of two types. Microconidia, which are most abundant, are small, colorless, oval shaped, and borne in chains and false heads. Macroconidia are produced sparingly and resemble generally the asexual spores of G. zeae. The ascospores, asci, and the perithecia are like those of G. zeae, except for minor differences. A closely related fungus, G. fujikuroi var. subglutinans, is also frequently associated With pink kernel rot. Microconidia are borne in false heads and never in chains. Both fungi are widely distributed and are weak parasites that frequently follow infections initiated by more vigorous pathogens. The fungi live over in the soil and on plant refuse from one season to another.

Some inbred lines are more susceptible than others to this disease. Inbred lines that tend to show "silk-cut" or "popped kernel" are more susceptible in that such breaks in the seed coat provide easy means for the fungus to establish infection. Inbred lines that transmit this tendency toward weak seed coats to their hybrids should be avoided where fusarium kernel rot is a problem.

NIGROSPORA COB ROT is widely distributed over the Corn Belt. The incidence of the disease is variable, but some may be found nearly every year.

The most typical symptom is the shredding of the cob. Shredding may begin at the tip or butt of the ear, but the latter site is far more common. The disease ordinarily is not conspicuous until harvest. Severely infected ears show a gray discoloration of the internal cob tissues because of growth of the fungus. The pith may be completely disintegrated, so that only the water-conducting strands remain intact. Chaff on normally red-Gobbed hybrids is brown instead of red as in healthy ears. Kernels on infected ears will show masses of black spores near their tips. Ears are light in weight and kernels are bleached and loose on the cob.

Nigrospora oryzae is the causal fungus. The black spores are oval to spherical in shape and are borne on short branches of the fungus strands. The fungus lives on dead plant refuse from one season to the next. No sexual stage of the fungus is known. The organism is not an aggressive parasite.

The disease develops when corn plants have been checked in their growth by frost, drought, stalk rot, or leaf blight. Corn appears to be more susceptible when grown on poor soil than corn that is well nourished.

The use of adapted, full-season hybrids tends to reduce the incidence of the disease. Kernels from infected ears, which often germinate and produce normal seedlings in warm soil, may give rise to weak seedlings when cold, wet weather follows planting.

GRAY EAR ROT has been found throughout most of the eastern part of the United States, but the disease has been serious only in local areas.

Early symptoms of gray ear rot resemble closely those of diplodia ear rot. Husks are bleached and adhere to each other because of the growth of the fungus. The ear often remains in an upright position until harvest. Infected ears have tiny black specks through the pith of the cob. Severely infected ears are slate gray in color and light in weight. Kernels from infected ears have small black specks or stripes beneath the seed coat.

Physalospora zeae is the fungus that causes gray ear rot. The organism produces no spores on the ears. All spore forms are found in large lesions on corn leaves. The black globe-shaped perithecia are embedded in the dead tissue of the leaf lesions. Asci are cylindrical and straight or slightly curved and contain eight ascospores. The colorless ascospores are narrow, elliptical, and one-celled. The asexual spores are colorless, one-celled, and oval. They are borne in black, flask-shaped pycnidia, which are embedded in the dead tissue in the leaf lesions.

The spores are carried by wind to corn leaves and ears where they germinate and start infection. The fruiting structures (pycnidia and perithecia), produced on leaves, overwinter and mature the following summer. Wet weather during and following silking favors development of the disease. Hybrids that hold ears in an upright position for a relatively long time and those having poor husk protection tend to be susceptible.

PHYSALOSPORA EAR ROT occurs in the Gulf States. The symptoms are somewhat similar to those of gray ear rot. The causal organism, Physalospora zeicola, is similar to P. zeae. Asexual spores are two-celled and olive brown. P. zeicola does not attack corn leaves. Perithecia and pycnidia are produced on cornstalks in the field. Warm, wet weather during and following silking favors development.

Rhizoctonia ear rot is known to occur only in Florida. In early stages of infection husks and kernels are covered with a pink mycelial growth of the fungus. Later the color becomes dull gray. Rhizoctonia a zeae is the fungus causing the disease. No spores are known in the life cycle of the organisms. The fungus is carried over from one season to another as dormant mycelium and sclerotic in the seed and on plant debris. The distribution of the disease suggests that it is favored by warm, humid weather.

A NUMBER OF other fungi attack and cause rotting of corn ears. These fungi, which are widely distributed, are not specialized in their parasitism and become prevalent only under conditions exceptionally favorable to their development. They often gain entrance to ears following insect injury.

Species of Penicillium cause a blue-green rot of kernels. A number of species of Aspergillus cause a powder black rot of kernels. A kernel rot caused by species of Hormodendrum is recognized by the deep greenish-black rot on the caps of the kernels. Warm, wet weather after the corn is mature favors development of the fungi on and between the kernels on ears in the field. All these fungi may cause storage rots if the corn is stored when its moisture content is too high.

ARNOLD J. ULLSTRUP, a graduate of the University of Wisconsin, is a plant pathologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering cooperating with the Purdue University Agricultural Experiment Station. He has been engaged in the study of the nature and control of the diseases of corn since 1938.