The epidemiology of tomato mosaic: Sources of TMV in commercial tomato crops under glass

Of the several possible sources of tomato mosaic virus, seeds and root debris in the soil are considered to be of greatest importance. A survey of 374,000 seedlings on ten commercial holdings found 0.05% of them infected, and although these were removed virus had been spread to other young plants which did not show infection when transplanted into the growing houses, seven of twenty-two of which contained a few infected plants when sampled shortly after planting. Virus overwintering on clothing, and debris on structures, are thought to be of minor importance, and smoking tobacco is seldom a source of infection for the tomato crop. A further survey of seventy-eight samples from tomato crops in Britain confirmed the 1960-61 survey: all were infected with tomato strains of TMV, none with tobacco strains, but one of the 187 infected seedlings referred to above was carrying a tobacco strain. Petunia was not as satisfactory as a special cultivar of White Burley tobacco for distinguishing between the tobacco and tomato TMV isolates. Observations and tests on a commercial holding showed that TMV was readily carried from plants in infected glasshouses into clean ones by workers, and once introduced, spread rapidly within the crop.     

Observations and experiments on the use of an avirulent mutant strain of tobacco mosaic virus as a means of controlling tomato mosaic

In 1973 tobacco mosaic virus (TMV) strain M II-16 was successfully used by growers in the United Kingdom to protect commercial tomato crops against the severe effects of naturally occurring strains of TMV. However, plants in many crops had mosaic leaf symptoms which were occasionally severe, so possible reasons for symptom appearance were examined. The concentration of the mutant strain in commercially produced inocula (assessed by infectivity and spectrophotometry) ranged from 28 to 1220 μg virus/ml; nevertheless all samples contained sufficient virus to infect a high percentage of inoculated tomato seedlings. Increasing the distance between the plants and the spray gun used for inoculation from 5 to 15 cm resulted in a significant decrease in the number of tomato seedlings infected. When M II-16 infected tomato plants were subsequently inoculated with each of fifty-three different isolates of TMV, none showed severe symptoms of the challenging isolates within 4 wk, although some isolates of strain o induced atypically mild leaf symptoms. In a further experiment, M II-16 infected plants showed conspicuous leaf symptoms only 7 wk after inoculation with a virulent TMV isolate. M II-16 multiplied more slowly in tomato plants and had a lower specific infectivity than a naturally occurring strain of TMV. More than 50% of plants in crops inoculated with strain M II-16 which subsequently showed conspicuous leaf mosaic contained TMV strain 1 or a form intermediate between strains o and 1. It is suggested that the production of TMV symptoms in commercial crops previously inoculated with strain M II-16 may result from an initially low level of infection, due to inefficient inoculation, which allows subsequent infection of unprotected plants by virulent strains. Incomplete protection by strain M II-16 against all naturally occurring strains may also be an important factor.     

Application of Vascular Puncture for Evaluation of Curtovirus Resistance in Chile Pepper and Tomato

Curtoviruses cause severe damage to tomatoes and peppers. Functional field resistance to curtoviruses in these plants is desirable but difficult to produce and difficult to screen for because it is time‐consuming and resistance could be achieved by developing resistance either to the virus or to insect feeding. To improve and speed curtovirus resistance testing in tomato (Solanum lycopersicum) and pepper (Capsicum annuum) plants, two puncture methods were developed and compared to leafhopper inoculation and feeding preference assays. The two puncture methods were adapted to introduce a modified Agrobacterium tumefaciens plasmid carrying a recombinant curtovirus into the meristem tissue of tomato plants and into newly germinated chile pepper seedlings. The puncture techniques were used to screen for resistance to curtoviruses in chile pepper and tomato breeding lines and varieties. Similarly, the peppers and tomatoes were assayed for curtovirus resistance using leafhopper inoculation and feeding preference, which was assessed by stylet sheath staining. Virus infection by puncture and leafhopper feeding was monitored using PCR and ELISA. ELISA was performed using an antibody to bacterially expressed coat protein. While pepper cvs Tabasco, NuMex Las Cruces cayenne and New Mexico 6‐4 were infected using both puncture and leafhopper inoculation methods, New Mexico 6‐4 had higher infection rates than the other two cultivars. Stylet sheath staining results suggest that leafhoppers prefer to feed on New Mexico 6‐4 rather than Tabasco and NuMex Las Cruces cayenne. Eight tomato cultivars were infected using meristem removal injection inoculation. Three tomatoes cultivars (CVF‐11, Saladmaster and Supersteak) were infected using leafhopper inoculation, although stylet sheath staining results suggested that the first two cultivars were not preferred by the insect vector. Our results suggest that puncture methods and leafhopper inoculation are successful in resistance screening, and both methods should be used as part of screening, because they assess different types of resistance.     

