Diseases of Carrots

Glenn S. Pound.

Carrots are grown the world over as a garden vegetable. They also are processed in great volume. In the United States several areas produce large acreages of carrots for fresh-market consumption, the bulk of this acreage being grown in Texas, Arizona, and California as winter crops. Acreages for canning are greatest in the Great Lakes States, notably New York and Wisconsin. Production of carrot seed is primarily in the Pacific Coast and Rocky Mountain States.

BACTERIAL BLIGHT of carrot has become an important disease of market carrots in the Southwest and carrot seed crops in the Pacific coast and Rocky Mountain areas.

First symptoms are yellow spots on the tips of leaf segments. They rapidly turn brown and get a water-soaked appearance. A yellowish halo often subtends the black center of the lesion. Entire leaf segments or leaflets may be killed and lower leaves die and dry up as the disease advances. In severe infections, long, dark-brown, water-soaked lesions develop on the petioles and main stem. A gummy bacterial substance frequently collects on them.

On seed plants the symptoms on the floral organs are most conspicuous. If infection occurs before the umbel emerges from the sheath or before it opens up, the entire umbel is usually killed outright. If the umbel opens before infection occurs, several of the umbellets may escape. A gummy exudate is usually present and frequently may cover the entire umbel.

The affected roots of market carrots may show small, water-soaked, greasy flecks or scablike lesions at any point on the surface. They first appear as brown or maroon spots, which may become raised pustules or sunken craters. A grayish ooze may cover the surface of the lesions. The larger craters usually crack open and are filled with soil particles embedded in the bacterial ooze. Often internal pockets are formed when surface lesions heal over to enclose the scab lesion. Another symptom is a marked constriction of the root, which may let it break in two at harvest.

The bacteria (Xanthomonas carotae) persist in the soil and are commonly carried with seed. Seed may be disinfected by soaking in hot water at 126 F. for 10 minutes. Control also requires adequate crop rotation.

BACTERIAL SOFT ROT is one of the most destructive diseases of carrot and other vegetables in storage or transit. It also may cause considerable rot before harvest, especially if organs are injured by insects or other diseases. It is caused by the bacteria Erwinia carotovora and E. atroseptica. It was one of the earliest bacterial diseases of plants to be studied. The bacteria attack storage organs of almost all vegetable crops, especially onion, cabbage, potato, carrot, parsnip, and celery. The latter bacterial species causes the important blackleg disease of potato. Leafy vegetables, such as spinach, are often attacked in the field and in storage.

The disease is characterized by a watery, smelly, soft decay of storage tissue.

The bacteria are common in most soils, particularly soils closely cropped with plants susceptible to attack. They normally invade plants through wounds. Injuries from harvesting, freezing, and insects make plants susceptible to attack. The bacteria must have relatively high moisture and high temperature for rapid development. Cool, dry storage or transit conditions therefore are helpful in control. A storage temperature just above the freezing point and a relative humidity below 90 percent check the development of the disease. Care in harvesting to avoid bruises is important. If vegetables are washed before storage or shipment, drying in warm forced air immediately after washing reduces the chances of soft rot.

Several species of maggot flies (especially Hylemya cilicrura and H. brassicae) carry the bacteria. Adult flies, which have the bacteria in their intestinal tract, lay eggs about the plant. As the larvae emerge they become contaminated with the bacteria, which cover the egg and are necessary for the normal development of the larvae. As the contaminated larvae bore into potato seed pieces or storage organs of other vegetables such as carrot roots and cabbage heads, they introduce the bacteria into the host tissue.

The pathogenicity of the soft rot bacteria depends on the production of an enzyme, protopectinase, which moves through the tissues ahead of the bacteria, loosening and destroying cells as it does so. Byproducts of bacterial growth cause the cell contents to flow into the intercellular spaces, where they are a nutrient medium for the bacteria. Thus the rot is watery.

THE ALTERNARIA AND CERCOSPORA BLIGHTS, leaf spot diseases caused by Alternaria dauci and Cercospora carotae, are world-wide in distribution and commonly occur together. In many areas, however, cercospora blight appears earlier in the season than alternaria. Cercospora is more severe on young leaves than on old leaves and thus builds up when the plants are relatively young. Alternaria blight is much more pathogenic on old leaves than on young leaves and does not become prevalent until the plants approach maturity. Temperature requirements are about the same for both fungi.

