An analysis of responses of resistant and of susceptible tomato plants to Verticillium infection

Comparative studies were made of the responses of resistant and of susceptible Gem tomato plants to infection by Verticillium albo-atrum. When inoculated through roots, there were striking differences in their responses. In susceptible plants, the foliar symptoms and amount of mycelium in the stem increased rapidly for some time. Then the mycelium started to disappear from the stem; this was accompanied by a check to the normal progress of symptoms, and by the formation of tyloses. In resistant plants, a limited invasion of the root and lower stem was accompanied by rapid and extensive tylosis. The mycelium soon disappeared from the stem and the plant then recovered from the initially mild symptoms. There was an inverse relationship between the amount of mycelium and the extent of tylosis in infected plants. The growth of susceptible plants was markedly reduced by infection. Total leaf area was much less because the newly produced leaves did not expand normally. The root system in infected plants was smaller because there were many fewer tertiary roots. In resistant plants infection stimulated growth. Tomato cuttings inoculated with conidia reacted similarly to root-inoculated plants. Hyphae grew well in the vascular system of susceptible cuttings whereas in resistant cuttings the pathogen started to grow but soon disappeared. Detached leaves of susceptible plants, inoculated through cut ends, wilted more than did leaves from resistant plants. It is suggested that resistance is mainly of the active type that develops after infection.     

Replication of tomato yellow leaf curl virus (TYLCV) DNA in agroinoculated leaf discs from selected tomato genotypes

The leaf disc agroinoculation system was applied to study tomato yellow leaf curl virus (TYLCV) replication in explants from susceptible and resistant tomato genotypes. This system was also evaluated as a potential selection tool in breeding programmes for TYLCV resistance. Leaf discs were incubated with a head-to-tail dimer of the TYLCV genome cloned into the Ti plasmid ofAgrobacterium tumefaciens. In leaf discs from susceptible cultivars (Lycopersicon esculentum) TYLCV single-stranded genomic DNA and its double-stranded DNA forms appeared within 2–5 days after inoculation. Whiteflies (Bemisia tabaci) efficiently transmitted the TYLCV disease to tomato test plants following acquisition feeding on agroinoculated tomato leaf discs. This indicates that infective viral particles have been produced and have reached the phloem cells of the explant where they can be acquired by the insects. Plants regenerated from agroinfected leaf discs of sensitive tomato cultivars exhibited disease symptoms and contained TYLCV DNA concentrations similar to those present in field-infected tomato plants, indicating that TYLCV can move out from the leaf disc into the regenerating plant. Leaf discs from accessions of the wild tomato species immune to whitefly-mediated inoculation,L. chilense LA1969 andL. hirsutum LA1777, did not support TYLCV DNA replication. Leaf discs from plants tolerant to TYLCV issued from breeding programmes behaved like leaf discs from susceptible cultivars.The Hebrew University of Jerusalem, Faculty of Agriculture, Department of Field and Vegetable Crops     

Ralstonia solanacearum extracellular polysaccharide is a specific elicitor of defense responses in wilt-resistant tomato plants

Ralstonia solanacearum, which causes bacterial wilt of diverse plants, produces copious extracellular polysaccharide (EPS), a major virulence factor. The function of EPS in wilt disease is uncertain. Leading hypotheses are that EPS physically obstructs plant water transport, or that EPS cloaks the bacterium from host plant recognition and subsequent defense. Tomato plants infected with R. solanacearum race 3 biovar 2 strain UW551 and tropical strain GMI1000 upregulated genes in both the ethylene (ET) and salicylic acid (SA) defense signal transduction pathways. The horizontally wilt-resistant tomato line Hawaii7996 activated expression of these defense genes faster and to a greater degree in response to R. solanacearum infection than did susceptible cultivar Bonny Best. However, EPS played different roles in resistant and susceptible host responses to R. solanacearum. In susceptible plants the wild-type and eps(-) mutant strains induced generally similar defense responses. But in resistant Hawaii7996 tomato plants, the wild-type pathogens induced significantly greater defense responses than the eps(-) mutants, suggesting that the resistant host recognizes R. solanacearum EPS. Consistent with this idea, purified EPS triggered significant SA pathway defense gene expression in resistant, but not in susceptible, tomato plants. In addition, the eps(-) mutant triggered noticeably less production of defense-associated reactive oxygen species in resistant tomato stems and leaves, despite attaining similar cell densities in planta. Collectively, these data suggest that bacterial wilt-resistant plants can specifically recognize EPS from R. solanacearum.     