Natural Infection of Tomato and Pelargonium in Germany by a Tombusvirus Originally Described from Pepper in Morocco

Two serologically apparently identical tombusviruses were isolated from glasshousegrown tomato plants showing mottle and malformation of the upper leaves and stunting of the shoot tips and, at a low rate, from a pelargonium plant, respectively. Both plant species were received from growers in southern Germany. In agar gel double diffusion tests and immuno-electrophoresis, the two virus isolates could not be distinguished from a tombusvirus (MPV) isolated from pepper in Morocco by F ischer and L ockhart (1977), although there were some minor differences in the reported host responses. In the tomato planting, strong symptoms were produced only temporarily. The majority of the 104 tomato plants of 15 different cultivars which had become infected by MPV following mechanical inoculation of the leaves showed only mild symptoms in contrast to a similar group of plants which developed severe stunting and foliar deformation when infected with the type strain of tomato bushy stunt virus. MPV was translocated only occasionally to the upper parts of tomato seedlings which had been grown in virus-containing soil, although root infections were more frequent. MPV is apparently not a major threat to tomato cultivation in Germany.     

Characteristics of Serially Produced Zoospore Suspensions of Polymyxa betae for Transmission of Beet Necrotic Yellow Vein Virus

Zoospore suspensions of Polymyxa betae were analysed for their potential as inocula to infect sugar beet plants with beet necrotic yellow vein furovirus. The infectivity could be maintained when zoospore suspensions were serially transferred. When zoospore-producing seedlings were individually transferred some of these seedlings lost their infectivity after several passages. Infectivity was first detected in suspensions within I day after inoculation of the plant by zoospores. The suspensions remained infectious for at least 10 h after removal of the plants producing viruliferous zoospores. Both the number of test plants infected and the concentration of virus that developed were greater at 25 C than at 20 C.     

Effects of Inoculation with a Mild Strain of Tomato Aspermy Virus on the Growth and Yield of Tomatoes and the Potential for Cross Protection

Mechanical inoculation of seedlings of 26 tomato cultivars with an attenuated strain of tomato aspermy virus (TAV-M, ATCC # PV361) resulted in development of only mild symptoms, which had largely disappeared by the time the infected plants had begun to set fruit. Yield trials revealed that plants inoculated with TAV-M when transplanted at the 6–8 leaf stage displayed no delay in fruit set and had yields only slightly less than uninoculated plants. Cross protection studies carried out in the greenhouse revealed that TAV-M has the potential to interfere with infection of tomato seedlings by a more severe TAV strain (ATCC # PV127). ‘Pik-Red’ seedlings inoculated with TAV-M showed a significant reduction in symptom expression when challenged with the Blencowe strain of TAV 21 days after the initial inoculation with TAV-M; interference was incomplete at 7 and 14 days.     

Beet leafhopper (Hemiptera: Cicadellidae) settling behavior, survival, and reproduction on selected host plants

Experiments were conducted to determine the settling behavior, survival, and reproduction of the beet leafhopper, Circulifer tenellus (Baker), when maintained on selected host plants. This leafhopper was recently identified in the Columbia Basin of Washington and Oregon as the probable vector of the beet leafhopper-transmitted virescence agent phytoplasma, causal agent of several vegetable crop diseases, including potato purple top. Plants selected for study were sugar beet, Beta vulgaris L.; radish, Raphanus sativus L.; dry bean, Phaseolus vulgaris L.; potato, Solanum tuberosum L.; carrot, Daucus carota L.; and tomato, Lycopersicon esculentum Mill. Leafhopper adults were confined on caged plants, and settling behavior was observed during a 72-h period and survival was monitored for 40 d. Also, oviposition and nymphal production were investigated by maintaining leafhoppers for approximately 90 d on each of the selected plants. Sixty to 100% of leafhoppers settled on all studied plants during the first 5 h, but settling on bean and tomato declined sharply thereafter. Leafhopper mortality was very high on bean and tomato, with 95 and 65% of the leafhoppers, respectively, dying in about a week. In contrast, 77, 90, and 95% of leafhoppers maintained on potato, sugar beet, and radish, respectively, survived until the end of the 40-d experimental period. Beet leafhopper oviposition and nymphal production and development only occurred on sugar beet, radish, and potato; reproduction was lower on potato.     