Symptoms of the two diseases are quite similar. Alternaria blight appears first as irregular brown spots with yellowish centers near the margins of the leaves and yellowish areas surrounding the spots. The leaflet tip or entire leaflet may be invaded and turn brown. Under severe infections entire fields may be bronzed as if scorched by heat.

Cercospora lesions are also usually marginal although any part of the leaf or petiole may be attacked. Spots are quite circular and usually have a whitish or tan center. Under very moist conditions, lesions of cercospora may be quite extensive and dark-colored, closely resembling alternaria. Both organisms attack petioles on which lesions are usually elongate. Neither organism attacks the fleshy root.

Spores of the two fungi are borne on the surface of the lesions. Those of alternaria are dark-colored, club-shaped, and have cross walls in both directions. Those of cercospora are colorless, long, and have cross walls only in a transverse direction. They are wind-borne and germinate over a wide range of temperature. Both fungi persist in the soil, in refuse of infected plants, and are commonly seed-borne. Seed can be freed of the organisms by soaking for 5 minutes in a 1 1,000 solution of mercuric chloride or dusting with New Improved Ceresan. Crop rotation and sanitation will reduce carry-over in the soil. Both diseases are amenable to control with fungicides. Bordeaux, fixed coppers, and the carbamates are all effective.

YELLOWS, a virus disease, is probably the worst disease of carrots. It is caused by the aster yellows virus.

Its first symptom is a yellowing of Young leaves as they emerge from the crown. Almost at the same time come Yellow side shoots, which may later give an appearance of witches'-broom to the plant. As the disease develops, the entire cluster of shoots may become a sickly yellow color and the older outside leaves may become bronzed or reddened and twisted. By late season the crown often becomes dead and blackened. Such roots are usually attacked by soft rot bacteria. The roots are smaller in size and a profusion of fine roots on the surface of the main root is not uncommon. Such unsightly roots are unsuited for bunching and the shortened, rosetted tops are often missed by mechanical harvesters. Affected roots are off-flavor and give a bitter, stringent flavor to canned products.

The virus attacks a wide range of cultivated and wild hosts. Two strains have been described, western and eastern, as being pathogenic and nonpathogenic to celery, respectively.

H. H. P. Severin, of the University of California, was first to show that the aster yellows virus was the cause of yellows in carrot. L. O. Kunkel, of the Rockefeller Institute, had earlier shown that the six-spotted leafhopper (Macrosteles divisus) was the vector of the disease on aster. Severin learned that the insect was also the chief vector of the western strain of the virus but that it also could be transmitted by at least 16 species of leafhoppers. No vector other than the six-spotted leafhopper has been reported in the East and Midwest.

The relationship of the virus to the vector has received much attention. Kunkel observed that insects were unable to transmit the virus until a lapse of a few days after feeding upon diseased plants. L. M. Black, of the University of Illinois, has shown that the virus multiplies many fold within the insect vector and an incubation period in the vector is apparently a necessary prelude to transmission. Infected hoppers may carry the active virus for 100 days or longer. Kunkel showed that if infected hoppers were subjected to exposure to an air temperature of about 90 F. for 12 days, the virus was permanently inactivated in the hoppers. With less exposure at 90 it was so reduced in some hoppers that several days were necessary before they could effect transmission. In still others, the virus was apparently changed so that when transmitted to the plant host a much milder disease occurred.

The virus may be carried from season to season in adult leafhoppers, but in most areas, especially the Northern States, overwintering occurs in perennial hosts. The six-spotted leafhopper overwinters in Northern States primarily as eggs laid in winter grains and grasses. The first-generation insects in the spring obtain the virus by feeding on diseased perennial plants. In 1952 Darrell Drake and R. K. Chapman, of the Wisconsin Agricultural Experiment Station, showed that in Wisconsin primary inoculum introduced in the spring by migrant leafhoppers is of more importance than the overwintering inoculum.

No variety of carrot has been shown to have any appreciable amount of resistance. The disease in carrots can be controlled by reducing the leafhopper vector with DDT sprays or dusts. Applications are made at intervals of 7 to 10 days beginning when hopper populations are prevalent and continuing until about 1 month before harvest.

GLENN S. POUND is associate professor of plant pathology in the University of Wisconsin and collaborator with the Bureau of Plant Industry, Soils, and Agricultural Engineering of the Department of Agriculture.