Beet Yellows Virus Research Articles

COMPARATIVE PHYSIOLOGICAL STUDIES ON THE GROWTH OF FIELD CROPS

Infection with beet yellows virus depressed the dry‐matter yield of sugar‐beet plants by decreasing both leaf area and net assimilation rate (N.A.R.). It did not reduce the number of leaves.The lower N.A.R. of infected plants may occur because photosynthesis is slowed by chlorosis of the leaves or by changes associated with it. Plants infected at the end of June had 30‐50% of their leaf area yellowed from mid‐August. Later infection caused less yellowing. The yellowing almost sufficed to account for the decrease in N.A.R., if yellowed parts of leaves do not photosynthesize. However, the similar diurnal fluctuations of carbohydrate in the laminae of healthy and infected leaves suggests that photosynthesis may not be much slowed by infection; if so, the decrease in N.A.R. indicates a large increase in respiration rate, especially of the root.The effects of yellows on leaf area and N.A.R. appear to be independent; late‐sown plants suffered a greater reduction of leaf area, but a smaller reduction in N.A.R., than early‐sown plants; similarly treated plants showed the same reduction of leaf area in two successive years, but the decrease in N.A.R. was much greater in the second.Most of the loss of dry matter was in the root; the dry weight of the petiole (including stem tissue) was also decreased but the dry weight of leaf lamina was little affected.Yellows greatly increased the reducing sugars in the leaf lamina, and caused smaller increases in sucrose and starch. These effects appeared when the leaves showed only etch symptoms. The increase in starch was greatest at this time, but the effect on sugar content subsequently increased with the development of yellowing.Yellows reduced the water content, nitrogen content and the arealweight ratio of the leaf lamina. The change in nitrogen content, but not the others, could be accounted for by the rise in carbohydrate.Yellows had no effect on the water content of petiole and root; it increased the nitrogen content of these parts. It reduced the sucrose content of the root, but the loss of sugar yield was mainly attributable to decreased root weight.Infection with beet mosaic virus decreased the dry weight of sugar‐beet plants only when nitrogenous fertilizer was applied. As with yellows infection, the loss of dry matter resulted from reductions in both leaf area and N.A.R. Unlike yellows, mosaic depressed the dry weight of leaf lamina, as well as of petiole and root.Mosaic had no effect on carbohydrate content, water content or arealweight ratio of the leaf lamina, or on water content of petiole and root. It increased the nitrogen content of all parts of plants that received no nitrogenous fertilizer, but not of nitrogen‐treated plants. It slightly increased the sucrose content of the root.Infection with beet mosaic virus at the end of June caused a 20 yo loss of sugar yield from plants that received nitrogenous fertilizer. Yellows infection of similar plants at the same date reduced the sugar yield by 50%.

Virus formation in chloroplasts

Examination with the electron microscope has revealed that the filamentous particles characteristic of leaves infected with beet yellows virus are frequently found in association with chloroplasts, and appear to have been extruded from them. Similar investigations have been carried out on tobacco mosaic virus. The assumption is made that at least some viruses are formed within chloroplasts.

THE SPREAD OF BEET YELLOWS AND BEET MOSAIC VIRUSES IN THE SUGAR‐BEET ROOT CROP I. FIELD OBSERVATIONS ON THE VIRUS DISEASES OF SUGAR BEET AND THEIR VECTORS MYZUS PERSICAE SULZ. AND APHIS FABAE KOCH

A survey of aphids and virus diseases of sugar‐beet root crops in eastern England was made between 1940 and 1948. Prior to 1943 the observations were made on fertilizer experiments; from 1943 onwards they were made on commercial fields selected for position in relation to beet and mangold seed crops. The incidence of beet yellows increased with increasing numbers of Myzus persicae, but not of Aphis fabae. The relation with M. persicae was sufficiently close to suggest that it is the most important, possibly the only important, vector of beet yellows virus. Beet mosaic virus also increased with increasing numbers of M. persicae, but the relation was not close enough to exclude the possibility of other vectors.Numbers of A. fabae on sugar beet were slightly, but consistently, depressed by the use of salt as a fertilizer. Other fertilizers had variable effects. Neither aphids nor virus are likely to be greatly affected by fertilizers.Beet yellows is most prevalent in areas where seed crops are grown, but within these areas nearness to individual seed crops did not appear to increase its incidence. M. persicae were more numerous on sugar beet in seed‐crop areas than elsewhere, and this alone might account for the prevalence of yellows. Beet mosaic virus is more closely associated with seed crops than is beet yellows. It is most prevalent near to seed crops within the seed‐crop areas.