Experiments on the Spread of Potato Virus X Between Plants in Contact

Experiments on the spread of five strains of potato virus X were made with seven potato varieties and with tomato plants both under glass and in the field. Spread by leaf contact between healthy and infected plants was confirmed, and it was also found that spread could occur between plants whose only contact was below ground.
The rate of spread was much greater in tomato than in potato plants, and virulent strains of the virus, which achieve a high concentration in infected plants, spread more rapidly than avirulent strains. In only one experiment with potatoes did more than 10% of the healthy potato plants exposed to infection become infected during one season.
Datura stramonium and tomato plants became infected when growing in soil containing sap or residues from X -infected plants.
It was common in the field for potato plants whose foliage gave no reaction for virus X at the end of the season to yield a mixed progeny of healthy and infected tubers. Such infections are thought to result from underground spread.
Attempts to transmit virus X from infected to healthy potatoes by means of Rhizoctonia solani failed. No examples of infection were found except when healthy plants came into direct contact with sources of the virus.     

THE PATTERN OF FIELD INFECTION OF POTATO BY THE BEET RINGSPOT STRAIN OF TOMATO BLACK-RING, A SOIL-BORNE VIRUS

The potential importance of the beet ringspot strain of tomato black-ring, a soil-borne virus, was assessed by growing stocks of Kerr's Pink potato for 1 year on infested land and subsequently on uninfested land. The incidence of infection in two stocks was 39 and 8% in the first year on uninfested land, and 29 and 5% after 2 years.
The virus was usually restricted to the roots of plants in the first year of infection, but a few plants showed black rings and spots in their leaves. In the second year, 20–55% of the plants grown from tubers set by symptomless, but infected, mother plants were infected: many of these showed leaf necrosis, others had stunted shoots, and cupped and distorted leaves; some were symptomless although systemically infected. In the third and fourth years, most of the progeny from plants which had symptoms or which were symptomless but systemically infected, contained the virus: nearly all such infected plants were stunted and distorted or were symptomless. Infection decreased the weight of tubers produced by plants with severe necrotic spotting but not the yield of plants with less necrosis. The number and weight of tubers per plant were decreased by 15 and 20% respectively, in symptomless systemically infected plants, and by 20 and 30% in stunted plants.     

Seed-transmission of nematode-borne viruses

Transmission through seed of crop and weed plants seems to be characteristic of nematode-borne viruses. It occurred with tomato black ring virus (TBRV) in nineteen species (thirteen botanical families), with arabis mosaic virus (AMV) in thirteen species (eleven families), with raspberry ringspot virus (RRV) in six species (five families), and also, in more limited tests, with tomato ringspot, cherry leaf roll and tobacco rattle viruses. A remarkable feature was that infected seedlings, except those containing tobacco rattle virus, often appeared healthy. The occurrence and extent of seed-transmission depended on both the virus and the host plant. In many progenies more than 10%, and in some 100%, of seedlings were infected. The viruses were transmitted through at least two or three generations of seed of those host species tested. After 6 years' storage, TBRV- and RRV-containing seed of Capsella bursa-pastoris and Stellaria media germinated to give infected seedlings. In controlled crossing experiments with strawberry and raspberry, virus was transmitted to seed from both male and female parents but, at least in raspberry, the presence of competing virus-free pollen much decreased the ability of pollen from infected plants to set seed. There was no evidence that healthy mother plants became infected when their flowers were pollinated with infected pollen.     

Trade-off between foliage and tuber resistance to Phthorimaea operculella in wild potatoes

Beet leafhopper, Circulifer tenellus (Baker) (Homoptera: Cicadellidae), is the only known North American vector of beet curly top virus (Geminiviridae), which causes major economic losses in a number of crops including sugar beet, tomato, beans, and peppers. Beet curly top virus is a phloem-limited, persistently transmitted, circulative geminivirus. The strain/species of curly top virus used in this study is the CFH strain, also referred to as beet severe curly top virus (BSCTV). The direct current (DC) electrical penetration graph technique was used to determine the specific stylet penetration behavior associated with inoculation of BSCTV. Viruliferous leafhoppers were allowed to feed on healthy 3–4-week-old sugar beet plants until specific electrical penetration graph waveforms were produced, at which point feeding was artificially terminated. A series of comparisons between leafhoppers that produced different combinations of waveforms clearly implicated waveform D1 as the only waveform correlated with inoculation of BSCTV. All successful inoculations contained waveform D1, and 56 out of 64 leafhoppers that produced waveform D1 successfully inoculated test plants. Eighty-five leafhoppers did not produce waveform D1 and none of these inoculated BSCTV. While the occurrence of waveform D1 appears to be necessary for BSCTV inoculation, there was no correlation between duration of waveform D1 and inoculation success rate. The correlation of waveform D1 and BSCTV inoculation found in this study implies that waveform D1 is associated with phloem salivation.     

