Potato Leafroll Virus Concentration Research Articles

First Report of Solanum sarrachoides (Hairy Nightshade) as an Important Host of Potato leafroll virus.

Potato leafroll virus (PLRV) (genus Polerovirus, family Luteoviridae) is of great economic importance. It has a very narrow host range consisting of only 21 known species, 14 of which are Solanaceae. Ten aphid species transmit PLRV in a persistent manner, but the green peach aphid (Myzus persicae Sulzer) is by far the most important vector and the only important vector in the Columbia Basin of the Pacific Northwest (3). Solanum sarrachoides (Sendtner) may be especially important in the epidemiology of PLRV because it is a predominant weed in potatoes (S. tuberosum L.), other annual crops, and waste areas of the Columbia Basin (1), and herbicides available for potato are relatively ineffective against this weed (2). Since our research began in 1974, we have routinely observed many S. sarrachoides plants throughout the Columbia Basin that acquired an abnormal erect stiff appearance in middle to late summer. Their leaflets curled upward at the margins and expressed interveinal chlorosis and necrosis and a reddened or purple color on abaxial surfaces. The presence of PLRV in symptomatic plants was routinely detected by double-antibody sandwich enzyme-linked immunosorbent assay and by density gradient sedimentation and light absorbance properties of the purified virus when we evaluated the host as a source of purified PLRV in 1980. A systematic study to satisfy the requirements of Koch's postulates to prove that PLRV causes the disease in S. sarrachoides was conducted in 1998. The virus produced typical PLRV symptoms on three young Physalis floridana, Datura stramonium, and potato plants when transmitted by the green peach aphid from each of four symptomatic S. sarrachoides collected from widely spaced potato fields in the Columbia Basin and from purified virus preparations from these plants. Back-transmission from each of these hosts and from purified preparations to S. sarrachoides plants observed the same symptoms. The green peach aphid readily colonized S. sarrachoides in the field, and populations on this host were typically higher than on adjacent potato plants in potato fields. The concentration of PLRV was approximately equal in infected S. sarrachoides and P. floridana plants, as was the efficiency of virus transmission from these hosts by individual green peach aphids. The aphids used in transmission efficacy assays were all the same clone and were reared on the host species assayed. The incidence of PLRV infection among S. sarrachoides in potato fields was typically higher than in the potato crop. Only two other summer annual hosts of PLRV, Amaranthus caudatus and S. nigrum, occur sporadically in potato fields of the Columbia Basin (1). The virus rarely infected these species. To our knowledge, this is the first report that S. sarrachoides is highly susceptible to PLRV and may play an important role in PLRV epidemiology in the Columbia Basin.

Resistance to potato leaf roll virus multiplication in potato is under major gene control

The concentration of potato leaf roll virus (PLRV), as measured by a quantitative enzyme-linked immunosorbent assay, in the foliage of potato plants (Solanum tuberosum) of cv 'Maris Piper' with secondary infection was 2900 ng/g leaf, whereas in clones G7445(1) and G7032(5) it was 180 ng/g leaf and 120 ng/g leaf, respectively. To examine the genetic control of resistance to PLRV multiplication, reciprocal crosses were made between the susceptible cultivar 'Maris Piper' and the two resistant clones, and the three parents were selfed. Seedling progenies of these families were grown to generate tubers of individual genotypes (clones). Clonally propagated plants were graft-inoculated, and their daughter tubers were collected and used to grow plants with secondary infection in which PLRV concentration was estimated. The expression of resistance to PLRV multiplication had a bimodal distribution in progenies from crosses between 'Maris Piper' and either resistant clone, and also in progeny from selfing the resistant parents, with genotypes segregating into high and low virus titre groups. Only the progeny obtained from selfing 'Maris Piper' did not segregate, all genotypes being susceptible to PLRV multiplication. The pattern of segregation obtained from these progenies fits more closely with the genetical hypothesis that resistance to PLRV multiplication is controlled by two unlinked dominant complementary genes, both of which are required for resistance, than with the simpler hypothesis that resistance is conferred by a single dominant gene, as published previously.

