Diseases that Seeds Can Spread

K. W. KREITLOW, C. L. LEFEBVRE, J. T. PRESLEY, AND W. J. ZAUMEYER.

SEEDS can spread plant diseases from a neighbor's farm to yours, from one State to another, and from a distant country to the United States.

Some disease pests may survive for years, safely lodged on or in a seed or on bits of stem or leaf mixed with the seeds.

Many seedborne diseases we cannot recognize when we examine the seeds, and we cannot detect them when we incubate them. Only by inspecting the growing crop can we be sure that the seeds are free of viruses, bacteria, and fungi, organisms that cause disease and are called pathogens.

Most seedborne parasites do not affect germination immediately. They do not kill the seeds but multiply on emerging seedlings, which may then succumb to the disease. Some seed lots that show high germination in tests are nearly destroyed when they are planted under conditions that favor development of the organisms they carry.

The control of seedborne diseases begins with the seed. It is easier and cheaper to eliminate a pathogen from a few pounds of seeds than to attempt to spray or dust entire fields of growing plants.

Some pathogens can be eliminated or their range of occurrence can be reduced by treating the seed with suitable chemical compounds, hot water, or fumigants. Seed-cleaning equipment can remove many lighter, disease-infected seeds and fragments of diseased plant parts carried with them.

Some seedborne diseases are not so prevalent in regions of low rainfall and relatively high temperature during the growing season. Seeds produced under such conditions usually are free of many of these destructive, disease-causing bacteria and fungi. The commercial production of seed of certain vegetable, ornamental, and forage crops therefore has been shifted from humid areas of the East and Midwest to irrigated, semiarid western areas.

Careful inspection and weeding out of diseased plants in fields destined for seed production greatly reduce the incidence of seedborne diseases.

Although hundreds of pathogens are known to be seedborne, effective control measures have eliminated some and reduced the incidence of others to the point where they are troublesome only occasionally.

We discuss here the seedborne diseases that occur oftenest or have special significance.

SOME OF THE worst diseases of vegetable crops are seedborne. In most instances, no varieties resistant to seed-borne diseases have yet been developed. For some, no chemical seed treatment gives satisfactory control. The vegetable grower therefore should make every effort to plant disease-free seeds.

Most of the seeds of beans and peas used by processors, market gardeners, and home gardeners before 1925 were produced in New England, New York, and Michigan. Frequent rainfall and high humidity there favor development of the three most important seedborne diseases of beans anthracnose, common bacterial blight, and halo bacterial blight, which are incited by Colletotrichum lindemuthianum, Xanthomonas phaseoli, and Pseudomonas phaseolicola, respectively. The environment is ideal also for the two major seedborne diseases of pea, ascochyta blight, caused by Ascochyta pisi, and bacterial blight, caused by Pseudomonas pisi.

Before western-grown seeds came to be used, losses from the three bean diseases in some years amounted to 30 or 40 percent of the crop, and severe outbreaks of ascochyta blight and bacterial blight of peas often were reported in the East and the Midwest. These diseases reduced yields and impaired the quality of the canned product.

Although bean seeds grown in the Western States are free of anthracnose, they are not free of the bacterial blight organisms in some years.

The bacterial blight diseases frequently are widespread in Nebraska, Colorado, Wyoming, and Montana and occur infrequently in California and southern Idaho.

Rain, hail, and high humidity following storms, which are responsible for the spread and development of the bacterial blight organisms, occur much less frequently during the growing season in parts of Idaho and California than in the other States. Consequently the production of seed of snap beans now centers in those sections. Seed of most of the bush types is grown in Idaho and that of the pole types in California.

Weather conditions in the Columbia Basin of Washington also are unfavorable for the development and spread of the three bean diseases, but curly top, a virus disease that kills beans of susceptible varieties, is widespread there. Since no garden varieties have yet been developed that resist curly top, no seed of garden beans is produced in the Columbia Basin. Several varieties of dry beans resistant to curly top are grown there.

The most important seedborne virus diseases of beans are common bean mosaic virus and a strain of it referred to as the New York 15 virus. Most of the varieties of snap and dry beans now grown are resistant to these two viruses, and the losses they once caused have been reduced. Certification of seed fields of dry beans also has been effective in reducing seed transmission of viruses in the few susceptible varieties still grown in 1961.