Studies on the nature of resistance in tomato plants to Verticillium albo-atrum

Extracts of healthy resistant and of healthy susceptible plants of tomato had the same effect on growth of Verticillium albo-atrum in vitro. Tracheal saps from resistant and from susceptible plants showed no difference in their effect on spore germination and mycelial growth of V. albo-atrum. Cuttings from resistant plants, inoculated with V. albo-atrum and fed with low concentrations of casamino acids, or glucose, at first wilted more than controls but soon recovered. Continuous treatment with dilute ethanol solutions for 2 weeks induced marked wilting in inoculated cuttings of resistant plants: treatment for shorter periods caused less severe symptoms, from which cuttings recovered slowly. Metabolic inhibitors did not break resistance of cuttings, but the pathogen survived longer in cuttings treated with sodium diethyldithiocarbamate, salicylaldoxime or 8-hydroxyquinoline than in controls. When one end of segments of stems of resistant plants was inoculated with the pathogen, and 48 h later the uninoculated end was placed near a colony of V. albo-atrum on agar, growth of the fungus colony towards the stem segment was sometimes inhibited. There was no such inhibition when segments from susceptible plants were used. Both tracheal sap and diffusates from segments of inoculated resistant plants supported less growth of germ tubes of V. albo-atrum than sap and diffusates from uninoculated plants. These differences were not obtained with the susceptible variety and production of fungitoxic substances in resistant plants after infection is inferred.     

Studies on the occurrence of tomato bushy stunt virus in English rivers

Tomato bushy stunt virus (TBSV) of unknown source was isolated from water of the River Thames, near Oxford. The isolate designated TBSV-T was mechanically transmissible to several tomato (Lycopersicon esculentum) cvs and to other species including Petunia hybrida, pepper (Capsicum annuum). eggplant (Solanum melongena), Nicotiana clevelandii, Chenopodium amaranticolor and C. quinoa in which it caused systemic symptoms. It caused no infection of globe artichoke (Cynara scolymus) or Pelargonium domesticum. The virus was not adsorbed to soil and could be isolated from leachate of soil in which systemically-infected tomato or C. quinoa plants were grown. Tomato plants became infected when grown in soil watered with virus suspensions. TBSV-T was infective after 10 min at 80°C but not at 90°C and when diluted to 10^-5 but not to 10^-6. Purified virus preparations contained C. 30 nm isometric particles. In gel-diffusion serological tests, TBSV-T reacted with homologous anti-serum and with antiserum to petunia asteroid mosaic virus but not to pelargonium leaf curl virus. Seed-borne infection (50–65%) of TBSV was demonstrated in plants grown from seed of symptomlessly-infected tomato fruit. TBSV was isolated from symptomlessly-infected tomato fruit imported from Morocco during October-April 1981. One of the isolates (TBSV-M) was indistinguishable from TBSV-T in host range, symptomatology and serological reactions. TBSV was also found in tomato plants growing extraneously in primary settlement beds at sewage works; such plants having been derived from undigested seeds in sewage. Because of its ‘alimentary-resistance’ in man, it is possible that one ecological route whereby TBSV enters rivers is by man's consumption of TBSV-infected tomatoes and eventual sewage dispersal into rivers.     

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.     

Whitefly population dynamics and evaluation of whitefly-transmitted tomato yellow leaf curl virus (TYLCV)-resistant tomato genotypes as whitefly and TYLCV reservoirs