An approach to field screening potato genotypes for potato leaf roll virus resistance

Eight potato cultivars and two advanced breeder selections were assessed for field resistance to the potato leaf roll virus (PLRV) following field exposures in which PLRV-infected Russet Burbank plants were used as inoculum sources within treatments. This screening protocol provided consistent PLRV resistance ratings despite year-to-year variation in PLRV pressure. Secondary disease incidence based on enzyme-linked immunosorbent assay (ELISA) of foliage from tuber progeny ranged from 0–87% in 1990 and 0–67% in 1991, and was consistent with reported PLRV resistance ratings for eight of ten genotypes. Agreement between visual assessment and ELISA on plants from harvested tubers was 94% in 1990 and 83% in 1991, for all genotypes. However, agreement data were inconsistent from year-to-year, with the exception of three genotypes. In both years, current season infection, based on ELISA of foliage, was detected in less than two percent of the plants and, was inadequate as a measure of secondary PLRV incidence. Green peach aphid (GPA) populations did not differ among genotypes at sampling times during the season, but the PLRV concentration in GPA colonizing Russet Burbank plots was significantly higher than in GPA colonizing any other genotype.

Symptom expression and accumulation of potato virus Y (PVYO) and potato leaf roll virus in thirteen potato cultivars

Necrotic local lesions developed in cvs. Matilda, Ostara, Record, Satuma, Stina, Hankkija’s (Hjan) Tanu and Hjan Timo and local ring spots in Olympia and Sieglinde (Siikli) following sap inoculation with the ordinary strain of potato virus Y (PVY0). Secondarily infected cvs. Ostara, Pito, Siikli and Hjan Timo developed leaf drop. No infected progeny was produced by Matilda, Saturna and Hjan Tanu. In contrast, Bintje, Puikula and Sabina developed neither local lesions nor systemic necrosis, but showed mosaic symptoms following primary and secondary infection by PVYO. The ELISA absorbance values for potato leafroll virus (PLRV) in Ostara, Pito and Saturna were less than 10% of those in the PLRV-infected Siikli. The ELISA values for PLRV in Olympia, Stina, Hjan Tanu and Hjan Timo were not significantly different from those of Siikli. The severity of the symptoms did not correlate with the concentration of PLRV in the potatoes.

Evidence of simple genetic control in potato of ability to restrict potato leaf roll virus concentration in leaves

The concentration of potato leaf roll virus (PLRV), measured by quantitative enzyme-linked immunosorbent assay, in foliage of plants of cv Maris Piper and clone G7445(1) with secondary infection was 2,700 ng/g leaf and 120 ng/g leaf, respectively. In experiments to examine the genetic control of their ability to restrict the multiplication of PLRV, reciprocal crosses were made between these two clones. Among 40 genotypes from the progeny of the crosses, about half had a low PLRV concentration in plants with secondary infection and the other half had a high concentration. The possibility of monogenic control of the character that restricts PLRV multiplication in such clones of Solanum tuberosum is discussed.

Unusually mild symptoms of potato leafroll virus in the progeny of late-infected mother plants

In a field experiment in which the date of potato leafroll virus (PLRV) inoculation was controlled, progeny plants derived from late infection of mother plants in the previous year showed much milder symptoms than progeny plants derived from mother plants that became infected earlier in the season. Plants with these milder symptoms contained as great a concentration of PLRV as progeny plants with more obvious symptoms derived from early primary infections.

