Infections of Corn Seedlings

Paul E. Hoppe.

The germination period is a critical one in the life of a corn plant. Corn seedlings normally are resistant to most parasitic diseases under the conditions that favor germination and early growth. But in cold, wet soils, a germinating corn kernel may be attacked by fungus parasites that cause the seed to decay and weaken the seedlings. The seedling diseases of corn are carried on the seed or in the soil.

Seed infection is less common in hybrid corn than in open-pollinated varieties partly because most of the hybrids involve inbred lines that were selected for resistance to ear and kernel rot diseases, caused by fungi that also attack seeds and seedlings. Also the prompt drying of hybrid seed corn checks the spread of initial infections.

Seed infection ordinarily can be detected only by germination or plating tests. Among the fungi concerned are Diplodia zeae, Gibberella zeae, Fusarium moniliforme, species of Penicillium and Aspergillus, and many others of lesser importance.

Their symptoms are generally similar. In cold soils the seed decay or seedlings die before they emerge. In warmer soils the seedlings usually emerge from the soil but they may be stunted because seminal and primary roots and the subcrown internode rot. The seedling dies when the fungus penetrates the central cylinder (stele) of the subcrown internode (mesocotyl), or invades the crown tissues before the roots arising in that region become established.

Diplodia, the most virulent and important of the seed-infecting fungi, causes severe disease, even in warm soils. In the United States it occurs most frequently in the warmer and more humid areas of the Corn Belt, and in Maryland, Delaware, southeastern Pennsylvania, and the Southern States.

Gibberella infection is prevalent in the cooler corn-growing areas, such as Minnesota, northern Iowa, and Wisconsin, particularly in seasons when wet weather occurs during the silking period. It also occurs commonly in Maryland and Delaware. Unlike Diplodia, the Gibberella fungus is short-lived and rarely can be found alive on old seed. Severe disease develops only in cold soils.

Infections with Fusarium moniliforme occur wherever corn is grown. Except for highly pathogenic strains reported in Florida, the fungus usually is weakly parasitic and is of only minor importance in the principal corn areas.

Infection with various species of Penicillium, Aspergillus, and many other fungi occurs most commonly when seed corn is stored with a high moisture content or under very humid conditions. Aspergillus infects seed corn also in relatively dry weather. Generally the storage rot fungi are only mildly pathogenic to germinating seeds. Wide differences in pathogenicity exist, however, among the various species and strains of Penicillium and Aspergillus. Penicillium oxalicum is unique in that it is most pathogenic in warm, dry soils.

SOIL-INHABITING FUNGI are the most common cause of seed rots and seedling diseases in corn. Their importance has increased because of the many kernel injuries that often occur in harvesting and processing the seed by modern machinery. Wounded kernels are particularly susceptible to attack by the soil fungi.

Many fungi attack a corn kernel that is lying dormant or is germinating slowly in cold soil. Among them are species of Fusarium, Helminthosporium, Sclerotium, Rhizoetonia, Trichoderma, and Pythium. Some are pathogenic only under unusual field conditions: Trichoderma viride attacks seed corn only at continuous temperatures exceeding 80 F., and Sclerotium bataticola is not pathogenic to seedlings in unsteamed or unsterilized soil.

Pythium species are the most common cause of seed decay and seedling disease in corn. Pythium was found in nearly all decayed corn kernels and diseased seedlings, following germination at low temperature, in a variety of Wisconsin soils. Nearly total seed decay occurred in untreated seed planted in Plainfield sand and peat soils.

The lowest amount of disease among the cropped soils was in seed planted in Superior red clay, where the seedlings generally emerged but were stunted. Differences in disease intensity in the various soils were attributed to the presence of different species of Pythium. The frequency that various species were isolated from Wisconsin soils, in descending order of their virulence or pathogenicity, was P. irregulars, 46; P. debaryanum, 5; P. ultimum, 67; P. paroecandrum, 2; P. splendens, 7; P. rostratum; 7; and P. vexans, 3.

Seed decay develops most rapidly in soils at temperatures of 48 to 50 F. That is too cold for corn, and untreated, susceptible seed is killed in 8 days or sooner by the more pathogenic species of Pythium. At a soil temperature of 40 , disease may be equally severe; because of slower growth of the fungus parasites at this low temperature, however, longer periods are required before disease actually affects the seed. Since cold, in itself, is not injurious to seed corn, kernels protected with an efficient fungicide will germinate and grow normally later when the soil warms up, even after having been exposed 3 weeks at 40 in wet soils.