Beet yellows virus.

SEVERAL plant viruses have been found to occur as elongated particles of variable length and 10–15 mµ in width. Leyon1 has published electron micrographs showing such rod-like particles with a width of about 10 mµ in sap from plants infected with beet yellows virus and in the specific precipitates produced by mixing the sap with its antiserum. We have also found rods in these two kinds of preparation. They do not occur in sap from healthy beet plants, and by analogy with other viruses which have been purified it could readily be assumed that the rods are the virus particles themselves. There are, however, other factors which make this assumption less tenable and suggest that the rods are not the only anomalous component in sap from infected plants.

The application of electron microscopy to the study of plant viruses in unpurified plant extracts.

SUMMARY: Rods of variable lengths occurred in sap from plants infected with tobacco mosaic, cucumber mosaic, potato X, potato Y, henbane mosaic, tobacco etch, and cabbage blackring viruses; the first two were about 15 mμ. wide and appeared rigid, the others were about 10 mμ. wide and apparently flexible. Sap from plants infected with tomato bushy stunt, tobacco ringspot and two tobacco necrosiss viruses contained spherical particles about 26 mμ. in diameter; two particles, one about 18 mμ. and the other about 37 mμ. in diameter occurred in sap from plants infected with a third tobacco necrosis virus. No specific particles were identified in sap from plants infected with tomato spotted wilt, potato leaf roll, cauliflower mosaic, tomato aspermy, sugar beet mosaic and sugar beet yellows viruses. Serologically related strains of any one virus were morphologically indistinguishable, but this has little diagnostic value because so also were some unrelated viruses.

Yellowing Disease of 'Family 41' Sugar Beet

IN a preliminary note1, Clinch, Loughnane and McKay describe a yellowing disease in a breeder's 'pure line' of sugar beet (family 41), which is transmissible through the seed and also by the aphis, Myzus persicœ. Recently, Clinch and Loughnane have published a paper2 on this disease with the title Seed Transmission of Virus Yellows of Sugar Beet and the Existence of Strains of this Virus in Eire. This title is, in our opinion, misleading, for it suggests that sugar beet yellows virus is transmitted by seed. There is, on the contrary, much evidence that it is not, and though the yellowing of family 41 may be transmitted through the seed, there is nothing to show that the condition is caused by beet yellows virus.

Factors affecting the loss of yield of sugar beet caused by beet yellows virus. II. Nutrition and variety

The effect of beet yellows virus on the yield of sugar beet was tested in the presence and absence of various fertilizers and farmyard manure. In general the manorial treatments increased the root and sugar yields of both infected and healthy plants, but the losses caused by infection increased proportionally as the mean yields increased. When the effects of nitrogen, farmyard manure and salt on mean yield were large, the losses caused by infection were increased more than proportionally.The fertilizers had little effect in varying the symptoms of the disease. There was some indication that the red colour associated with some varieties of sugar beet was intensified by deficiency of phosphate and potash.The effect of fertilizers on rate of spread of infection was variable, and again appeared to depend largely upon the magnitude of the fertilizer effects on mean yield. Nitrogen, phosphate and potash occasionally had positive effects, and salt had a fairly consistent negative effect. None of the effects appeared to be of any economic importance.