Restricted distribution of potato leafroll virus antigen in resistant potato genotypes and its effect on transmission of the virus by aphids

SUMMARYEnzyme‐linked immunosorbent assay was used to measure the concentration of potato leafroll virus (PLRV) antigen in different parts of field‐grown secondarily infected plants of three potato genotypes known to differ in resistance to infection. The antigen concentration in leaves of cv. Maris Piper (susceptible) was 10–30 times greater than that in cv. Pentland Crown or G 7445(1), a breeder's line (both resistant). Differences between genotypes in antigen concentration were smaller in petioles and tubers (5–10‐fold) and in above‐ground stems (about 4‐fold), and were least in below‐ground stems, stolons and roots (about 2‐fold). PLRV antigen, detected by fluorescent antibody staining of tissue sections, was confined to phloem companion cells. In Pentland Crown, the decrease in PLRV antigen concentration in leaf mid‐veins and petioles, relative to that in Maris Piper, was proportional to the decrease in number of PLRV‐containing companion cells; this decrease was greater in the external phloem than in the internal phloem. The spread of PLRV infection within the phloem system seems to be impaired in the resistant genotypes.Green peach aphids (Myzuspersicae) acquired < 2800 pg PLRV/aphid when fed for 4 days on infected field‐grown Maris Piper plants and < 58% of such aphids transmitted the virus to Physalis floridana test plants. In contrast, aphids fed on infected Pentland Crown plants acquired <120 pg PLRV/aphid and <3% transmitted the virus to P. floridana. The ease with which M. persicae acquired and transmitted PLRV from field‐grown Maris Piper plants decreased greatly after the end of June without a proportionate drop in PLRV concentration. Spread of PLRV in potato crops should be substantially decreased by growing cultivars in which the virus multiplies to only a limited extent.

Restricted multiplication of potato leafroll virus in resistant potato genotypes

SUMMARYPlants of a range of potato genotypes differing in rating for field resistance to potato leafroll virus (PLRV) were inoculated with the virus by grafting or by aphids (Myzus persicae). Plants of all genotypes tested became infected by each inoculation method and PLRV was detected by ELISA in the upper leaves of all genotypes within 26 days after grafting. Most genotypes with high resistance ratings developed only mild primary and secondary symptoms whereas those with low resistance ratings developed more pronounced symptoms. However, one genotype (G7461(4)) with a high resistance rating was very severely affected. The concentrations attained by PLRV in genotypes with high resistance ratings were only 1–10% of those in genotypes with low resistance ratings. These differences in virus concentration were found in young leaves of plants with primary or secondary infection, whether inoculated by grafting or by aphids and whether grown in the glasshouse or the field. In older leaves, differences in virus concentration between genotypes were at least as pronounced as those in younger leaves. In contrast, PLRV concentration in vascular tissue at the heel end of tubers of plants with primary infection was similar for all the genotypes tested. Although low PLRV concentration was consistently associated with high resistance rating it is not the only form of resistance to PLRV occurring in potato.

Detection of Potato Leafroll Virus in Primarily Infected Tubers by Enzyme‐Linked Immunosorbent Assay

The effects of artificially breaking dormancy on the detection of potato leafroll virus (PLRV) in tubers of four field‐grown potato cultivars with primary infection were studied by ELISA. Treatment with Rindite increased the concentration of PLRV at the rose end of tubers to a level which exceeded in the susceptible cv. Sieglinde that of the heel end and was in cultivars Sirco and Dirus almost as high as that of the heel end. PLRV concentration was highest at four weeks after Rindite treatment and remained on a much higher level in treated tubers from August till November than in untreated ones. At 14 weeks after breaking dormancy, PLRV dropped in treated tubers to a very low level which was similar to that of untreated ones. With sprouting tubers of cv. Jetta, PLRV occurred at significantly higher concentrations in samples from rose‐end eye tissue than in those from heel‐end tissue next to an eye. The reliability of detecting PLRV was higher in treated tubers than in untreated ones and did not depend upon the tuber end tested. With the exception of cv. Sieglinde, all infected tubers which could not be detected by ELISA in the tuber test originated from late‐inoculated plants. PLRV was reliably detected in sprouts. ELISA on leaves of field‐grown plants with primary infection gave very unsatisfactory results, irrespective of the time of inoculation.

