Characterization and Experimental Host Range of a Brazilian Tomato Isolate of Tomato severe rugose virus

We report the complete molecular characterization of the DNA‐A and DNA‐B of a Brazilian tomato isolate of Tomato severe rugose virus (ToSRV) and the experimental host range of the virus determined using whitefly transmission tests. Genome analysis showed that ToSRV has a close evolutionary relationship with Tomato rugose mosaic virus. Of 33 plants species inoculated with viruliferous Bemisia tabaci biotype B, 13 species were susceptible to ToSRV, nine asymptomatically. Therefore, ToSRV disease management strategy should include the control of infected weeds close to tomato fields.     

Settling preferences of the whitefly vector Bemisia tabaci on infected plants varies with virus family and transmission mode

Virus infection may change not only the host‐plant phenotypic (morphological and physiological) characteristics, but can also modify the behavior of their insect vector in a mutualistic or rather antagonistic manner, to promote their spread to new hosts. Viruses differ in their modes of transmission and depend on vector behavior for successful spread. Here, we investigated the effects of the semi‐persistently transmitted Tomato chlorosis virus (ToCV, Crinivirus) and the persistent circulative Tomato severe rugose virus (ToSRV, Begomovirus) on alighting preferences and arrestment behavior of their whitefly vector Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) Middle East Asia Minor 1 (MEAM1) on tomato plants (Solanum lycopersicum L. cv. Santa Clara, Solanaceae). The vector alighting preferences between infected and uninfected plants in choice assays were apparently influenced by the presence of ToCV and ToSRV in the whiteflies or by their previous exposure to infected plants. The observed changes in vector behavior do not seem to benefit the spread of ToCV: non‐viruliferous insects clearly preferred mock‐inoculated plants, whereas ToCV‐viruliferous insects landed on mock‐inoculated and ToCV‐infected plants, indicating a partial change in insect behavior – ToCV was able to directly affect the preference of its vector B. tabaci, but this change in insect behavior did not affect the virus spread because viruliferous insects landed on mock‐inoculated and infected plants indistinctly. In contrast, ToSRV‐viruliferous insects preferred to land on mock‐inoculated plants, a behavior that increases the probability of spread to new host plants. In the arresting behavior assay, the majority of the insects remained on mock‐inoculated plants when released on them. A greater number of insects moved toward mock‐inoculated plants when initially released on ToCV‐ or ToSRV‐infected plants, suggesting that these viruses may repel or reduce the nutritional quality of the host plants for B. tabaci MEAM1.     

Molecular Detection and Complete Genome Sequences of Tomato chlorosis virus Isolates from Infectious Outbreaks in China

Tomato chlorosis virus (ToCV) is a whitefly‐transmitted, phloem‐limited, bipartite Crinivirus. In 2012, severe interveinal symptoms characteristic of ToCV infections were observed in greenhouse tomato plants in the Shandong province of China. High levels of infestation by whiteflies (Bemisia tabaci), which transmit ToCV, were also observed on tomato plants in all the greenhouses investigated. The presence of ToCV was confirmed by specific RT‐PCR either in the sampled plants or in the whiteflies collected from the ventral surface of the leaves of diseased plants. The complete genomic nucleotide sequences (RNA1 and RNA2) of the Shandong isolate of ToCV (ToCV‐SDSG) were determined and analysed. ToCV‐SDSG RNA1 consisted of 8594 nucleotides encompassing four open reading frames (ORFs). ToCV‐SDSG RNA2 consisted of 8242 nucleotides encompassing nine ORFs. Phylogenetic analysis suggests that the Chinese ToCV‐SDSG isolate is most similar to the ToCV‐Florida isolate.     

