ROOT KNOT NEMATODES cause distinct knots or galls on the roots of sugar beets. These are generally more numerous on the lighter type soils than on the heavier ones, but may be a serious problem wherever sugar beets are grown. They also are controlled by soil fumigation or by crop rotation, but since no cyst is formed shorter rotation periods can be used. Since several species of root knot nematodes attack sugar beets and these differ in their host relationships, no definite rotation system can be specified. Rotation details must be worked out on a local basis.

AMONG THE MINOR ROOT DISEASES is verticillium wilt, caused by Verticillium albo-atrum. It occurs to a limited extent in Colorado and Nebraska. It is serious in the Yakima Valley of Washington. The foliage becomes yellow and dries. Roots show only slight evidence of small, blackened fibrils here and there in the flesh. The lateral root through which the fungus entered the plant usually can be detected because it becomes black and water-soaked. Probably crop rotations with several years between sugar beet crops would reduce the incidence of the disease.

Crown gall, caused by Agrobacterium tumefaciens, is an overgrowth that usually takes the form of a smooth gall or knot on the shoulder or central part of the beet root. It is caused by the same bacterial organism that attacks fruit trees and other plants. Occasional affected plants have been found in nearly all sugar beet districts. The overgrowth excites attention because frequently the gall is as large as the beet root itself. No control measures are necessary as the disease is limited on sugar beets.

Texas root rot, caused by Phymatotrichum omnivorum, attacks sugar beets in New Mexico and Arizona, on soil in which Texas root rot, well known as a cotton disease, occurs. The tissue of an affected plant is rotted completely. The fungus produces a thin, feltlike coating of hyaline or yellowing mycelium on the surface of the root. It advances on the root surface as a whitish or yellow fan-shaped growth. Sugar beets are not grown for sugar production to any extent in areas where Texas root rot is common. No loss in the seed-producing areas of Arizona or New Mexico has been reported.

STORAGE ROTS are caused by Phoma betae, Botrytis cinerea, Rhizopus nigricans, Rhizoctonia solani, several species of Fusarium, and other fungi.

To prevent cold weather from interfering with harvesting, factories in nearly all the beet-growing districts must accept deliveries of sugar beet roots in excess of their capacity for prompt processing so huge tonnages of roots are piled at the factories and their receiving stations.

The roots stored in the huge piles are alive; like all living things, they respire. Respiration involves the oxidation of the sucrose in the root. Temperature influences the rate of respiration;therefore, if the storage piles are kept cold, the loss of sugar from respiration is at a minimum. Because respiration produces heat, if a storage pile contains layers of dirt and masses of leaves and trash, the dissipation of heat by air currents may not take place and a chain of conditions is set up that leads to the generation of heat in pockets of the pile. First, respiration is accelerated; finally, roots become subject to fungus rots, and the rotting organisms feeding on the rich sugar stores of the roots produce excessive heating in the piles. These manifest themselves as "hot spots." In some years the losses from storage rots are staggering.

Experiments by Myron Stout and Charles A. Fort a few years ago started active prevention of sugar losses from stored sugar beets by cooling the roots by forced ventilation of the piles with chill night air. Engineers at beet sugar factories promptly capitalized on this finding, and today piles of sugar beets at the factories very commonly have ventilating ducts laid through them. Thermostats are located at critical points to control the operation of blowers. Cold air is forced through the piles by blowers as the temperatures within the piles may require.

Sugar losses still occur with roots in storage piles, and after a period in storage such losses may sharply rise. Studies to reduce losses further are continuing. Beets grown with abundant nutrients, particularly phosphorus and nitrogen, and with adequate soil moisture, are resistant to storage rots, particularly to Phoma betae. Drying in the field before piling has bad effects on keeping qualities. High temperatures have been shown to break down the resistance of roots to the attack of organisms that ordinarily are saprophytes unable to attack a live root. Thus, a period of 60 F. is enough to change the character of the sugar beet root so that the bread mold, Rhizopus nigricans, can rot the tissue.

Roots have been exposed to invasion by Phoma betae and Botrytis cinerea, nerea, chief rotters of roots in storage, and the most resistant roots selected. Progenies from such selections keep better than parents. Sugar beets with good keeping quality may be bred, since apparently this is governed by the genetic make-up of the sugar beet.