Tests for transmission of four potato viruses through potato true seed

The Andean potato calico strain of tobacco ringspot virus (TRSV-Ca) was detected in 2–9% of potato seedlings grown from true seed from plants of cv. Cara and clone G5998(6) infected with TRSV-Ca. Similarly, a potato isolate of the oca strain of arracacha virus B (AVB-O) was detected in 4–12% of progeny seedlings of cv. Cara and clone D42/8 infected with AVB-O. Potato virus T (PVT) passed through 33–59% of seed from PVT-infected cv. Cara, but only 0–2% infection was detected in seedlings from seed of PVT-infected clone D42/8. By contrast, no infection was detected in seedlings grown from seed from plants of G5998(6), D42/8 or cv. Cara infected with Andean potato latent virus strains Hu (APLV-Hu) or Caj (APLV-Caj), although both strains passed through seed of Nicotiana clevelandii. AVB-O, PVT and TRSV-Ca were detected in all tests of pollen from flowers of infected potato plants, but APLV-Hu and APLV-Caj were detected less frequently. AVB-O and PVT were transmitted through 2% and 8% respectively, of seed from healthy potato plants pollinated with pollen from infected plants. However, no transmission through seed was detected when pollen from TRSV-Ca infected plants was used. None of the four viruses were transmitted to healthy potato plants pollinated with pollen from infected plants. APLV-Hu caused exceptionally severe symptoms in the cv. Cara plants used for seed production, but the Bolivian strain of PVT induced only mild symptoms rather than the severe systemic necrosis previously reported for the type of strain of PVT in this cultivar. No symptoms developed in potato seedlings infected with TRSV-Ca, AVB-O or PVT through the seed.     

The relationships of some viruses causing necrotic diseases of the potato

Potato virus B , and some other viruses with reactions in potato varieties different from any previously described, are strains of virus X . All produce intracellular inclusions which vary with different hosts and virus strains. Except with virus B, the inclusions are larger and more frequent in potato than in tobacco or tomato. All give systemic infection when inoculated to tobacco, tomato and potato varieties in which they are carried or cause mosaic symptoms; some give systemic infection when inoculated to varieties in which they cause top-necrosis, whereas others give only local lesions.
Potato virus C is a strain of virus Y: in tobacco and a few potato varieties both produce similar symptoms, but in those varieties in which Y causes leaf-drop streak, C causes top-necrosis. C causes systemic infection when inoculated to tobacco and to potato varieties in which it causes mosaic symptoms, but not when inoculated to potato varieties in which it causes top-necrosis. Virus C was not transmitted by M . persicae. Viruses C and Y produce a few small intracellular inclusions in potato and tobacco.
Virus A is not related to Y or X : no inclusions were found in plants infected with A alone.     

The effect of plant age and infection with virus yellows on the survival of Myzus persicae on sugar beet

Survival of Myzus persicae confined in clip-cages on mature leaves of sugar beet declined as the plants aged. Death of aphids was often preceded by the appearance of a black deposit in the aphids' stomachs, which may have been the cause of death. Both the rate of death and the proportion of aphids dying with black deposits was significantly less when plants were infected with beet yellows virus or beet mild yellowing virus, by comparison with healthy plants. The implication of these phenomena on the onset of mature plant resistance is discussed.     

THE INFLUENCE OF DENSITY OF PLANT POPULATION ON THE INCIDENCE OF YELLOWS IN SUGAR-BEET CROPS

The incidence of yellows virus in sugar-beet crops was reduced by increasing the density of plant population. The variations in plant population were obtained by differences in row width and singling distance. The differences in susceptibility between large- and small-topped varieties, and between early and late sown crops, could not be attributed solely to differences in plant size. It is suggested that close planting would increase the yields of sugar beet and reduce the losses caused by yellows virus. Roguing infected plants during the early part of the growing season did not reduce the incidence of disease.     