Field studies on the spread of African cassava mosaic

The spread of African cassava mosaic disease (ACMV) into healthy cassava fields was recorded at weekly intervals. In addition, 21 yellow water traps were placed in one field and the number of whiteflies caught was recorded twice a week. The number of Bemisia spp. feeding on cassava was also estimated. The results indicate that the pattern of disease spread is related to the pattern of infestation with Bemisia. Airborne whiteflies carried by the south-west prevailing wind alighted preferentially on cassava plants along the upwind edges (south and west borders) of the plantings. The pattern of incidence of mosaic disease resembled that of whiteflies. Along the SW-NE diagonal, there was a gradient of disease incidence with a maximum at the SW corner block. Similar gradients occurred in three different fields and they were maintained throughout the 6-month study, although gradually flattening with time. There were indications that the reservoirs both of the virus and of the vectors were located some distance upwind from the experimental fields.     

The Spread of African Cassava Mosaic Virus into and within Cassava Fields

Only a small proportion of Bemisia tabaci collected in totally infected cassava fields at a site in Côte d'lvoire transmitted African cassava mosaic virus (ACMV) to test plants. Nevertheless, the monthly increase in disease incidence in an experimental planting was directly related to numbers of adult whiteflies counted on plants 6 weeks earlier. In plots at different spacings, the greatest incidence of ACMV expressed as a percentage of the total stand occurred at the lowest plant density. Much speread into the spacing trial and into two other experimental plantings occurred from outside sources and followed downwind gradients. By contrast, spread from ACMV-infected sources within plantings was limited. It occurred in all directions but over distances of only a few metres. These contrasting patterns of spread are attributed to the differentbehavior of B. tabaci above and within the crop canopy. It is concluded that contamination of cassava fields in the coastal forest area of Côte d'Ivoire is due mainly to rapid spread from outside sources which leads to internal foci that contribute to some further, although limited, spread. These findings are discussed in relation to possible control strategies based on the release of healthy cuttings, dense planting and subsequent roguing. Such measures are unlikely to be effectve in the coastal forest region of Côte d'Ivoire and adjacent countriesunless varieties are grown with greater resistance to infection than those currently used.     

Begomoviruses Infecting Tomato Crops in Panama

The key regions in Panama involved in open field‐ and greenhouse‐grown commercial tomato production, including the Chiriquí, Veraguas, Herrera, Los Santos, Coclé and Panama Oeste provinces, were surveyed for the incidence and distribution of begomoviruses in the growing seasons of 2011 and 2012. The surveys took place in 14 of the 51 districts of the above‐mentioned provinces and comprised all relevant tomato production areas of the provinces. A total of 28 tomato plots were surveyed. The exact location of each plot was geo‐referenced using a hand‐held Global Positioning System unit. In total, 319 individual tomato plants (181 in 2011 and 138 in 2012) were sampled. Plants displayed diverse combinations of virus‐like symptoms of different severity, including necrosis, yellowing, mosaic, mottling, rolling, curling, distortion and puckering of leaves, reduced leaf size, and stunted growth. DNA was extracted from each plant for a subsequent polymerase chain reaction (PCR) analysis, using two sets of degenerate primers able to detect members of the genus Begomovirus. The samples displaying a positive reaction were subsequently analysed with specific primer pairs to identify the affecting begomoviruses. A total of 42.3% of all collected samples showed a positive signal to PCRs. Three begomovirus species were detected with the species‐specific set of primers; in particular, in the samples obtained in 2011, Potato yellow mosaic Panama virus (PYMPV), Tomato leaf curl Sinaloa virus (ToLCSiV) and Tomato yellow mottle virus (TYMoV) were detected, while in the 2012 samples, only PYMPV and ToLCSiV were found. To our knowledge, this is the first reported incidence of ToLCSiV and TYMoV in Panamanian tomato crops.     