BEET MOSAIC occurs in Colorado, Nebraska, Utah, Idaho, Oregon, California, and other Western States. This virus disease produces mottling of the young leaves of sugar beet. Sometimes veinlet clearing develops on the youngest leaves of an affected plant. The green peach aphid (Myzus persicae) and other aphids transmit the virus from plant to plant.

The disease is apparently limited to the sugar beet, red garden beet, chard, and mangel-wurzels. Spinach and other plants have been artificially inoculated.

Other than in California, mosaic is chiefly to be found close to infected commercial seed fields or near breeding stations where mosaic-affected roots are carried over winter for transplanting in spring. Ordinarily it is not prevalent beyond the flight zones of aphids usually a matter of a few miles. In California, mosaic is very common. Since sugar beet crops over- lap from year to year and seed crops of both sugar beet and red garden beet are grown, there is no break in the cycle of development. Furthermore, the aphid vectors are abundant and have year-round activity. The absence of mosaic in the more eastern beet-growing sections, such as Michigan, Ohio, and Minnesota, is not understood. It is probable that wherever the virus has been introduced, a cycle Of development has been broken because affected plants have not been overwintered.

Any damage that sugar beet mosaic may do to sugar beets grown for sugar has not been appraised. It is known that plants affected with mosaic are Significantly poorer than healthy plants as seed producers. As yet no control measures are employed against sugar beet mosaic, or its insect vectors.

SAVOY, which is a virus disease that is transmitted by the lacewing bug, Piesma cinerea, curls and distorts the leaves and dwarfs the root of sugar beet. The disease also affects red garden beet, chard, and mangel-wurzel, but is not known on other hosts. Usually the disease appears as a plant here and there in the field, but 10 to 15 percent Savoy has been found in fields bordering wood lots or weedy areas where the insect could overwinter. The disease occurs from Maryland and Virginia westward through Wyoming and Montana, but it has not been found in sugar beets west of the Rocky Mountains although the insect vector occurs throughout the United States.

The most definite symptom of Savoy is the swelling and thickening of the veins. Their growth is reduced and they show up as prominent network on the under side of the leaves. The leaves curl and roll downward. The tissue between the veins bulges out; the whole leaf becomes whiplike and distorted. The roots show darkened vascular rings, the flesh between becoming glassy white. Affected plants make very limited growth, and the roots show much reduced sugar storage.

Ordinarily the disease is minor in its effects, the lessened growth being compensated by greater growth of the neighboring plants. No direct control measures are employed against the disease.

VIRUS YELLOWS, long known as the most serious disease attacking sugar beets in Europe, was found in 1951 in Michigan, Colorado, Utah, Oregon, and California. It is probably widespread in the United States and may have been here for some time but overlooked. This virus is spread by aphid vectors, of which the green peach aphid, Myzus persicae, is most important. Only the older leaves show the disease symptoms. They turn a greenish yellow, particularly at their tips. The veins stay greener than the interveinal tissue. The leaf blades become thick and brittle. Apparently the disease does its damage to the sugar beet plants by interfering with the movement to the root of the food elaborated in the leaves. If a plant is attacked early, its root growth is checked and storage of sugar is strongly reduced. With early attack, sugar production may be cut from 40 to 50 percent. If the attack occurs in August, damage may be much less.

Totally infected fields were found in California in 1951 and 1952. Instead of a normal green color, the fields were canary yellow. The condition has been known for several years in California as "Salinas yellows," but now is identified as virus yellows. Because infection was total, there was no measuring stick of healthy fields for determining effects of the disease. In 1952 the disease was prevalent in Colorado, where it appeared in late August. Early records indicate the disease probably occurred in Colorado as far back as 1940.

Studies in California have shown that the disease may strikingly reduce root weight, and cause strong declines in sucrose percentage. In light of present knowledge, virus yellows may rank with curly top as a serious menace.

Control measures now in use in Europe are not likely to be of particular applicability in the United States. There is some evidence that breeding for resistance to virus yellows may be fruitful, but no resistant varieties have as yet been developed.

GEORGE H. COONS was loaned in 1924 and 1925 to the Bureau of Plant Industry, Soils, and Agricultural Engineering by the Michigan State College, where he was professor of botany, to initiate the research program of breeding sugar beet varieties resistant to curly top and to leaf spot. After his return to the College, he continued as plant pathologist on a half-time basis with the Bureau until 1929, when he left Michigan State College to become principal pathologist in charge of sugar beet research Projects in the division of sugar plant investigations.