Sweetpotato whitefly, Bemisia tabaci (Gennadius), and whitefly-transmitted tomato yellow leaf curl virus (TYLCV) are major threats to tomato production in the southeastern United States. TYLCV was introduced to Florida from the Caribbean islands and has spread to other southern states of the United States. In Georgia, in recent years, the incidence of TYLCV has been steadily increasing. Studies were conducted to monitor population dynamics of whiteflies in the vegetable production belt of Georgia, to evaluate TYLCV-resistant genotypes against whiteflies and TYLCV, and to assess the potential role of resistant genotypes in TYLCV epidemiology. Monitoring studies indicated that the peak incidence of whiteflies varied seasonally from year to year. In general, whitefly populations were not uniformly distributed. Tomato genotypes exhibited minor differences in their ability to support whitefly populations. TYLCV symptoms were visually undetectable in all but one resistant genotype. The infection rates (visually) in susceptible genotypes ranged from 40 to 87%. Greenhouse inoculations with viruliferous whiteflies followed by polymerase chain reaction (PCR) indicated that up to 100% of plants of resistant genotypes were infected, although predominantly symptomless. TYLCV acquisition by whiteflies from TYLCV-infected genotypes was tested by PCR; TYLCV acquisition rates from resistant genotypes were less than from susceptible genotypes. Nevertheless, this difference did not influence TYLCV transmission rates from resistant to susceptible genotypes. Results emphasize that resistant genotypes can serve as TYLCV and whitefly reservoirs and potentially influence TYLCV epidemics.     

Further studies on a British form of pea early-browning virus

An isolate of pea early-browning virus from Britain (PEBV (B)) has tubular particles most of which are either about 103 or 212 mμ long with sedimentation coefficients of 210 and 286 S respectively. Both types show cross-banding at intervals of 2.5 mμ. Virus preparations containing only the shorter particles were not infective. PEBV (B) was transmitted to pea seedlings by both adult and juvenile Trichodorus primitivus (de Man) (Nematoda) and persisted for 32 days in T. primitivus kept without plants. In two experiments T. primitivus failed to transmit a Dutch isolate (PEBV (D)), which is distantly related serologically to PEBV (B). PEBV (B) was transmitted by nematodes to cucumber roots more readily in soil at 20d? than at 24d? C., and more readily at 24d? than at 29d? C. When transmitted by inoculation of sap, PEBV (B) and PEBV (D) caused similar symptoms in some pea varieties but differed in virulence towards others. Thirty-one varieties resistant to natural infection with PEBV in The Netherlands were susceptible to PEBV (B) when manually inoculated with sap or when grown in naturally infested soil from one site; twenty-six of these varieties did not become infected in soil from a second site, in which several other varieties that are susceptible in The Netherlands were infected. Varieties should therefore be tested for resistance by growing them on many infested fields. All but one of the pea varieties resistant to PEBV in The Netherlands became infected with the English form of tomato black ring virus when grown in soil containing infective Longidorus attenuatus Hooper.     

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.     

Temporal Effects of a Begomovirus Infection and Host Plant Resistance on the Preference and Development of an Insect Vector,Bemisia tabaci,and Implications for Epidemics

Persistent plant viruses, by altering phenotypic and physiological traits of their hosts, could modulate the host preference and fitness of hemipteran vectors. A majority of such modulations increase vector preference for virus-infected plants and improve vector fitness, ultimately favouring virus spread. Nevertheless, it remains unclear how these virus-induced modulations on vectors vary temporally, and whether host resistance to the pathogen influences such effects. This study addressed the two questions using a Begomovirus-whitefly-tomato model pathosystem. Tomato yellow leaf curl virus (TYLCV) -susceptible and TYLCV-resistant tomato genotypes were evaluated by whitefly-mediated transmission assays. Quantitative PCR revealed that virus accumulation decreased after an initial spike in all genotypes. TYLCV accumulation was less in resistant than in susceptible genotypes at 3, 6, and 12 weeks post inoculation (WPI). TYLCV acquisition by whiteflies over time from resistant and susceptible genotypes was also consistent with virus accumulation in the host plant. Furthermore, preference assays indicated that non-viruliferous whiteflies preferred virus-infected plants, whereas viruliferous whiteflies preferred non-infected plants. However, this effect was prominent only with the susceptible genotype at 6 WPI. The development of whiteflies on non-infected susceptible and resistant genotypes was not significantly different. However, developmental time was reduced when a susceptible genotype was infected with TYLCV. Together, these results suggest that vector preference and development could be affected by the timing of infection and by host resistance. These effects could play a crucial role in TYLCV epidemics.     