Most of our pea seed originates in southern Idaho, the Palouse section of northern Idaho, and the Columbia Basin. Ascochyta blight and bacterial blight rarely occur there because of low rainfall.

As practically all the bean and pea seeds of the market and processing varieties used in the United States now are grown in a few of the Western States, anthracnose and the bacterial blights of snap beans and ascochyta and bacterial blight of peas have been reduced to minor importance throughout the country. These diseases caused severe and widespread damage to both crops in the 1930's and cost the American farmer millions of dollars each year in crop losses.

Lima beans grown in the South and East sometimes are affected by two seedborne diseases bacterial spot, caused by Pseudomonas syringae, and stem anthracnose, caused by Colletotrichum truncatum. These diseases differ from bacterial blight and anthracnose of snap and dry beans.

Most of our seeds of lima beans are grown in California, where the environment does not favor the development of these diseases and they are not known to occur. Even though disease-free seed is used, stem anthracnose sometimes causes severe losses in the South and in some of the Eastern States. The causal fungus may over-winter on lima bean refuse and infect a crop the following year if the environment is ideal for the development of the disease and strict crop rotation is not practiced.

Seed of cabbage, cauliflower, rutabaga, and turnip, like seed of beans and peas, once were produced commercially in the East and Midwest. Because of the destructiveness of two seedborne diseases black rot, caused by the bacterium Xanthomonas campestris, and blackleg, caused by the fungus Phoma lingam, both of which spread and develop only in humid, rainy weather production of seeds was shifted to the Pacific Coast States. Because of low rainfall during the time these vegetables are growing in the seedbed and as transplants in the field, the organisms causing the two diseases do not become established.

Bacterial canker of tomato, a seed-borne disease caused by Corynebacterium michiganense, is found in field-grown tomatoes from New Jersey to California and in several Southern States. The disease can be controlled by fermenting the seed and pulp for 72 hours before extraction. Soaking freshly extracted seed in acetic acid is also effective. Inspection and certification of seed fields has greatly reduced the importance of the disease.

Another seedborne bacterial disease, angular leafspot of cucumber, caused by Pseudomonas lachrymans, occurs mainly in humid regions. Seed grown in arid parts of the interior of California usually is free of the organism.

Late blight, Septoria apii-graveolentis, a destructive foliage blight of celery, affects the seeds and is widely distributed by this means. Since the fungus in infected seed usually dies before the seed loses viability, 3-year-old seed is recommended as the most effective control measure.

Lettuce mosaic virus causes considerable damage in the coastal valleys of California and losses in all parts of the United States. Fewer than 1 percent of the seeds are infected with the virus; the disease therefore causes little damage unless the virus is transmitted by aphids from infected seedlings or weeds to lettuce plants.

No resistant varieties are available but losses from the disease are being reduced by roguing diseased plants from seed fields and producing seed in fields free from wild lettuce species and other weeds infected with the virus. Isolation of seed fields from other lettuce fields is also recommended.

Many of the most destructive diseases of oilseed crops are seedborne. They occur primarily in the more humid regions of the United States with the possible exception of bacterial blight of cotton, which is most severe in semiarid areas. Because of climatic requirements and for economic reasons, much of the total oilseeds are produced in humid sections where rainfall is moderate.

The production of soybeans in the South Central and Southeastern States has increased. This shift of production from the North Central States has accentuated the seedborne disease problem in this crop. Such diseases as purple seed stain, target spot, wildfire, and bacterial pustule, caused by Cercospora kikuchii, Corynespora cassiicola, Pseudomonas tabaci, and Xanthomonas phaseoli, respectively, are more prevalent and destructive in the South than elsewhere.

Other seedborne diseases of soybean, such as brown spot, frogeye, downy mildew, and bud blight, caused by Septoria glycines, Cercospora sojina, Peronospora manshurica, and the tobacco ringspot virus, respectively, are hazards. Varieties of soybeans resistant to several of the important seedborne diseases are available and are usually recommended for areas where these diseases occur.

The gradual shifting of the center of cotton production toward the Southwest has increased the problem of bacterial blight, caused by Xanthomonas malvacearum.

Although the seedborne diseases anthracnose and wet weather blight, caused by Colletotrichum gossypii and Ascochyta gossypii, are largely controlled by the use of chemical seed protect-ants, diseased crop residues frequently are the source of new epiphytotics.