i \i *ji?*f\ of The College of UNIVERSITY OF CALIFORNIA VIRUS DISEASES of SUGAR BEETS IN CALIFORNIA EDWARD S. SYLVESTER H. H. P. SEVERIN CALIFORNIA AGRICULTURAL -it Station i Service CIRCULAR 422 Six virus diseases (at least) affect sugar beets in California. The most important is curly top, which affects many other crops as well. The virus is carried by the beet leaf- hopper, Circulijer tenellus (Baker). Sugar-beet mosaic, western cucumber mo- saic, and sugar-beet yellow-net are three other viruses of some importance in the field. All three are carried by aphids, chiefly the green-peach aphid, Myzus persicae (Sulzer), and perhaps occasionally and locally by the bean or dock aphid, Aphis jabae Scop. The fifth disease, yellow vein, is found only occa- sionally; so far it is more or less a curiosity. Recently a sixth and possibly seri- ous disease, beet yellows, has been reported from California and other sections of the United States. Although little is known concerning beet yellows in Cali- fornia, some facts concerning this possible threat will be touched upon. The amount of any particular disease varies from year to year. This circular will tell you the symptoms of each, its vectors, what plants are used in transmis- sion, and how the virus is carried. A final section will treat briefly the control of these six virus diseases. The Authors: Edward S. Sylvester is Assistant Professor of Entomology and Assistant Entomolo- gist in the Experiment Station. H. H. P. Severin is Entomologist Emeritus in the Experiment Station. [21 VIRUS DISEASES OF SUGAR BEETS IN CALIFORNIA CURLY TOP Outbreaks of this disease in sugar-beet crops date back some 50 years. The dis- ease distribution and severity of infec- tion are correlated with the distribution and abundance of its vector, the beet leaf- hopper. Local and general losses can be extremely severe during periods of out- breaks. Symptoms. Curled leaves, with in- wardly rolled edges, showing swollen veins, are the most reliable symptoms in the field. Coupled with these is the pres- ence of vein-clearing. By this is meant that the small veins and veinlets of the leaves have the property of transmitting light, and therefore are clearly visible when held up to the light. After infection the development of symptoms follows a somewhat general pattern. First there is vein clearing, then leaf-edge rolling and curling, coupled with vein swelling. Usually, as the disease progresses, the swollen veins will give rise to little nipple-like swellings along the veins and veinlets. At times a sticky brownish fluid may be exuded, which will collect in droplets along the leaf stalks. If the roots of affected plants are sliced, brownish rings, indicating degenerative changes in the vascular tissues, are evi- dent. When young plants are infected in the pre-thinning, or immediate post-thinning stage, they will make little further growth, but will remain as tight balls of stunted leaves. However, if infection is late in the growing season, especially in resistant varieties of beets, little in the way of symptoms can be seen. A mild vein clearing may be noted, with some slight indication of vein swelling. Symptoms may develop any time from 3 days to 1 or 2 weeks following inocula- tion. The symptoms in seed beets are similar to those in crop beets, and since consider- able stunting occurs a great loss of yield may take place, both in quantity and quality of seed produced. In either case Sugar beet (Beta vulgaris) naturally infected with curly top, showing rolling and curling of the leaves. [3 Left: Curly top symptoms on sugar beet leaf, showing swelling of the veins and development of wartlike elevations. Right: vein clearing on sugar beet due to virus infection. This type of symptom can be produced by curly top, beet mosaic, or yellow-net. Taken alone this symptom is not often reliable for determining a specific disease. Vein clearing may also be found with severe strains of virus yellows. the damage suffered by a particular plant is largely dependent upon its age. If the plant is young, damage may be severe; if it is mature, little effect will be noted. Vector. The virus is carried by the beet leafhopper. This is a small active in- sect about % inch long. The color varies from pale yellow (spring) to dark tan or brown (fall and winter). In the fall, usually in early October, the insects fly to the overwintering sites in the foothills and canyons of the San Joaquin Valley. This coast-range foothill breeding area extends from Mt. Diablo, in the Bay Area, to the Tehachapi Mountains. The foothills of the Tehachapi and Sierra Nevada Mountains as far north as Round Valley near Lindsay are also natural breeding areas, as are the plains of the middle San Joaquin Valley and most of Kern County. After flying to the overwintering areas, the insects feed on various range weeds until the late winter and early spring. Beginning in middle or late December and extending until the end of April, the spring generation develops from the eggs laid by the overwintering adults. Usually sometime during early April through May or June these nymphs mature and begin to fly to the cultivated fields. If the numbers are high, because of favorable weather conditions, and a large per cent of the insects have virus, then much dam- age can be done by the virus-laden insects as they disperse into the cultivated valley floor, in search of favorable plants upon which to feed and in which to lay eggs. ACTUAL COLOR PHOTOS of symptoms of four of