SOME FACTORS AFFECTING THE TRANSMISSION OF LEAF-ROLL VIRUS BY APHIDS

Datura tatula is a more suitable host than potato for studying the factors influencing the transmission of potato leaf-roll virus by Myzus persicae ; it is more easily infected, provides a better source of virus for feeding aphids, produces symptoms more quickly and over a longer period of the year.
Loughnane's (1943) claim that leaf-roll virus is transmitted by starved aphids that feed for only 5 min. on infected potato plants was not confirmed. The shortest infection-feeding time in which M. persicae aphids became infective was 2 hr.; such aphids did not infect healthy plants in the first 2 days and, when transferred to a series of healthy plants at intervals, infected only few. The ability to cause infections was increased by increasing the length of infection feeding. Aphids fed for many days on infected plants could infect healthy plants in the first 15 min. of test feeding, and they continued to cause infections for long periods.
Aphids became infective more readily when feeding on recently infected Datura tatula , showing only slight symptoms, than on older plants with pronounced chlorosis; similarly, young potato sprouts showing no symptoms were better sources of virus for aphids than older plants showing severe leaf roll.
The differences in severity of symptoms shown by potato plants with leaf roll in the field mainly occur because of differences in virulence of accompanying strains of potato virus X , but isolates of leaf-roll virus were found that also varied in virulence.     

The impact of Meu1-mediated resistance in tomato on longevity, fecundity and behavior of the potato aphid, Macrosiphum euphorbiae

The effect of the tomato resistance gene, Meu1, on feeding, longevity, fecundity and developmental rate of the pink biotype of the potato aphid, Macrosiphum euphorbiae (Thomas) (Hemiptera, Aphididae), was determined using nearly isogenic tomato (Lycopersicon esculentum Mill, Solanaceae) lines. Aphid mortality was significantly higher on resistant plants, with 60% of the aphids dying by the 4th day of exposure. By the 10th day, all the aphids on the resistant plants were dead whereas 100% of the aphids on susceptible plants were alive. Meu1-mediated resistance resulted in significantly decreased fecundity with a ten-fold decrease in the net fertility rate (4.5 and 45.7 progeny per aphid on resistant and susceptible tomato, respectively). A qualitative analysis showed that honeydew was produced by aphids on resistant and susceptible plants, suggesting that aphids initiate feeding on both plant types. However, significantly lower quantities of honeydew were present when aphids were caged on resistant plants. There were also significant differences in aphid location on resistant and susceptible leaves. Experiments evaluating behavior in less than 24 h showed that aphids left resistant leaves after relatively short exposure (3–6 h). Aphids transferred from resistant to susceptible tomato at intervals between 3 h and 24 h resumed feeding as evidenced by presence of honeydew. Although the mechanism by which Meu1-mediated resistance operates is not yet known, our data suggest that resistance factors act rapidly after initiation of feeding and that lower fecundity and longevity are related to reduction in aphid feeding.     

Properties of some defective strains of tobacco mosaic virus and their behaviour as affected by inhibitors during storage in sap

From the type strain of tobacco mosaic virus, defective strains were isolated that produced chlorotic or ringspot type symptoms in tobacco and were difficult to transmit without carborundum in the inoculum. Their concentration was less than 0–1 μg/ml of sap instead of the usual 2 mg/ml with the type strain. Phenol extracts of infected leaves were a little more infective than extracts in buffer, whereas phenol extracts of leaves infected with type strain were very much less infective than extracts in buffer. Electron microscopy of infective sap rarely showed any virus particles, but preparations concentrated by ultracentrifugation contained virus particles, many of which were broken or seemed inadequately assembled. Changing the ambient temperature at which infected plants were kept from 20 to 35°C did not increase the amount or improve the appearance of the virus. Some of the strains were inactivated during heating for 10 min between 70 and 80 °C. Undiluted sap lost its infectivity in 3 days at 20 °C, as did the type strain when diluted to 0–1 μg/ml in sap from healthy leaves. This is because substances that inhibit infection were produced by microbes in the sap. The ability of sap from healthy leaves to inhibit infection increased by more than twenty-five times when left 3 days at 20 °C. Infectivity of appropriate mixtures of type strain and aged sap was restored by diluting them in buffer. Sodium azide at 0·02% in sap prevented formation of the inhibitor. The infectivity of the defective strains increased when inoculated together with the type strain.     