Patterns of spread of Tomato spotted wilt virus in field crops of lettuce and pepper: spatial dynamics and validation of control measures

Patterns of spread of Tomato spotted wilt virus (TSWV) were examined in lettuce and pepper plantings into which thrips vectors spread the virus from external virus sources. These plantings were: 1) seven separate field trials into which TSWV ‘infector’ plants of tomato were introduced alongside or near to plantings of lettuce or pepper, and 2) three commercial lettuce plantings into which spread from nearby external infection sources was occurring naturally. The vector thrips species were Frankliniella occidentalis, F. schnitzel and Thrips tabaci, at least two of which were always present. Spatial data for plants with TSWV infection collected at different stages in the growing period were assessed by plotting gradients of infection, and using Spatial Analysis by Distance IndicEs (SADIE) and maps of spatial pattern. Despite the persistent nature of TSWV transmission by thrips vectors, in both lettuce and pepper plantings there was a steep decline in TSWV incidence with distance from external infection sources that were alongside them. The extent of clustering increased over time and was greatest closest to the source. The relationship between percentage infection and assessment date suggested that spread was predominantly monocyclic with only limited polycyclic spread. Development of isolated clusters of infected plants distant from TSWV sources within both crops was consistent with only limited polycyclic spread. Spread to lettuce was greater downwind than upwind of virus source, with magnitude and proximity of source determining the amount of spread. When 15 m wide fallow or non-host (cabbage) barriers separated TSWV sources from lettuce plantings, spread was slower and there was much less clustering with the latter. In commercial lettuce plantings, spread was favoured by TSWV movement within successive side-by-side plantings. The spatial data from the diverse scenarios examined enabled recommendations to be made over ‘safe’ planting distances between external infection sources of different magnitudes and susceptible crops that were short-lived (e.g. lettuce) or long-lived (e.g. pepper). They also helped validate the inclusion of isolation and ‘safe’ planting distances, planting upwind, prompt removal of virus sources, avoidance of side-by-side plantings, and deploying intervening non-host barrier crops as control measures within an integrated disease management strategy for TSWV in field vegetable crops.     

Evaluation of barrier plants for the cultural control of tomato yellow leaf curl disease

Barrier cropping plays an essential role in controlling insect pests and insect-transmitted diseases in cultural control. It has been proven efficient in suppressing the spread of nonpersistently transmitted viruses. For suppressing the spread of persistently transmitted viruses, barrier cropping is not considered an effective control strategy because barrier plants cannot act as a virus sink to purge the virus in the vector. However, few successful cases of barrier cropping suppressing the spread of persistently transmitted viruses have been reported. The objectives of the present study were to screen candidates (cucumber, okra, Chinese kale, soybean, and corn) for potential barrier plants to control tomato yellow leaf curl Thailand virus (TYLCTHV) and examine whether prefeeding on these plants can reduce the virus titer in its vector, Bemisia tabaci, thus reducing TYLCTHV transmission. The results revealed that nonviruliferous whiteflies preferred cucumber and okra to tomato, whereas viruliferous whiteflies preferred cucumber to tomato. Although prefeeding on cucumber, okra, and Chinese kale did not reduce the titer of TYLCTHV in viruliferous whiteflies, the vector transmission rate decreased after the whiteflies fed on Chinese kale. It implies that planting Chinese kale as a barrier plant for tomato cultivation may reduce the incidence of TYLCTHV. In addition, the preference to cucumber plants may reduce the incidence of whiteflies acquiring TYLCTHV from virus-infected tomato plants and of viruliferous whiteflies inoculating the virus into healthy tomato plants, thereby reducing the disease incidence. Further field trials of barrier cropping using the candidate plants are warranted.     

Effects of Tomato chlorosis virus on the performance of its key vector,Bemisia tabaci,in China

Tomato chlorosis virus (ToCV), which is a newly emerged and rapidly spreading plant virus in China, has seriously reduced tomato production and quality over the past several years. In this study, the effect of ToCV on the demography of the whitefly, Bemisia tabaci biotype Q (Hemiptera: Aleyrodidae), fed on infected and healthy tomato plants was evaluated using the age‐stage, two‐sex life table. When reared on ToCV‐infected tomato plants, the fecundity, length of oviposition period and female adult longevity of B. tabaci biotype Q decreased significantly, while the pre‐adult duration significantly increased compared to controls reared on healthy tomatoes. Consequently, the intrinsic rate of increase (r) and finite of increase (λ) of B. tabaci biotype Q on ToCV‐infected tomato plants significantly decreased compared to those on healthy tomatoes. Population projection predicted that a population of B. tabaci biotype Q fed on ToCV‐infected tomatoes increases slower than on healthy plants. These findings demonstrated that ToCV infection decreased the performance of B. tabaci biotype Q on tomato plants.     

