Bacteria and Fungi on Seedlings

David C. Neal.

Cotton seedlings are subject to attack by several fungus and bacterial diseases, especially when soil temperatures are low and wet weather prevails in the spring following planting.

Some of the diseases are caused by strong pathogens, which deplete the stand to such an extent that replanting becomes necessary. The additional expense for more seed and labor for replanting and a reduction in yield (which ranges as high as 15 percent) because of late planting are considerable.

Many species of fungi, bacteria, and nematodes have been encountered by pathologists in studies of seedling diseases of cotton, but little is known regarding the pathogenicity of some of them. I limit this report mainly to the pathogens whose distribution over the Cotton Belt and effects on seedlings are well known.

The name sore shin apparently was first used to describe the symptoms produced by damping-off of cotton seedlings in Alabama by George F. Atkinson in 1892. The disease is caused by the soil-inhabiting fungus, Rhizoctonia solani. It occurs throughout the Cotton Belt and is considered to be the most serious seedling disease of cotton in Oklahoma, Texas, and the Western States. It is also frequently prevalent and destructive in the Mississippi terrace and delta soils of Arkansas, Louisiana, Mississippi, and western Tennessee and in other heavier soils of the middle South. It attacks cotton seedlings only under conditions favorable to the fungus and unfavorable to the plant, such as cold, moist weather. The disease is also more severe when seedlings are damaged by thrips.

When seedlings are attacked by the sore shin fungus, usually within 5 or 10 days after planting, dark- to reddish-brown cankers develop on the stems near the soil line. In severe attacks, the cankers encircle the stems or penetrate so deeply that the plants fall over and die. The attack usually occurs in early-planted cotton before the formation of true leaves, but the fungus may persist and continue to restrict root development and top growth until the plants begin to flower. Infection also occurs on the leaves late in the season in some States. Some defoliation results, but that is of little economic importance.

Tests have been made of various chemical and fungicidal materials for possible control of sore shin of cotton, but we had no satisfactory control measure in 1953. Certain practices will reduce losses.

First is the thorough preparation of the seedbed so that rows are well pulverized at the surface, firm beneath, and slightly raised-4 to 6 inches to give good drainage.

Second is the planting of seed that is certified as to viability and has been treated according to accepted practice for control of seed-borne diseases.

Third is to avoid planting too early. Early plantings usually are subjected to cool nights and moist weather which favor attack by the disease. With the newer and more effective insecticides now available for insect control, the planting date for cotton could be delayed with safety in many areas for 10 or 14 days or until the soil warms up.

Fourth is speeding up germination by using reginned or acid-delinted seed and liberal fertilization so as to give the seedlings a vigorous start.

Other soil-borne organisms that attack cotton seedlings are Fusarium oxysporum, F. vasinfectum, the wilt fungus; Fusarium moniliforme and other species of Fusarium, which cause rots and blights; Thielaviopsis basicola, the cause of root rot; species of Pythium that cause damping-off; Sclerotium bataticola and S. rotfsii, which cause rots; and the parasitic nematodes Meloidogyne incognita var. acrita (which causes root knot), Pratylenchus leiocephalus (the meadow nematode), species of Trichodorus (which cause stubby root), and other nematodes, such as Helicotylenchus nannus and Rotylenchulus reniformis, which affect cotton and are known to be associated with the incidence of fusarium wilt.

Most of those organisms are pathogenic on seedlings and cause damage in some areas, but, with the exception of the root knot nematode, which frequently reduces seedling stands of Egyptian cotton in Arizona and New. Mexico, they are of minor importance in the complex of seedling diseases.

THE ANTHRACNOSE FUNGUS, Glomerella gossypii, is the main pathogen responsible for seedling blight and damping-off in all of the cotton-growing States east of the 40-inch rain belt of Oklahoma and Texas.

In a survey conducted from 1938 to 1941 by pathologists of the Department of Agriculture in 14 States, the anthracnose fungus was found to be the predominant organism affecting cotton seedlings throughout the Southwestern and Mississippi Valley States. Its distribution on seedlings also coincided with the area affected with anthracnose boll rot. It occurred in 81.2 percent of seedling samples, but in Texas and Oklahoma the occurrence of the fungus was limited to the eastern parts of the States. The absence of the disease in the western belt is apparently due to dry conditions and high temperatures that prevent the survival of the fungus during the interval between the seedling blight stage and the boll rot stage.

Seed become contaminated at the gin with anthracnose spores, which are present in such trash as infected leaves, bracts, stems, and bolls. All such material may carry heavy loads of spores, which are mixed with the seed in ginning and adhere to the fuzz or linters.

In studying the relation of germination to contamination of cotton seed by the anthracnose fungus, Richard Weindling and P. R. Miller, of the Department of Agriculture, find no relation between the size of spore loads of Glomerella gossypii on a given sample of seed and the percentage of germination of the seed. They emphasize, however, that the size of the spore load seems to influence the amount of post-emergence damping-off when the seeds are planted.

When anthracnose-infested seed are planted, the seed-borne spores become active and may attack the germinating seedling. Infected seedlings may be killed before emergence or after emergence or they may survive and overcome the disease. Affected seedlings have reddish or dark-brown lesions on the stems below the soil line and frequently on the roots. The cotyledons the seed leaves also may be attacked, the disease producing brownish spots that enlarge before the plant wilts and succumbs. If cool, moist weather prevails for several days after planting, infested seedlings continue to blight and damp-off to the extent that stands become skimpy and replanting is necessary.