Isolates of Beet Mosaic Virus With Different Degrees of Virulence

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1 Isolates of Beet Mosaic Virus With Different Degrees of Virulence C. vv. B ENNETT' Received /0" tj/.blication June 24, 196J Introduction Beet m osaic has been reported from all countries where sugar beets are grown commercially. It is the J']1ost common and widespread of the virus diseases of this crop. It occurs in all the commercial beet areas or Cali fornia and many fields have high percentages uf infection at harvest. Symptoms usually are mild and tend to be masked dur ing the warmer part at the season. In general. the disease has n ot been considered a highly important factor in beet production. In recent years, however. more ev i dence that beet mosaic is ca pable of causing measurable reduction in yield under some conditions in both the seed and the root crop has accumulated. It is possible that a part of this apparent increase in importance of mosaic as a disease of sugar beet is due to increased prevalence of more virulent strains of the causal virus. Observations and studies have been made during the past few years to determine whether more highly virulent strains of the virus may be involved in losses caused by this disease. Beets in the seed-producing area near Tehachapi, California, have shown uniformly more severe mottling and leaf distortion than in other areas. In 1959 and 1960 plants showing marked symptoms of mosaic were se lected at Tebacbapi and tested on potted plants in the greenhouse. Symptoms produced by virus from some of these selections proved to be more severe than those produced by isolales from beets from the Salinas Valley. In other parts of CalifOlnia beet plants with sym ptoms m ore severe than average have also been found. One of these plants, which showed unusuall y severe symptoms, was selected from a field near Stockton, California, in 1961 for more extensive study. The virus was transferred from this plant to greenhouse plants for further comparisoll with a stock \'irus culture' from the Salinas Valley. The results of these comparisons are presented in this report. The.salinas Valley isol ate is designated "common m osaic virus isolate (C)" and the isolate from Stockton, "severe mosaic vi rus isolate (S)." 1 Plant Pathologist, Crops Reseorch Division. Ag ricultural R esearch Service. U. S. Departm ent of Ag ricultu re.

2 28 JOURNAL OF THE A. S. S. B. T. It is of interest in this connection that in 1%2 Marx and Beiss (1)" reported the occurrence of a more virulent strain of beet mosaic virus in Germany where beet mosaic h;1s been considered a factor of importance in beet production for several years. Descriptions of Symptoms Common Mosaic. In the field, symptoms are evident on sugar beet plants throughout the year. They tend to be more marked, however, in the spring and fall or during periods of rapid growth. Typical symptoms consist of different patterns of mottling varying from small more or less circular chlorotic spots to larger chlorotic areas that collectively cover most of the leaf surface. There is little leaf distortion and the disease usually does not noticeably dwarf tbe plant in the held (Figure 1A). In the greenhouse, first symptoms of disease appear on the young leaves and may consist of translucency of veins similar to that produced by curly top virus except that the translucent areas produced by mosaic are broader. Soon the young leaves shuw chlorotic areas of various sizes, shapes, and degrees of yellowing. Frequently no vein translucency is evident and chlorotic spots on tbe young leaves are the first evidence of disease. Severe Nlosaic. As with common mosaic, the first symptom of disease may consist of vein translucency or chlorosis. In some plants, veins may show necrosis. Mottling of young leaves, sometimes in the absence of vein chlorosis, soon appears. Leaves may be ddormed and show green blisters (Figure I B). As infected plants continue to grow, leaves show various stages and degrees of deformity and mottling, or they may show mottling without deformity. Mottling is more intense than in plants with common mosaic. Some plants show degrees of leaf deformity and chlorosis comparable to those produced on beets by cucumber mosaic virus. Chlorotic areas are deeper yellow than similar areas produced by the C isolate. Comparison of Two Mosaic Virus Isolates Transmission. Both isolates are readily juice transmissible but a bigher percentage of infection was obtained with isolate S than with isolate C. Both isolates produce local lesions on sugar beet and Cheuopodium amaranticolor. Tests with sugar beet in which half of the leaf "vas inoculated 'with isolate C and the other half was inoculated "with isolate S, gave an average of 16 local lesions per half-leaf with isolate C and ~ I local lesions with isolate S in 30 single-leaf tests. Similar tests with C. amaranticolot gave averages of 22 and 41 for the C and S isolates, respectively. 2 Number in parentheses refers 10 literature ciled.

3 VOL. 13, No. I, APRIL Figure l.-(a) Sugar beet leaves from a plant infected with common beet mosaic virus isolate (C) showing typical mottling. (B) Sugar beet leaves from a plant infected with virulent isolate (S) of beet mosaic virus showing mottling, chlorosis, and deformities, characteristic of the more severe symptoms produced by this isolate.