Patterns of spread of Carrot virus Y in carrot plantings and validation of control measures

Patterns of spread of Carrot virus Y (CarVY) were examined in carrot plantings in Western Australia into which naturally occurring aphid vectors spread the virus from external infection sources. Within three field trials, CarVY ‘infector’ plants were introduced between or at different distances from carrot plantings. There was a marked decline in CarVY incidence over distance from adjacent introduced infection sources. Clusters of infected plants that enlarged and coalesced were concentrated next to such sources but, later, isolated, expanding clusters formed further away. With a small external virus source, initial spread into the edge of a planting was less extensive than with a larger source. When 15‐m‐wide fallow areas separated a CarVY source from carrot plots, spread was much slower than when the separation was only 1 m; it was also slower upwind than downwind of this source. The data collected help validate the inclusion of isolation and ‘safe’ planting distances, intervening fallow, planting upwind, prompt removal of virus sources, avoidance of side‐by‐side plantings and manipulation of planting date within an integrated disease management strategy for CarVY in carrots.     

Analysing spatial patterns of spread of Lettuce necrotic yellows virus and lettuce big-vein disease in lettuce field plantings

Spatial patterns of spread of lettuce big‐vein disease (LBVD) and Lettuce necrotic yellows virus (LNYV) were examined in two plantings each consisting of two blocks of lettuce. LBVD came from planting land infested with viruliferous Olpidium brassicae resting spores, while LNYV was introduced by aphid vectors from external sources consisting of LNYV‐infected sowthistle (Sonchus oleraceus) weeds. Clustering of LBVD was obvious in an area where the soil was heavily infested with only sporadic occurrence elsewhere. There was a steep decline in LNYV incidence over distance from a concentrated external weed source, with clustering of LNYV‐infected plants at the crop edge closest to it. There was no evidence of secondary spread with LBVD or LNYV.     

Suppressing spread of Tomato spotted wilt virus by drenching infected source or healthy recipient plants with neonicotinoid insecticides to control thrips vectors

Field experiments were done to determine whether drenching plants with two systemically active neonicotinoid insecticides, thiamethoxam and imidacloprid, suppresses spread of Tomato spotted wilt virus (TSWV) by thrips vectors. Separate treatments to TSWV ‘infector’ tomato (source) and healthy lettuce (recipient) plants provided information on the relative importance of targeting control at virus acquisition by nymphs versus virus transmission to healthy plants by adults. Drenches were applied either to seedlings just before transplanting or to soil around plants. The thrips vectors recorded were Frankliniella occidentalis, F. schultzei and Thrips tabaci, but F. schultzei and T. tabaci predominated. Overall ratios of external to internal TSWV spread into and within plots without insecticide ranged from 1 : 2.3 to 1 : 2.8 between field experiments. Applying thiamethoxam as a soil drench to both young source plants and recipient seedling transplants suppressed TSWV incidence by 86%, while such application to either young source or recipient seedlings diminished incidence by 67–70%. When thiamethoxam was applied either as a soil drench to old source plants and concurrently as a seedling drench to recipient plants or as a seedling drench to recipient plants alone, incidence was suppressed by 65–73% and 54–73%, respectively. Thiamethoxam applied as a soil drench to old source plants diminished incidence by only 33% or not significantly. Imidacloprid applied either as a soil drench to old source plants and concurently as a seedling drench or as a seedling drench alone, suppressed TSWV incidence by 90–92% and 80% respectively. Although adult vector thrips and nymph numbers were low, fewer adults and/or nymphs were sometimes recorded due to insecticide application. Drenching healthy seedlings with neonicotinoid insecticides just before transplanting can be an effective chemical control measure to include in integrated disease management strategies to suppress TSWV epidemics in short‐duration crops like lettuce.     