Corn germinates rapidly in warm soils, and sound seed is highly resistant to the soil-borne diseases. Kernels having crown wounds are attacked, however, by Pythium even in the warm soils, and the seedlings may become severely stunted.

Soil moisture has less influence than temperature on disease of seedlings. Disease is severe in cold soils over a wide range of soil moistures with marked reductions in level of disease only in soils that are extremely dry. In experiments in Wisconsin it was found that disease was heaviest in peat soil that had been wetted to 60 percent of its water-holding capacity.

Inbred lines of corn differ widely in their inherent resistance to seed rot diseases. In severe tests of Wisconsin inbreds, seedling stands have ranged from nothing to more than 70 percent when the highest quality of seed was used. Corn inbreds also differ in the extent of physical injuries sustained in rough handling of the seed. Resistance to breakage is important because injured kernels become so susceptible to attack by soil fungi.

The reaction in corn hybrids to seedling diseases is influenced by the inbred lines used in their combination. The female, or seed parent, sometimes exerts the stronger influence in resisting disease, but inherent resistance in either parent is important.

Inferior seed corn inferior because of immaturity, frost injury, old age, improper curing or storing, or physical injury is susceptible to attack by soil fungi.

Kernel injury is the most serious. Injured seeds have little chance of survival when they are planted in cold soils unless treated with an effective fungicide. Broken seed coats over or near the germ are the most damaging; next are chipped crowns and broken tip caps.

Kernel injuries occur during all the steps of mechanical harvesting and Processing. Most damage comes during shelling. In some commercial lots of seed all kernels are injured. Reductions in field stands and ultimate yields usually are nearly proportionate to the amount of injury to kernels.

COLD TESTS first were used by commercial companies to test seed lots to see if they would germinate in cold soil. They essentially are tests against soil-borne diseases. Improved testing methods enabled corn breeders to evaluate their stocks for resistance and pathologists to test fungicides.

The walk-in refrigerator method, older and widely used, requires large refrigeration equipment and a warm room or greenhouse. The corn kernels are planted in soil in flats or pans that are kept usually for 6 to 10 days in a large refrigerator maintained at about 50 F. Following the low-temperature incubation, the flats are moved to a warm place, where germination is completed. Data are recorded when seedlings have reached the three- to four-leaf stage of growth. A complete test requires about 3 weeks.

The paper doll method is easy and rapid and permits extensive testing without large refrigerators or greenhouse facilities. The kernels are placed on wet paper towels on which soil is spread thinly. The towels then are rolled into "rag dolls" and incubated in covered refrigerator pans in an ordinary electric refrigerator for the desired time, at 48 to 50 . Afterward the pans are stored at room temperature. Germination is recorded 2 or 3 days later. The paper doll technique permits the testing of fungicides in 9 days-6 days in refrigerator and 3 at room temperature.

SEEDLING DISEASES of corn are controlled by seed treatment, the use of good seed, and the use of resistant varieties.

Proper seed treatment is the most practical. Mercury-containing compounds have been replaced largely by organic nonmercurials, among them Arasan, Thiram, Phygon, and Orthocide. Properly applied as dusts or slurries, they give good protection under severe conditions. Proper application calls for adequate dosages and good materials.

Quality of seed is important. Seed corn should be mature and harvested before heavy freezes occur. Injury from frosts usually is correlated with the moisture content of the seed the higher the moisture the greater the damage. Improved methods of harvesting and processing may reduce kernel wounds. Reducing the cylinder speed of shellers and the use of rubberized surfaces in processing machinery eliminates much injury to seed.

Care is necessary in curing and storing seed. Temperatures exceeding 110 F. during artificial drying are injurious. Seed should be stored where it is cool and dry.

PAUL E. HOPPE, a native of Wisconsin, has been engaged in corn disease research since 1929, when he became affiliated with the division of cereal crops and diseases, Bureau of Plant Industry, Soils, and Agricultural Engineering. He has specialized on the diseases which occur in the more northern areas of the Corn Belt, particularly on seedling diseases.