Selection of resistance breaking strains of tomato spotted wilt tospovirus

In glasshouse tests, sap from plants infected with 15 different isolates of tomato spotted wilt tospovirus (TSWV) from three Australian states was inoculated to nine genotypes of tomato carrying TSWV resistance gene Sw-5 or one of its alleles. A further two resistant tomato genotypes were inoculated with four isolates each. The normal response in resistant genotypes was development of necrotic local lesions in inoculated leaves without systemic invasion, but 22/752 plants also developed systemic reactions in addition to local hypersensitive ones. Using cultures from two of these systemically infected plants and following four cycles of subculture in TSWV resistant tomato plants, two isolates were obtained that gave susceptible type systemic reactions but no necrotic spots in inoculated leaves of resistant tomatoes. When these two isolates, Da_WA-1d and To_TAS-1d, were maintained by repeated subculture for 10 successive cycles in Nicotiana glutinosa or a susceptible tomato genotype, they still induced susceptible type systemic reactions when inoculated to resistant tomato plants. They were therefore stable resistance breaking isolates as regards overcoming gene Sw-5. When resistance-breaking isolate Da_WA^-1ld multiplied together with original isolate Da_WA-l in susceptible tomato, it was fully competitive with the original isolate. However, when Da_WA-ld and To_TAS-ld were inoculated to TSWV resistant Lycopersicon peruvianum lines PI 128660R and PI 128660S and to TSWV resistant Capsicum chinense lines PI 152225, PI 159236 and AVRDC CO0943, they failed to overcome the resistance, producing only necrotic local lesions without systemic infection. Thus, although the ease of selection, stability and competitive ability of resistance breaking isolates of TSWV is cause for concern, L. peruvianum and C. chinense lines are available which are effective against them. The effectiveness of the resistance to TSWV in nine tomato genotypes was examined in a field experiment. Spread was substantial in the susceptible control genotype infecting 42% of plants. Resistance was ineffective in cv. Bronze Rebel, 26% of plants developing infection. In contrast, it held up well in the other eight resistant genotypes with only 1–3 or no plants of each becoming infected. Accumulated numbers of Thrips tabaci, Frankliniella occidentalis and F. schultzei were closely correlated with TSWV spread.     

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.     

STUDIES ON THE HOST RANGES OF VIRUSES FROM THEOBROMA CACAO L.

Only plants in the natural order Tiliales were susceptible to experimental infection by viruses found infecting cacao. Of sixty-three species in the Tiliales colonized with infective mealybugs, twenty-eight became infected with the New Juaben virus. The ease with which different species contracted infection and the severity of symptoms varied considerably. The host ranges of seven other viruses were all contained in that of the New Juaben virus. The viruses could be divided into three groups according to their host ranges, and it is suggested that host ranges may prove helpful in attempts to classify cacao viruses. Four species of Corchorus were all rapidly killed by any of the eight viruses.     

线虫诱导番茄抗性对烟粉虱发育及存活的影响

有害生物为害植物后可以诱导后者产生防御抗性,进而对同株植物上其它生物产生一定的负面影响。本文以蔬菜生产中的两大重要有害生物——南方根结线Meloidogyne incognita和烟粉虱Bemisia tabaci为研究对象,在实验室内, 利用不同浓度（200头/株、500头/株、1000头/株）的南方根结线虫侵染番茄,分别在侵染后的第3、7、11 d接种烟粉虱,研究南方根结线虫为害番茄后诱导的植物抗性对叶部害虫烟粉虱生长发育及存活等特性的影响。结果表明,受三种浓度的南方根结线虫危害3 d、7 d和11 d后的番茄植株上,与对照植株上相比烟粉虱卵至成虫的发育明显延长;当500头/株和1000头/株浓度南方根结线虫危害3 d 和7 d 后,该番茄植株上烟粉虱的发育历期与对照相比差异达到显著水平,而在南方根结线虫危害11 d的番茄上,尽管烟粉虱发育历期也随接种南方根结线虫浓度的增加而延长,但处理组与对照组之间的发育历期均无显著差异。在南方根结线虫危害3 d、7 d后的番茄上接种烟粉虱,后者卵-成虫的存活率较对照组番茄上明显下降,且差异达到显著水平,而南方根结线虫不同为害时间、不同为害浓度的处理之间对烟粉虱存活率的影响差异并不显著。研究表明,当南方根结线虫与烟粉虱分别在根部和叶部为害同一株植物时,前者通过植物对后者的影响是负面的,二者之间的互作关系应属于拮抗类型。     