Expression of the Full‐length Coat Protein Gene of Tomato leaf curl Taiwan virus is Not Necessary for Recovery Phenotype in Transgenic Tomato

Transgenic tomato plants expressing full‐length (CPV1) and truncated coat protein (CP) gene (CPV2) of Tomato leaf curl Taiwan virus (ToLCTWV) were generated by Agrobacterium‐mediated transformation. Transgene integration and expression was confirmed by PCR and Southern blotting and Northern analysis, respectively. Resistance was evaluated both in plants of T0 and T1 progenies using viruliferous whiteflies under two different inoculum pressures (10–15 and 40–50 whiteflies/plant). Upon inoculation with ToLCTWV using viruliferous whiteflies, various levels of phenotypic reaction were observed. No complete resistance was observed in any of the plants tested. The reaction of the transgenic tomato lines carrying full‐length and truncated CP gene to ToLCTWV phenotype was (i) susceptible as non‐transgenic control, (ii) delayed symptom expression, (iii) complete susceptible (from delayed symptom expression phenotype) and (iv) recovered phenotype (either plants from symptom expression as non‐transgenic plants or delayed symptom expression phenotype). Dot blot quantification of the ToLCTWV using the replicase gene as a probe revealed that the recovered phenotypes accumulated a low level of ToLCTWV, and virus concentration was gradually reduced from 10 to 14 weeks postinoculation. The possible mechanisms of CP‐mediated resistance are discussed.     

Detection of Ralstonia solanacearum in Soil and Weeds from Commercial Tomato Fields Using Immunocapture and the Polymerase Chain Reaction

A detection assay for Ralstonia solanacearum in soil and weeds was developed by combining immunocapture and the polymerase chain reaction (IC‐PCR). Anti‐R. solanacearum polyclonal antibodies were produced in a white female rabbit and Dynal^® super‐paramagnetic beads were coated with purified immunoglobulinG (IgG). Using IC‐PCR, the 718 bp target DNA was amplified at a detection threshold of approximately 10⁴ colony‐forming units (CFU) bacteria per millilitre of suspension. DNA was not amplified in soil suspensions derived from autoclaved and non‐autoclaved soils, which contained R. solanacearum at 1–10⁵ CFU/g soil. However, a positive PCR result was obtained when bacteria in the soil suspensions were first enriched in nutrient broth. IC‐PCR detected R. solanacearum in tomato stems 24 h after inoculation by stem puncture with a suspension containing approximately 10⁵ CFU/ml. IC‐PCR detected the bacterium in 28 of 55 (51%) weeds and 10 of 32 (31%) soil samples. Of the weeds, Physalis minima, Amaranthus spinosus and Euphorbia hirta had the highest incidence of infection. R. solanacearum was not detected in soil taken from fallow fields, but it was discovered in some weed species. Symptomless tomato and pepper plants collected from the fields in which tomato bacterial wilt had previously occurred were found to contain R. solanacearum. These discoveries suggest that weeds and latent hosts may play a role in the survival of R. solanacearum between cropping cycles.     