the diseases discussed here will be found on pages 6 and 7. [4] * This spring generation, now widely spread over the cultivated areas, lays eggs in suitable hosts, and the cycle of summer generations begins, extending from late May until the middle of Octo- ber, when the leafhoppers return to the wintering places. The number of generations is some- what variable, but there are at least three — overwintering, spring, and sum- mer. Under very favorable conditions there can be a partial second generation in the spring and also in the summer, as well as many overlapping broods, due to the long egg-laying period of the females. The eggs, deposited singly in slits in the stems and leaf stalks of host plants, are oval to sickle-shaped, colorless when first deposited, later on turning pale greenish or yellow; the length is about ]->2 of an inch. The eggs hatch in 11 to 50 or more days, depending upon the temperature. The young nymphs, which look like tiny wingless adults, shed their skins 5 or 6 times before they become mature, the period between molts vary- ing from 2 to 14 days. Males live up to 4 months, while the females may live as long as 9 months. The longer-lived fe- males usually live through the winter. Plants concerned in disease spread. The virus affects and damages a number of crops, one of the most impor- tant being the sugar beet. Losses due to curly top have also been reported from time to time by growers of tomatoes and melons. The entire list of plants that can be infected with the virus is long. Of special importance are the chenopodia- ceous and some other weeds including saltbushes, Russian thistle, atriplex, mes- quite, filaree, peppergrass, malva, and lambsquarters. Some of these weeds serve as favorable perennials upon which the insects overwinter, some are favored an- nuals used in breeding large populations, and some aid in supplying the new spring generation of insects with a source of virus, insuring the presence of virus- carrying leafhoppers in the dispersing populations. How the virus is carried. Once a leafhopper has fed on a diseased plant and acquired a charge of virus, it can carry the virus for the rest of its life. A minute is time enough for the insect either to pick up a virus charge or to inoculate a healthy plant. There is good evidence indicating that if an insect has fed only once on a diseased plant, its power to transmit the virus lessens as the days pass, since apparently the virus does not multiply in the insect's body. Nor is the virus transmitted through the egg; insects must feed on a diseased plant be- fore they can become carriers. BEET MOSAIC Beet-mosaic virus disease is world- wide in distribution. In California the disease can be found in practically all sections where beets are grown, but it is especially common in the coastal areas and in the delta region of the Sacramento Valley. When weather conditions favor the build-up of large aphid populations, the virus can infect great portions of the beet crops in the middle and northern sections of the San Joaquin Valley. One outstanding outbreak occurred in 1927. Losses in general are slight, although some reduction in yield has been sug- gested in the cooler coastal regions. The damage is much greater on seed beets, especially upon the transplanted seed crops, where severe losses in seed yield occur. Symptoms. A splotchy light-and- dark-green mosaic pattern, especially noticeable in the early part of the grow- ing season, is the most prominent symp- tom. The symptoms of mosaic are most pronounced on the middle-aged leaves. These symptoms vary considerably with growing conditions, becoming less and less noticeable as the weather becomes [5] Curly top Beet mosaic warmer until they finally fade, leaving many small light green rings on the older leaves. The rings are often unnoticed be- cause of their faintness. During cooler seasons, the mosaic pattern may be brighter and last longer, and some stunt- ing may occur. Red ring and line patterns may develop on the older leaves. Veinlet lightening or clearing, as in most virus diseases, starts the symptom development, occurring on the youngest leaves and rapidly developing into the mottle phase and finally into the ring pattern. The incubation period can vary from 1 to 3 weeks. Vectors. The virus is carried by aphids, mainly by the green-peach aphid, although others can carry the virus, in- cluding the black-bean or dock aphid, commonly a pest on seed beets. The green-peach aphid, one of the most common in California, feeds and breeds on many plants. Some of the host plants for vectors include peach, apricot, cab- bage, celery, cucumber, lettuce, spinach, beets, potatoes, tomatoes, dock, malva, mustard, and sowthistle. The aphid has two main forms in Cali- fornia, the wingless and the winged. The young are born alive (that is, the females do not lay eggs) and look like tiny wing- less adults. A mature wingless female may be % inch long. The general body form is pear-shaped, and the color varies from a dark green to a very pale green to almost pink. The winged form has a black head and thorax, and the abdomen is light green with a black patch on the top side. The rate of reproduction is high, and the insects can mature in a few days under ideal conditions. Low winter tempera- tures as well as high summer tempera- tures are unfavorable for rapid develop- ment. The spring and occasionally the fall are the seasons most favorable to in- crease in aphid population. [6] ' *,. Western cucumber