4 30 JOURNAL OF THE A. S. S. B. T. Incubation Period in the Plant. Several tests were made in which beet plants of the same size and age were inoculated by rubbing one leaf of each plant with juice trom plants infected with isolates C or S. The results of one such test in which the plants were inoculated at 3 different ages, are shown in Table 1. The average time for appearance of symptoms on the inoculated plants was longer in plants inoculated with isolate C than with isolate S in each age class. This may reflect more rapid multiplication of the S-isolatc or it may indicate a greater plant sensitivity to this isolate. Table I.-Incnbation periods of severe (S) and common (C) mosaic virus isolates in sugar beet plants illoculaled in difierent IStages of growth. Stage of growth Plants infected oc beet plants of 40 inoculated with at titne of indicated isolate A,,'cragc incubation period inoculation (Number) (Days) S C S C 2 leaf leaf eaf Effect of Isolates C and S on Growth of Sugar beet in the Greenhouse_ Seven greenhouse tests were made in which plants of the variety US 75 in 6-inch pots were inoculated in about the 4-leaf stage with the respective virus isolates. The plants were held for 49 to 82 days after inoculation and then harvested and weighed. The S-isolate produced more severe symptoms in all tests and the plants were somewhat more yellow tban those inoculated with the C-isolate. Many plants had deformed leaves. and the plants were more obviously stunted than those inoculated with the C-isolate. Reduction in weight varied considerably in the different tests but the S-isolate caused more injury than the C isolate in all tests. Each isolate caused appreciable reduction 111 yield (Table 2). The percentage reduction in weight of plants in these tests is undoubtedly higher than that expected in the field. However, there is some evidence that very early infection with mosaic virus in the field might lead to appreciable reduction in wei~ht of rools at harvest. Infection of field plants after they have attained considerable size probably causes much less damage. Rate of growth of plants may also inauence the percentage reduction in yield. This may account for the smaller amount of damage

5 VOL. 13, No. I, APRIL Table 2.-R cducuon in weight o[ ~ u ga r beet plants ca ll,cd by severe (S) and com JliOn (C) mosaic virus isolates under grcenhous(' colldi lions. T es t No. Time froln inoculation to harvest (Days) Plants with each treatment t (,< uluber) Average weight at harvest o( plants inoculated wirh indicated virus isolate (Grams) S C Check I H I '[ !i All plants were in 6 inch pots: I plath rer pot in test 7, 2 plants per pot in test 5, and 4 p lants per pot in all other tests. to inoculated plant in Test 7, in which there 'was only 1 plant per pot, which provided space For good growth from time of inoculation to harvest. Properties of Virus lsolates. The two isolates, C and S, produce such markedjy different symptoms under some concitions that it seemed desirable to make property tests to obtain further informalion as to whether they actually are strains of the same virlts or distinct and unrelated viruses. Thermal inactivation tests were made by pl ac ing Juice trom beet plants infected with the respective isolates in small thinwall ed test tubes and immersing these in a water bath held at the desired temperature for 10 minutes. The juice was then used to inoculate beet plants in the 4- to 6-Jeaf stage. There was less infection from juice held 10 minutes at 56 and 58 C and no infection was obtained from juice held at 60 "C. There ',vas no significant difference hetween the thermal inactivation points of the tvvo isolates. In tests in which beet juice from infected plants was diluted with water to various degrees, a slight amount of infection was obtained with each isolate at a dilution of 1-4,000, bul no infection was obtained from dilutions of 1-10,000. Tests in which juice from diseased plants was placed in Haslzs and held at room temperature (21-23 C) and used for inoculating beet plants at daily intervals, indicatecl a gradual decline in infectivity over a period of 6 days and loss of inkctivity after 7 days. Higher percentages of inlection were obtained, however, with the S-isolate than with the C-iso]ate over the 6-d ay period of activity.

6 32 JOlJRNAL OF THE A. S. S. B. T. The results of property studies show that the two isolates are similar in the properties tested, indicating that botb are strains of beet mosaic virus. Summary and Conclusions Results of studies of beet mosaic virus over the past 2 years at the U. S. Agricultural Research Station at Salinas, California, indicate that this virus probably occurs under natural conditions in the form of a number of variants or strains that differ in virulence on sugar beet. Highly virulent strains of this virus may be capable of causing measurable reductions in yield of sugar beets, particularly if infection occurs in the early stages or growth of the beet plant. Literature Cited (I) MARX, RCTH, and U. BEISS Nachweis von Stammen beim Mosaikvirus der Beta-Ri.iben. Phytopath. Z. 44 (I):