Factors affecting the loss of yield of sugar beet caused by beet yellows virus: I. Rate and date of infection; date of sowing and harvesting

Field experiments were carried out to measure the effect on the yield and composition of sugar beet of infection with beet yellows virus, and to determine how the effect varies with rate and date of infection and with date of sowing and harvesting of the crop.Control measures designed to prevent spread of infection within experimentally infected plots and introduction of infection from sources outside the experiment, were not completely effective. Consequently the numbers of plants which became infected differed from those prescribed by the experimental treatments. The effects of infection were, therefore, estimated by means of regressions on the numbers of plants observed to become infected on successive dates.After correcting the observed yields for accidental infections by means of the regression coefficients, it was found that the effect of infection on the yield of sugar was linearly related to the rate of infection, i.e. the loss of yield of sugar caused by infection was proportional to the percentage of infected plants. There was no compensation for loss in infected plants by increased growth of healthy neighbours.The effect of infection on yield of sugar decreased linearly with later date of infection, falling to values not significantly different from zero at the date of harvest. The loss of yield caused by infection was therefore approximately proportional to the interval of time between the date of infection, as shown by the appearance of symptoms, and the date of harvest.

The transmission of beet mosaic and beet yellows viruses by aphides; a comparative study of a nonpersistent and a persistent virus having host plants and vectors in common.

The vectors of beet mosaic virus are optimally infective when they have fed for only a few minutes on the infected plants after a period of fasting. After infection feeding, infectivity is very rapidly lost when the vectors feed on healthy plants, but while it remains a single vector can infect several plants. Infectivity is lost much more slowly when the vectors fast after infection feeding. In this behaviour beet mosaic virus resembles Hy 3, potato Y , cucumber 1, and other aphis-transmitted viruses which have been called the non-persistent group. It resembles these viruses also in its physical properties. In some secondary characters beet mosaic differs from the other non-persistent viruses more than they differ from each other. It is retained longer by the fasting vectors, and infectivity of the vectors may increase considerably with increasing infection feeding time, in the absence of preliminary fasting, though it rarely reaches the optimal level. With beet yellows virus infectivity of the vectors is not affected by preliminary fasting, but always increases with increasing feeding time on both infected and healthy plants. Infectivity increases with increasing feeding time on the healthy plants whatever the infection feeding time, and therefore there is always a delay in the production of optimum infectivity by the aphides after cessation of infection feeding. Infectivity is more rapidly lost from the fasting than from the feeding vectors. The properties indicate that beet yellows belongs to the persistent group of viruses, although its persistence in the fasting vectors is only about the same as that of beet mosaic, which is a non-persistent virus. The main basis of distinction between the two types seems not to be the time for which they are retained by vectors, but the effect of preliminary fasting. Beet yellows and beet mosaic viruses have the same vector and host plant and therefore the differences in their behaviour are properties of the viruses them selves, and are not induced by the conditions in which they are transmitted.

Some causes of chlorosis and necrosis of sugar‐beet foliage

The symptoms and characteristics of two virus and one fungus disease and four nutritional disorders of sugar beet which cause chlorosis and necrosis of the foliage are described. The causes of the diseases and methods of distinguishing between them have been investigated by analytical, pathological and field experimental methods.Experiments in which diagnosis was confirmed by serological and spectrochemical methods show that the two often easily confused diseases, sugar‐beet yellows virus and magnesium deficiency, can be visually distinguished.Sugar‐beet yellows virus reduces the potassium, but slightly increases the magnesium content of the leaves.Magnesium deficiency symptoms are associated with a low magnesium content of the foliage, but may be induced by salt applications without greatly affecting the magnesium analysis.‘Potash’ deficiency symptoms are often, but not necessarily, associated with a low potassium analysis and may actually be caused solely by a deficiency of sodium. In the field, symptoms are induced by sulphate of ammonia and phosphate applications and may be prevented in some cases by the application of either salt or muriate of potash, in others by salt only. Some interchangeability of the functions of potassium and sodium in the plant is suggested.Manganese deficiency symptoms are associated with a low manganese content of the leaves, which can be readily increased by spraying or injection with manganese sulphate solution, but a high concentration of manganese in the foliage, such as sometimes occurs naturally on acid soils, has a toxic effect.