Quantitative studies on the uptake and retention of potato leafroll virus by aphids in laboratory and field conditions

SUMMARYEnzyme‐linked immunosorbent assay (ELISA) was adapted for the efficient detection and assay of potato leafroll virus (PLRV) in aphids. Best results were obtained when aphids were extracted in 0.05 M phosphate buffer, pH 7.0, and the extracts incubated at 37 °C for 1 h before starting the assay. Using batches of 20 green peach aphids (Myzus persicae), about 0.01 ng PLRV/aphid could be detected. The virus could also be detected in single aphids allowed a 1‐day acquisition access period on infected potato leaves. The PLRV content of aphids depended on the age of potato source‐plants and the position of source leaves on them. It increased with increase in acquisition access period up to 7 days but differed considerably between individual aphids. A maximum of 7 ng PLRV/aphid was recorded but aphids more usually accumulated about 0.2 ng PLRV per day. When aphids were allowed acquisition access periods of 1–3 days, and then caged singly on Physalis floridana seedlings for 3 days, the PLRV content of each aphid, measured subsequently, was not strongly correlated with the infection of P. floridana. The concentration of PLRV in leaf extracts differed only slightly when potato plants were kept at 15, 20, 25 or 30 °C for 1 or 2 wk, but the virus content of aphids kept on leaves at the different temperatures decreased with increase of temperature. PLRV was transmitted readily to P. floridana at all temperatures, but by a slightly smaller proportion of aphids, and after a longer latent period, at 15 °C than at 30 °C.The PLRV content of M. persicae fed on infected potato leaves decreased with increasing time after transfer to turnip (immune to PLRV). The decrease occurred in two phases, the first rapid and the second very slow. In the first phase the decrease was faster, briefer and greater at 25 and 30 °C than at 15 and 20 °C. No evidence was obtained that PLRV multiplies in M. persicae. These results are compatible with a model in which much of the PLRV in aphids during the second phase is in the haemocoele, and transmission is mainly limited by the rate of passage of virus particles from haemolymph to saliva.The potato aphid, Macrosiphum euphorbiae, transmitted PLRV much less efficiently than M. persicae. Its inefficiency as a vector could not be ascribed to failure to acquire or retain PLRV, or to the degradation of virus particles in the aphid. Probably only few PLRV particles pass from the haemolymph to saliva in this species.The virus content of M. euphorbiae collected from PLRV‐infected potato plants in the field increased from early June to early July, and then decreased. PLRV was detected both in spring migrants collected from the plants and in summer migrants caught in yellow water‐traps. PLRV was also detected in M. persicae collected from infected plants in July and August, and in trapped summer migrants, but their PLRV content was less than that of M. euphorbiae, and in some instances was too small for unequivocal detection.

Application of enzyme‐linked immunosorbent assay to the detection of potato leafroll virus in potato tubers

SUMMARYFactors affecting the detection of potato leafroll virus (PLRV) by enzyme‐linked immunosorbent assay (ELISA) in tubers of field‐grown potato plants with primary or secondary infection were studied. The reactions of extracts of virus‐free potato tubers were minimised by pre‐incubating the extracts at room temperature and by careful choice of the dilution of enzyme‐conjugated globulin. PLRV was reliably detected in tubers produced by secondarily infected plants of all six cultivars tested. PLRV concentration was greater in heel‐end than in rose‐end vascular tissue of recently harvested tubers but increased in rose‐end tissue when tubers stored at 4°C for at least 5 months were placed at 15–24°C for 2 wk.PLRV occurred at greater concentration in tubers from plants of cv. Maris Piper with natural or experimentally induced primary infection than in tubers from secondarily infected plants; again PLRV concentration was greater in heel‐end than in rose‐end vascular tissue. Plants whose shoots were infected earliest in the growing season were invaded systemically and produced the greatest proportion of infected tubers; plants infected late in the season also produced infected tubers but PLRV was not detected in their shoot tops. PLRV concentration in tubers from the earliest‐infected plants was less than in tubers from later‐infected plants. PLRV was detected reliably by ELISA in tubers from progenies that were totally infected but was not detected in all infected tubers from partially infected progenies.ELISA is suitable as a routine method of indexing tubers for PLRV, although the virus will not be detected in all infected tubers produced by plants to which it is transmitted late in the growing season.