Bacillus subtilis QST 713 Confers Protection to Tomato Plants Against Pseudomonas syringae pv tomato and Induces Plant Defence‐related Genes

Bacterial speck, caused by Pseudomonas syringae pv. tomato (Pst), is an economically important disease of tomato, resulting in yield loss of marketable fruit. Management of bacterial speck is a challenge in commercial production fields due to the limited efficacy of current disease management strategies, as the pathogen acquires resistance to antibiotics and fixed copper bactericides and host resistance has not proven durable. Therefore, it is essential to develop alternative disease management strategies, like biological control. In this study, the efficacy of the commercially available biocontrol agent Bacillus subtilis QST 713 along with copper hydroxide was tested against Pst under greenhouse conditions. QST 713 reduced significantly disease severity and incidence compared to control and the copper hydroxide treatment; subsequently, the Pst population was lower in the QST 713‐treated plants compared to control. In parallel, QST 713 and copper hydroxide increased plant height compared to control and mock plants. Furthermore, the quantitative PCR analysis of PR1a, PR1b and Pin2 expression suggests a positive role for Pin2 in the plant protective activity of QST 713, as Pin2 expression was significantly higher in the QST 713‐treated plants challenged with Pst compared to the control Pst‐inoculated plants.     

Epidemiology of tobacco leaf curl virus in India

The incidence of disease caused by tobacco leaf curl geminivirus (TbLCV) in ten tobacco growing areas of India ranged from 1.2% to 77%. The highest incidence of disease was observed in Andhra Pradesh (77%) followed by Gujarat (59%), Karnataka (17%), Bihar (11.6%) and West Bengal (5.4%). Under field conditions, an average of 32 adult whiteflies (Bemisia tabaci) per plant were recorded in Andhra Pradesh followed by Gujarat (20), Karnataka (12), Bihar (8) and West Bengal (5). In sequential sowings at Bangalore, all the plants were infected within 90 days in plots planted from February to June. Infection in plots planted later was progressively less. There was a positive correlation between whitefly catches and the final incidence of leaf curl disease in plantings. TbLCV was transmitted by Bemisia tabaci to 35 plant species, including Beta vulgaris, Capsicum annuum, Carica papaya, Cymopsis tetragonoloba, Lycopersicon esculentum, Sesamum indicum, Phaseolus vulgaris and Petunia hybrida. Forty five TbLCV isolates from different parts of India induced four distinct types of symptoms on tobacco cultivars Samsun and Anand 119. Group 1 isolates caused severe curling and cup-shaped enations; group II isolates induced pale green leaves, pit-like depressions and thorny enations: group III isolates caused leathery leaves, narrow and tiny protruding enations between the veins, and group IV isolates induced irregular thickening and swelling of veins and green flap-like enations on veins. Nylon net covers protected tobacco seedlings in nursery beds for 45 days. Ricinus communis and Helianthus annuus sown around the tobacco nursery bed as barrier crops attracted adult whiteflies and decreased the number found on tobacco.     

Relationship Between Ralstonia solanacearum Diversity and Severity of Bacterial Wilt Disease in Tomato Fields in China

A field survey was conducted to determine the relationship between Ralstonia solanacearum diversity and severity of bacterial wilt disease in tomato plants grown in plastic greenhouses. Both vegetative and reproductive stages of the plants were surveyed, and the symptoms were empirically categorized into five scales: 0 (asymptomatic): 1st, 2nd, 3rd and 4th. The bacterial wilt pathogen was isolated from infected plants at each disease scale; pathogenic characteristics and population densities of the bacterial strains were assessed. Two hundred and eighty‐two isolates were identified as R. solanacearum, which were divided into three pathogenic types, virulent, avirulent and interim, using the attenuation index (AI) method and a plant inoculation bioassay. Ralstonia solanacearum was detected in all asymptomatic and symptomatic tomato plants, with population numbers, ranging from 10.5 to 86.7 × 105 cfu/g. However, asymptomatic plants harboured only avirulent or interim R. solanacearum, whereas tomato plants displaying 1st or 2nd disease degree contained interim and virulent strains. Additionally, 3rd and 4th degree plants harboured only virulent strains. The disease was more severe in vegetative‐stage plants (disease severity index (DSI) 0.20) with higher total numbers of interim and virulent R. solanacearum strains than those in reproductive‐stage plants (DSI 0.12). Three pathotypes of R. solanacearum coexisted in a competitive growth system in the tomato field, and their distribution closely correlated with the severity of tomato bacterial wilt.     