mosaic Yellow-net Winged females spread the infestations over wide areas. These winged aphids are developed when local conditions be- come unfavorable for existing colonies, and their function is to search out more favorable spots and establish new colo- nies. Build-up occurs rapidly in the spring and early summer; during this time the apid problem becomes acute. As the summer progresses, temperature and natural enemies take their toll, and the aphids decline in numbers. In some areas an increase in aphid numbers may occur with the approach of fall and winter, but usually the abundance of natural enemies and the diminishing of favorable host plants, together with the approach of cold weather, limit the fall build-up. Plants concerned in disease spread. The virus has a limited host range, compared to that of the curly-top virus. Beta maritima (the plant from which the sugar beet is derived) beets, Swiss chard, spinach, New Zealand spin- ach, and some tobacco species are suscep- tible to infection, along with such weeds as atriplex and saltbush. Some types of clover can be infected. The vector host range is wide, as men- tioned before — much wider than that of the virus — but several plants are shared in common by both the virus and the vectors. How the virus is carried. All stages and forms of the aphids can carry the virus. An aphid has to feed only a few seconds to pick up a charge of virus, and this charge is rapidly lost during further feeding. Consequently, only a very few healthy plants can be inoculated by any one aphid that has fed upon a diseased plant. In order for an aphid to be active in the spread of much virus, it must be able to feed frequently upon diseased plants. As is the usual case, the virus is not trans- ferred from the female aphid to her young. [7] ... */" * Beet mosaic symptoms on sugar beet. Left: Mosaic pattern of green blotches in a faint chlorosis of leaf. Right: chlorotic blotches in green portion of the leaf. * tjP;. ™ Symptoms of western cucumber mosaic on sugar beets. Left: coarse, bright yellow and green mosaic pattern. Right: mottle pattern with blisterlike elevated areas of green tissue, and longi- tudinal distortion of the leaves. [8] WESTERN CUCUMBER MOSAIC Western cucumber mosaic is an aphid- carried virus which has been present for a long time in the San Joaquin Valley. The disease is not found in the coastal fog-belt areas. This virus is most fre- quently found in crops other than beets — for example, various cucurbits. How- ever, in 1940 in the middle San Joaquin Valley a fairly extensive outbreak oc- curred in sugar beets. Because of the stunting and extreme chlorosis induced by the virus, yield losses can occur in localized areas. Nor- mally the amount of infection is slight, and losses are negligible. Symptoms. Large irregular patches of bright yellow tissue contrasted with dark green areas of the leaf are the symp- toms most readily seen. Symptoms are of a mosaic type, but the pattern is coarser and more contrasting in coloration than that found in beet mosaic. Leaf distor- tion is common, as is the development of fairly extensive blister-like areas of green tissue; necrosis of large portions of the older leaves is frequent. The main diagnostic features are the brilliance and coarseness of the mosaic pattern, the blister-like elevations, and the longitudinal distortion of the leaf shape. The incubation period of the disease varies from 1 to 3 or more weeks. Vectors. This is an aphid-carried virus, and the same species that transmit the beet-mosaic virus carry Western Cu- cumber mosaic. Plants concerned in disease spread. The host range of the virus is fairly large. Crops found infected in vary- ing degrees include celery, cucumber, lettuce, melon, spinach, and tomatoes, in addition to beets. Many weeds and some ornamental plants have also been found susceptible. The host range of the aphid vectors is the same as was discussed in connection with the beet-mosaic virus. How the virus is carried. Aphids spread this virus in much the same man- ner as they do the beet-mosaic virus. (See discussion on page 7.) BEET YELLOW-NET This aphid-borne virus disease, strik- ing in its early stages of symptom de- velopment, occurred rather recently in California. The first example to attract attention came from the delta area near Rio Vista in 1945. Other infections have been found near Isleton (delta area), Mountain View and Hollister (coastal area), and the Imperial Valley, near El Centro. No extensive outbreaks have been reported, and under present conditions the virus is of little economic importance. Symptoms. An intense yellow chlo- rosis of the veins and veinlets of affected leaves is the most commonly found symp- tom, especially in spring. At the height of their brilliance infected plants stand out in the fields in distinct contrast to the surrounding normal green plants. Later the symptoms tend to fade, so that it is often difficult to relocate infections once known to be present. Initially the first few affected leaves, with their veinal yellowing, closely re- semble leaves showing the early vein- lightening symptom due to beet-mosaic virus infection. The early stages of the two diseases are in many cases indis- tinguishable. Multiple infections with the two viruses are not uncommon. In cases of confusion, it is usually only a matter of a few days until the characteristic symptoms of the two diseases permit sep- aration. In mosaic the mottle develops; in yellow-net, the vein and veinlet