An avrPto/avrPtoB mutant of Pseudomonas syringae pv. tomato DC3000 does not elicit Pto-mediated resistance and is less virulent on tomato

AvrPto and AvrPtoB are type III effector proteins expressed by Pseudomonas syringae pv. tomato strain DC3000, a pathogen of both tomato and Arabidopsis spp. Each effector physically interacts with the tomato Pto kinase and elicits a hypersensitive response when expressed in tomato leaves containing Pto. An avrPto deletion mutant of DC3000 previously was shown to retain avirulence activity on Pto-expressing tomato plants. We developed an avrPtoB deletion mutant of DC3000 and found that it also retains Pto-specific avirulence on tomato. These observations suggested that avrPto and avrPtoB both contribute to avirulence. To test this hypothesis, we developed an deltaavrPtodeltaavrPtoB double mutant in DC3000. This double mutant was able to cause disease on a Pto-expressing tomato line. Thus, avrPto and avrPtoB are the only avirulence genes in DC3000 that elicit Pto-mediated defense responses in tomato. When inoculated onto susceptible tomato leaves and compared with wild-type DC3000, the mutants DC3000deltaavrPto and DC3000deltaavrPtoB each caused slightly less severe disease symptoms, although their growth rate was unaffected. However, DC3000deltaavr PtodeltaavrPtoB caused even less severe disease symptoms than the single mutants and grew more slowly than them on susceptible leaves. Our results indicate that AvrPto and AvrPtoB have phenotypically redundant avirulence activity on Pto-expressing tomato and additive virulence activities on susceptible tomato plants.     

VIRUS DISEASES OF CACAO IN WEST AFRICA

Of the tested plants which are indigenous to West Africa, three species in the Bombacaceae and four in the Steruliaceae are susceptible to one or more of four viruses from cacao, three occurring in the Gold Coast and one in Nigeria. These species are less affected than cacao by the viruses; some show transient leaf symptoms and others are symptomless carriers. The development of spines on the stems of Ceiba pentandra seedlings is suppressed by infection with virus 1 A.
In general, the indigenous species are more difficult to infect than cacao, and mealybugs do not become infective as readily when feeding on them as when feeding on infected cacao. The availability of the viruses to vectors seems to be correlated with severity of symptoms, and transmission from infected plants to cacao becomes less frequent with increasing duration of infection.
Ceiba pentandra trees were found naturally infected in the Gold Coast and Nigeria. In the Western Province of the Gold Coast, Cola chlamydantha trees in cacao farms and forests were naturally infected with viruses apparently identical with those causing swollen shoot of cacao there.
There is little doubt that C. chlamydantha trees are an important source of virus for cacao trees. Whenever possible these and other alternative hosts growing near to cacao should be destroyed.     

Bacterial soft rot of tomato in plastic greenhouses in Crete

During recent years a new disease has been noticed on tomatoes grown in Polythene greenhouses in Crete. Early symptoms are yellowing of the lower leaves, and a yellow brown discoloration of the pith and stem xylem. As leaves wilt and die there is progressive yellowing towards the top of the plants. A progressive disintegration of the cortical tissues follows which results in a soft rot and a longitudinal splitting of the stem running mainly upwards. Soft rot of the fruits rarely appears. Severely infected plants may wilt and die, but other less affected plants often survive and yield normally. Very vigorous plants grown under humid conditions are more susceptible. Often more than 20% of the plants are infected. Isolations were made from stem (xylem, cortex and pith), from leaf xylem and from fruits of infected tomato plants collected throughout the island from 1979 to 1985. Bacteria of the genus Erwinia and Pseudomonas were consistently isolated. On the basis of physiological and biochemical characters of 49 representative pathogenic isolates, 22 were identified as Erwinia carotovora subsp. carotovora, 10 as Erwinia carotovora subsp. atroseptica, four as Pseudomonas viridiflava and 13 as Pseudomonas fluorescens biotype I. All disease symptoms were reproduced when artificial inoculations were made with the above isolates in the laboratory (20°C and 100% r.h.) on 3–4 week tomato plants and in a commercial greenhouse on 4–5 months tomato plants. Bacteria used for inoculations were reisolated. Results indicated that the disease symptoms as described may be caused by four different bacteria species.