Factors affecting the detection of potato leafroll virus in potato foliage by enzyme‐linked immunosorbent assay

SUMMARYUsing antiserum globulins that reacted only weakly with plant materials, potato leafroll virus (PLRV) at 10 ng/ml was detected consistently by enzyme‐linked immunosorbent assay (ELISA). The reaction with PLRV particles was slightly impaired in potato leaf extracts that were diluted less than 10‐1 but not at greater dilutions. Antiserum globulins that reacted more strongly with plant materials could be used satisfactorily for coating microtitre plates but were unsuitable for conjugating with enzyme.The detection end‐point of PLRV, in leaf sap of potato cv. Cara plants grown from infected tubers in the glasshouse, was about 10‐2 and the virus was reliably detected in extracts of composite samples of one infected and 15 virus‐free leaves. PLRV concentration was much less in extracts of roots or stolons than in leaf extracts. The virus was detected in infected leaves of all 27 cultivars tested.PLRV was readily detectable 2 wk before symptoms of secondary infection developed in field‐grown plants of cv. Cara and Maris Piper and remained so for at least 5 wk. Its concentration was slightly greater in old than in young leaves and was similar to that in glasshouse‐grown plants. In field‐grown plants of cv. Maris Piper with primary infection, PLRV was detected in tip leaves 21–42 days after lower leaves were inoculated by aphids; in some shoots it later reached a concentration, in tip leaves, similar to that in leaves with secondary infection. Symptoms of primary infection developed in the young leaves of some infected shoots but were inconspicuous and were not observed until at least a week after PLRV was detected by ELISA.

Potato leafroll virus concentration in the vascular region of potato tubers examined by enzyme-linked immunosorbent assay (ELISA)

Enzyme-linked immunosorbent assay (ELISA), used in conjunction with a new rapid extraction method, showed that potato leafroll virus (PLRV) concentration in the vascular region of infected potato tubers decreases from the heel to the rose end. Lower virus concentration at the rose than at the heel end was found not only in dormant tubers but also in tubers three weeks after breaking dormancy although the difference was then less pronounced. These results were obtained from plants with both primary and secondary infection by one of two French virus isolates which behave differently in respect of either accumulation in the ants or in their serological properties or both.

Latent Period and Retention of Potato Leaf-Roll Virus in its Vector, Myzus persicae

Green peach aphids, Myzus persicae Sulz. fed on the Physalis floridana Rydb. plants infected with potato leaf-roll virus (PLRV) for 3 hours, began to transmit the virus 9 hours after leaving the source plants. Aphids injected with extracts or blood from the viruliferous insects were also enable to transmit PLRV 6 hours after injection. The length of the latent period and the frequency of virus transmission in its aphid vector appeared to depend on the virus concentration within the vector. Inoculativity in the transmission of aphids tended gradually to increase till 24 hours after acquisition feeding or injection of extracts and blood from viruliferous insects.PLRV was detected in extracts and blood from aphids fed on the infected P. floridana plants for more than 3 hours, but no virus was recognized in them from insects given an hour's acquisition feeding. Even though the concentration of PLRV in the aphid vectors increased with the prolonged acquisition feeding, it decreased gradually with the time after leaving the source plants. The frequency of virus recovery from blood of the injected aphids also tended similarly to decrease gradually with the time after injection of the virus into aphids.The latent period of PLRV in the aphids vectors appeared to represent the time required for the virus to circulate within the insect body.