Recent research progress on Xanthomonas ampelina1

Bacterial blight of grapevine (caused by Xanthomonas ampelina) occurs in Greece, France, Spain, Italy, Turkey, Portugal, USSR and South Africa. It is a serious, chronic vascular disease which is widespread in some areas of grape production, affecting commercially important cultivars. There is evidence that the disease is presently spreading into new areas in some countries. The actual distribution of blight may be much wider, since its symptoms may be confused with those of other diseases. It is therefore essential to confirm the presence of X. ampelina by isolation and identification. X. ampelina is an anomalous member of the genus Xanthomonas which is readily distinguished on the basis of a few tests. The pathogen may also be rapidly detected in grapevine sap using the indirect immunofluorescence technique. The susceptibility of shoots to infections, under Cretan conditions, is highest from November to late January and very low during February and March. It has been found that up to 50% of apparently healthy canes obtained from diseased vineyards were latently infected. Transmission of the pathogen through soil and roots during the control of Daktulosphaira vitifolii or frost damage by flooding has been demonstrated in France. Several of the more widely grown cultivars were found to be very susceptible to the pathogen while no completely resistant cultivar has been detected. Control trials based on four sprays with a number of chemicals were unsuccessful. Control policy should be directed: (a) to preventing spread of the disease, by exclusive use of certified material; (b) to minimizing the disease in affected areas by suitable sanitary, cultural and preventive measures.     

Susceptibility of Broccoli Cultivars to Bacterial Head Rot: In Vitro Screening and the Role of Head Morphology in Resistance

Head rot is a major disease of broccoli caused by the soft rot pathogens Pseudomonas fluorescens and Erwinia carotovora. Two in vitro pathogenicity tests were evaluated as a methods to identify broccoli cultivars susceptible or resistant to bacterial head rot. One test used mature heads excised from the plant and inoculated with squares of cotton lint which had been soaked in a bacterial suspension. The other test involved stab-inoculating axenically grown seedlings. With the excised head test, susceptible cultivars showed a black soft rot, whilst less susceptible or moderately resistant cultivars showed only watersoaking, or browning and slight softening of the tissue. No cultivar was completely resistant. Ten cultivars were tested, and their susceptibility ratings corresponded with previously recorded field data, with one exception. This laboratory test could be used to screen for susceptibility to head rot in broccoli breeding programmes. The seedling test distinguished differences in aggressiveness among bacterial isolates but not cultivar susceptibility. Increasing head size correlated negatively with disease resistance. Head shape, i.e. cultivars which showed a domed shape rather than a flat shape, was positively correlated with disease resistance. Thus small domed heads are more resistant to head rot than large flat heads. Other morphological characteristics, viz. floret prominence and number, and sepal stomatal number were not correlated with host resistance.     

Laboratory Evaluation of Citrus Cultivars Susceptibility and Influence of Fruit Size on Fortune Mandarin to Infection by Alternaria alternata pv. citri

Young leaves of 62 citrus cultivars were inoculated with conidia of three Spanish isolates of Alternaria alternata pv. citri, the causal agent of brown spot of citrus. Hybrids with Dancy mandarin, King mandarin or their derivates as a parent, grapefruit cultivars and the mandarin cultivars Guillermina, Emperor, Clemenpons and Esbal were highly susceptible to the pathogen. Satsuma cultivar Clausellina and orange cultivars, with the exception of Sanguinelli, were slightly susceptible. Lemon and lime cultivars were not susceptible, with the exception of Mexican lime (Citrus aurantifolia), which was slightly susceptible. Although this study shows a range of potential hosts for this pathogen, to date the only affected cultivars in Spain are Fortune and Nova mandarins, and Minneola tangelo. The susceptibility of Fortune fruits decreased as diameter increased, being susceptible through the whole season. This was confirmed with field observations in autumn where fruit infections have been detected when the diameter reaches 6–7 cm.     

Susceptibility of European pear cultivars to Pseudomonas syringae pv. syringae using immature fruit and detached leaf assays

The susceptibility of thirty-three pear cultivars and two pear rootstocks to four virulent strains of Pseudomonas syringae pv. syringae was evaluated by inoculating detached immature fruits and young leaves. The four strains were similarly virulent and did not show cultivar specificity although they were isolated from different pear cultivars and exhibited different biochemical profiles. The most frequently planted pear cultivars, Conference, Abate Fetel, General Leclerc, Williams, D. Comice, El Dorado, Alexandrine, B. Anjou, Passe Crassane and the rootstock OHxF 333 were susceptible to P. syringae pv. syringae. Maximal severity values were obtained on 'Preguystar' leaves (about 90%). The rootstock Winter Nelis was less susceptible. Results with immature fruit and detached leaf assays agreed with field observations on cultivar susceptibility to bacterial blast. However, the detached leaf test gave a more accurate prediction and has the advantages that symptoms develop quickly (48 h), and leaves are available for a longer period of time than fruits. This method is proposed as a rapid and reproducible screening system of cultivar susceptibility to bacterial blast of pear.     