First Report of a Croton yellow vein mosaic virus (CYVMV) Associated with Tomato Leaf Curl Disease in India

Leaf curl disease symptoms were observed in tomato crop grown in a tomato field at Matera district of Bahraich, India, in March 2013 with an 85% disease incidence. The infected plants exhibited leaf curl symptoms accompanied with puckering, vein swelling and stunting of the whole plant. PCR carried out with begomovirus coat protein gene and DNA beta‐specific primer sets resulted in positive amplification of ~775 bp and 1.35 kbp, respectively, with all symptom‐bearing plant samples. BLASTn and phylogenetic analyses of CP gene sequences showed highest and close relationship with Croton yellow vein mosaic virus (CYVMV) isolates, while the phylogenetic study of betasatellite sequence showed distinct relationships with other begomovirus associated betasatellites reported from India and abroad. This is a first report of a CYVMV associated with tomato leaf curl disease in India.     

Temporal and spatial spread of Lettuce mosaic virus in lettuce crops in central Spain: factors involved in Lettuce mosaic virus epidemics

Lettuce mosaic virus (LMV) is transmitted by aphid vectors in a nonpersistent manner as well as by seeds. The virus causes severe disease outbreaks in commercial lettuce crops in several regions of Spain. The temporal and spatial patterns of spread of LMV were studied in autumn 2002 in the central region of Spain. Symptomatic lettuce (var. Cazorla) plant samples were collected weekly, first at the seedling stage from the greenhouse nursery and later outdoors after transplantation. The exact position of symptomatic plants sampled in the field was recorded and then material was tested by enzyme‐linked immunosorbent assay to assess virus infection. Cumulative spatial data for infected plants at different growth stages were analysed using spatial analysis by distance indices. For temporal analysis, the monomolecular, Gompertz, logistic and exponential models were evaluated for goodness of fit to the entire set of disease progress data obtained. The results indicated that the disease progress curve of LMV epidemics in the selected area is best described by a Gompertz model and that the epidemic follows a polycyclic disease progression. Our data suggest that secondary cycle of spread occurs when noncolonising aphid species land on the primary infected plants (probably coming from infected seed) and move to adjacent plants before leaving the crop. The role of weeds growing close to lettuce fields as potential inoculum sources of virus and the aphid species most likely involved in the transmission of LMV were also identified.     

Temporal Dynamics of Powdery Mildew (Oidium neolycopersici) and its Effects on the Host Growth Dynamics of Tomato

Controlled glasshouse experiments were conducted to investigate the temporal progress of powdery mildew and its effects on host dynamics of tomato, without and with one fungicide application. Healthy tomato transplants (5‐ to 6‐week old) were artificially inoculated with powdery mildew, and disease progress as well as host growth were monitored in both fungicide sprayed and unsprayed treatments and compared with non‐inoculated plants. Actual disease severity on a plant basis increased in unsprayed plants reaching maximum severity in the proportionate range of 0.53–0.83. One fungicide spray significantly reduced the maximum disease severity by two‐ to fourfolds. Despite adjustments for defoliation, declines in the proportion of disease severity between successive assessments were evident. Whereas the estimated growth rates of diseased plants were significantly lower than that of healthy plants, no significant differences were observed in the maximum leaf area formed of inoculated and non‐inoculated plants. A considerable effect of the powdery mildew epidemics was manifested through hastened shrivelling and defoliation of diseased leaves within the tomato canopy. An average of 18–29% and 40–52% of leaves had abscised from the plant canopy at the last date of assessment in sprayed and non‐sprayed plants, respectively. Accordingly, defoliation accounted for 14–33.3% and 58.3–63.1% losses in leaf area of sprayed and non‐sprayed plants, respectively. Duration of healthy leaf area and yield of inoculated plants were also significantly reduced by powdery mildew epidemics.