Effect of virus yellows on yield of some monogerm cultivars of sugar beet

The yield of plants of monogerm cultivars of sugar beet artificially infected with both beet yellows and beet mild yellowing viruses was, on average, depressed 2–7% for every 100 ‘infected plant weeks’, equivalent to c. £25/ha at 1976 prices. The cv. Vytomo, previously recommended to growers as being tolerant of infection by virus yellows, had a high sugar content and abundant foliage but in field trials its actual yield of sugar was no greater when infected, and lower when virus-free, than that of some other monogerm cultivars.     

Effects of virus genotype and temperature on seed transmission of nepoviruses

Transmission of different nepoviruses through chickweed (Stellaria media) seed was differently affected by ambient temperature during seed production. Raspberry ringspot and tomato black ring (Scottish isolate) viruses were similarly and frequently transmitted at 14 , 18 and 22 ^oC, whereas arabis mosaic virus was transmitted most frequently at 14 ^oC, and strawberry latent ringspot and tomato black ring (German isolate) viruses at 22 ^oC. When infected by seed-borne nepoviruses, seedlings of S. media and other species were symptomless at 15–25 ^oC, and the viruses were therefore detected by inoculating sap to Chenopodium quinoa indicator plants. However, typical symptoms of arabis mosaic and tomato black ring viruses were induced by growing Nicotiana tabacum, N. clevelandii and C. quinoa seedlings infected with seed-borne virus at 33–37 ^oC during the third and fourth weeks after sowing, preceded and followed by periods at 15–25 ^oC. The proportion of N. tabacum seedlings developing symptoms was the same as that of untreated seedlings yielding sap-transmissible virus. Seed transmissibility of pseudo-recombinant isolates of raspberry ringspot and tomato black ring viruses, containing RNA-i from one virus strain and RNA-2 from another strain, depended greatly on the transmissibility of the strain contributing RNA-i. The source of RNA-2 had an additional but smaller influence. The satellite RNA (RNA-3) of tomato black ring virus was seed-transmitted in S. media and its occurrence in cultures did not affect the frequency of transmission of the virus. Results of testing the infectivity of extracts of seed from infected mother plants suggested that failure of seed transmission reflected failure to become established in the seed, not subsequent inactivation. Whereas seed transmissibility of raspberry ringspot virus is primarily dependent on information carried in RNA-i, transmissibility by nematode vectors, another property of major ecological importance, is determined by RNA-2. In the field, selection pressures presumably can act independently on the two parts of the genome but evidence was also obtained of selection for mutual compatibility of RNA-i and RNA-2.     

A Graft Mimic Strategy for Verticillium Resistance in Tomato

Grafting vegetables for disease resistance has increased greatly in popularity over the past 10 years. Verticillium wilt of tomato is commonly controlled through grafting of commercial varieties on resistant rootstocks expressing the Ve1 R-gene. To mimic the grafted plant, proteomic analyses in tomato were used to identify a suitable root-specific promoter (TMVi), which was used to express the Ve1-allele in susceptible Craigella (Cs) tomato plants. The results indicate that when infected with Verticillim dahliae, race 1, the transformed plants are comparable to resistant cultivars (Cr) or grafted plants.     

Strains of Prunus necrotic ringspot virus in hop (Humulus lupulus L.)

Purified preparations of Prunus necrotic ringspot virus (NRSV) from hop plants formed two light-scattering zones when centrifuged in sucrose density gradients; the upper and lower zones contained particles 25 mμ and 31 mμ in diameter respectively whose sedimentation coefficients were 79 S and 107 S. NSRV isolates from hop were of two distinct serological types: ‘A’ strains, serologically very closely related to NRSV isolates from apple; and ‘C’ strains more nearly related to NRSV from cherry. The variety Fuggle is tolerant to hop mosaic (not related to NRSV) and different selections of apparently healthy female plants usually contained A strains; but C strains were usually isolated from nettlehead-diseased plants. Either A or C strains occurred in male plants grown with the hop-mosaic tolerant varieties. In mosaic-sensitive varieties (Goldings and Bramlings) apparently healthy female plants tested were usually infected with C strains; either A or C types occurred in mosaic-sensitive male plants. NRSV was not detected in the seventy-four hop seedlings obtained from virus-infected plants. Some varieties developed nettlehead when infected with NRSV (A) or (C) + the hop form of arabis mosaic virus, but not with NRSV (A) or (C) alone. Others developed nettlehead when infected with arabis mosaic virus + NRSV (C) but not with arabis mosaic + NRSV (A). A and C strains can multiply together in the same hop plant. There is evidence of partial antagonism, however, and the fluctuating behaviour of the nettlehead syndrome probably reflects changes in the relative concentration of the two serotypes.