Saving Our Sugar Beets

by EUBANKS CARSNER and F. V. OWEN

ONLY 30 years ago curly top of sugar beets seemed an insurmountable barrier to progress of the beet-sugar industry in much of the West. One authority at that time even advised the abandonment of infested and susceptible areas. Had that been necessary, we would not have the sugar and sugar-beet seed now produced in a vast territory in 12 Western States, and their settlement and development would have been retarded.

But now, in retrospect, curly top appears to have been a blessing in disguise. The need to conquer it prompted much research, which led to the development of varieties adapted to American conditions and the establishment of a new industry, growing sugar-beet seed. The two results completed the integration of the American beet-sugar industry and made it independent of Europe.

The violent vicissitudes through which sugar-beet growing and the beet-sugar industry in the Western States passed before curly-top resistant varieties were produced can be brought out best in the record of a representative region. Farmers in the Burley area of southern Idaho started growing sugar beets in 1912. Average yields were low at first but gradually improved with increasing experience, and the acreage was expanded because of the sugar shortage during the First World War. In 1919 came the first outbreak of curly top. From then until 1935 yields dropped recurringly to disastrous levels—in 1924, 1926, 1931, 1934. Areas in Arizona, California, Colorado, Nevada, New Mexico, Oregon, Texas, Utah, and Washington suffered in much the same way.

The deep dips in the curve of average yields in southern Idaho indicate only a part of the losses that curly top caused in those years. Average yields of 5 to 6 tons an acre meant heavy losses to processors and the growers who carried their beets through harvest. Besides, a big acreage was plowed up. The average yield in 1934 was 4.88 tons an acre from 2,754 acres harvested—but in that season, of 21,389 acres planted to beets, 18,635 were abandoned. Contrast with this the record for 1941, a season in which damage from curly top also caused a conspicuous decline. It would have been a disastrous year if varieties of relatively high resistance had not been used. That year 22,005 acres were planted, 21,418 were harvested, and the average yield was 13.45 tons an acre.

The progress that has been made in the control of curly top through breeding resistant varieties can be indicated by results obtained in 1941 on experimental plots. Under the drastic disease conditions of that season a standard European variety once widely used in this country failed completely; U. S. 1, the first resistant variety to be released to growers, gave 6.31 tons to the acre, and the latest commercial variety, the second release of U. S. 22, yielded 16.61 tons. The test plots were purposely planted late to get more severe exposure to curly top. There was a good deal of curly-top damage that year in the commercial fields and, even with the resistant varieties now in use, curly-top losses of economic importance occur under some conditions. However, it can be stated with assurance that crop failures of sugar beets due to curly top need no longer .be feared anywhere. It is not uncommon now to have the resistant varieties yield 25 and 30 tons an acre even when exposed to curly top, under conditions that cut the production of the old susceptible varieties to unprofitable levels.

Mass selection has been used to breed the varieties thus far released for commercial use. But the method involves much more than it did when breeding for curly-top resistance was started 29 years ago. We used to depend on naturally occurring epidemics to differentiate degrees of resistance. Now we regularly induce epidemics artificially in the breeding field regardless of the severity or mildness of the injury in the surrounding territory.

We introduce an abundance of the curly-top virus into the breeding field early in the spring. There are several ways to do it. In the first place, as many viruliferous leafhoppers (carriers of the curly-top virus) as possible are held over winter on hardy food plants, like annual mustards, spinach, and sugar beets, which are planted late in summer next to the field to be used for the sugar-beet plots the following spring. Leafhoppers held over in this way move into the young beets in the spring and inoculate beets here and there.

A second important measure is setting out in the field infected mother beets that have been carefully stored over winter. We usually also introduce virus into the experimental field by releasing viruliferous beet leafhoppers that are reared in insectaries. The newly infected young plants or the transplanted, diseased mother beets serve as reservoirs of curly-top virus. From these reservoirs the spring-brood leafhoppers invading from natural breeding grounds in the desert obtain a supply of virus which they then pass on to the healthy beets they feed upon. The presence of an adequate source of virus in the experimental field at the time of the spring invasion accelerates the rate of development of the curly-top epidemic in the field because many of the invading leafhoppers carry no virus with them.

Planting the breeding plots late is another way we use to bring about severe epidemics. Beets generally are more susceptible to infection and injury when they are small. The leafhopper vector is more active under warm temperatures. And the curly-top virus multiplies more rapidly under relatively high temperatures when the beets are growing rapidly. Plants that withstand such severe epidemics give rise to highly resistant varieties.

But besides resistance to curly top, extensive areas, especially in California, require sugar beets that do not bolt and that resist downy mildew. Some varieties like that have been bred and are in wide use. Better adapted varieties are needed in several districts, however, and efforts are being directed toward producing them.