yellow- ing will persist. Severe cases have been noted, in which there is complete chlorosis of some of [9] . ■:,■•»%•■:><■■ Symptoms of yellow-net on sugar beet. Left: Leaf from naturally infected plant, showing chlorosis of veins and veinlets. Right: A more advanced stage of the chlorosis. the older leaves. No noticeable stunting or malformation is connected with the disease, and recovery from symptoms usually occurs. The incubation period ranges from 10 to 30 or more days. Vectors. The virus is transmitted mainly by the green-peach aphid. Other aphid species have been experimentally implicated but have been judged of little importance in the field spread. Plants concerned in disease spread. The yellow-net virus has a lim- ited host range: only beets and Swiss chard have been satisfactorily shown to be susceptible, and no weed hosts have been found. Beets persisting from one season to the next have been known to carry over the virus. The vector host range is the same as that which was discussed in the case of the green-peach aphid which carries the beet-mosaic virus. How the virus is carried. In con- trast to the relationships found in con- nection with the two mosaic viruses, the aphid vectors of yellow-net, once infec- tive, remain so for long periods of time, often for life. One insect can inoculate many plants, although not every plant fed upon will necessarily be infected. To pick up a charge of virus, an aphid must feed for at least five minutes; to inocu- late a plant, it must feed for at least 15. The longer the feeding periods, the more chance there is for the insects either to pick up or to inoculate virus. As in the case of the other two viruses yellow net is not transferred from the female to the young aphids. « ACTUAL COLOR PHOTOS of symptoms of four of the diseases discussed here will be found on pages 6 and 7. [10] BEET YELLOW VEIN The yellow-vein disease of sugar beets is rarely found in beet fields. The re- ported cases are from southern Califor- nia. Little is known concerning the dis- ease. Although the symptoms are strik- ingly similar to those of yellow-net, there is reported with the yellow-vein disease a fairly characteristic asymmetrical stunting, during which half the plant grows fairly normally while the other half is severely stunted. The disease is graft-transmissible. In- sect vectors are unknown, but the green- peach aphid has so far failed to transmit the virus during tests. The same applies to leafhoppers tested. BEET YELLOWS This serious virus disease of sugar beets has caused varying losses in sugar- beet crops on the European continent and the British Isles. Recent reports state there is some evidence that a mild strain of the disease occurs in California. Thus it might be wise to discuss the disease briefly at this point. Symptoms. Diagnosis is difficult, since the symptoms are at times almost identical with symptoms which can result from un- favorable soil conditions or attack by fungus disease. Thick, yellow, brittle leaves which break easily when crushed are the main symptoms. The degree, extent, and type of symptoms depend to a large extent upon the severity of the virus strain in- volved. At first, in young plants, a pass- ing vein-clearing may occur. The chlo- rosis is interveinal, continuous, and bright yellow to orange yellow. Necrosis of the tips and edges of the older affected leaves sometimes develops. In some va- rieties the necrosis is in the form of tiny dots, or "freckles," over the leaf surface. In some cases the "freckles" are scarlet ; when these occur, they are considered to be of specific diagnostic value, as they are unknown in connection with any dis- order other than virus yellows. Vectors. The virus is aphid-borne, the green-peach aphid being the chief vector. The virus is carried in much the same manner as the yellow-net virus. (See page 10 for discussion.) CONTROL Usually beet mosaic, beet yellow- net, and western cucumber mosaic are of minor economic importance. It is doubtful that direct control measures such as insecticidal application would give adequate return for dollars invested. Effective chemicals for killing aphids are on the market, but their value in con- trolling aphid-transmitted virus diseases has not been satisfactorily demonstrated. The sporadic and unpredictable seasonal variations in the number of aphids and amount of disease are apparently too great to make chemical control practical. A curly top control program has ex- isted for several years under the direc- tion of the State Department of Agricul- ture, with the financial and technical as- sistance of the beet-sugar processors. The aim of the program is to reduce the num- ber of leafhoppers which overwinter so that only small populations are left for spring flights. Two methods of attack are used: first, local concentrations of over- wintering insects and spring populations are sprayed with chemicals to reduce their number; and second, the stands of Russian thistle, an important late-season- weed food plant, are being reduced. Early planting of beets, which insures large plants at the time of the spring flights, definitely helps to reduce loss. [11] c „„, where early ptotrng aetedule. ear. «•— J, „mbe, of boa. pl.nU ..»!• C W- SSi'SK: 3JS. W. - — - "~ es Department of Agriculty (■California :ni«l I'm!.-.! Stat.- IVnai tm.m ... -a b , ■> t pension work in Agriculture and Home Economics Coll.*.- »f A*- <■'"-; ^H Earl Coke, Director, California Agricultural Extonnon . ,r 7£m-3,'53(A3033)AA