Present status of verticillium wilt of olive in Andalucía (southern Spain)1

Verticillium wilt caused by Verticillium dahliae is a disease highly prevalent in newly established olive orchards in Andalucía, southern Spain. Two syndromes of the disease occur in Andalucia, namely apoplexy and slow decline. Apoplexy is characterized by quick dieback of twigs and branches while slow decline consists of rapid drying out of inflorescences together with leaf chlorosis and necrosis. Systematic disease observations carried out in two experimental orchards planted with susceptible cv. Picual indicated that natural recovery of diseased trees occurred over time. Infection and vascular colonization of olive plants by V. dahliae were studied in susceptible (Picual) and resistant (Oblonga) cultivars inoculated with a mildly virulent or a highly virulent cotton-defoliating isolate of V. dahliae. Disease symptoms developed 24–32 days after inoculation in cv. Picual, but at that time plants of cv. Oblonga remained free from symptoms. However anatomical observations and isolations indicated that systemic infections by the two isolates had occurred to a large extent in both cultivars.     

Assessment of Pierce's disease susceptibility in Vitis vinifera cultivars with different pedigrees

Pierce's disease (PD) of grapevine is caused by the bacterium Xylella fastidiosa. In this study, an integrated approach was applied to assess PD susceptibility among different Vitis vinifera cultivars that incorporated disease severity, bacterial pathogen abundance and loss of stem xylem hydraulic conductivity. It was hypothesized that levels of PD susceptibility in V. vinifera can be attributed in part to the host anatomical features that are shaped by its pedigree background. Two popular wine grape cultivars were initially selected from the occidentalis group, Merlot and Cabernet Sauvignon, and one from the orientalis group, Thompson Seedless. The more recently bred table grape cultivar Scarlet Royal, that has mixed pedigree parentage, was also included. PD susceptibility was compared to the known PD resistant b43-17 V. arizonica/candicans wild grape species from North America. The data indicated that Thompson Seedless was ranked as the most susceptible to PD because it significantly exhibited the most severe disease symptoms at 12 weeks post-inoculation and hosted the highest X. fastidiosa titre of the cultivars, and lost over 90% of its stem hydraulic conductivity. In contrast, the other three cultivars displayed less susceptibility to PD. The way in which the xylem anatomy could impact PD susceptibility in V. vinifera cultivars is discussed, together with how grape pedigrees and their cognate centre of domestication may have influenced xylem anatomical features. This work provides a reference framework to further test the hypothesis that V. vinifera cultivars with wide xylem vessels may be more prone to PD decline.     

A rapid technique for screening banana cultivars for resistance to Xanthomonas wilt

The banana Xanthomonas wilt disease (BXW) has threatened the livelihood of millions of farmers in East Africa. Use of resistant varieties is the most cost-effective method of managing this bacterial disease. A reliable and rapid screening method is needed to select resistant banana varieties. An in vitro screening method was developed for early evaluation of Xanthomonas wilt resistance using small tissue culture-grown plantlets. Eight cultivars of banana were screened with sixteen isolates of Xanthomonas campestris pv. musacearum using this method. There were significant differences (P < 0.0001) in susceptibility among the various banana cultivars tested, whereas no significant difference (P = 0.92) in pathogenicity was observed between the pathogen isolates. The cv. Pisang Awak (Kayinja) was found to be highly susceptible and Musa balbisiana resistant. Nakitembe was found to be moderately resistant while cvs Mpologoma, Mbwazirume, Sukali Ndiizi, FHIA-17 and FHIA-25 were susceptible. The susceptibility of these cultivars was further tested in vivo by artificial inoculation of potted plants with similar results. This study shows that an in vitro screening test can serve as a convenient, cheap and rapid screening technique to discriminate BXW-resistant from BXW-susceptible banana cultivars.     

Bacterial fruit blotch of melon: screens for disease tolerance and role of seed transmission in pathogenicity

Bacterial fruit blotch (BFB) of cucurbits, caused by Acidovorax avenae subsp. citrulli, is a serious threat to the watermelon and melon industries. To date, there are no commercial cultivars of cucurbit crops resistant to the disease. Here we assessed the level of tolerance to bacterial fruit blotch of various commercial cultivars as well as breeding and wild lines of melon, using seed-transmission assays and seedling-inoculation experiments. Selected cultivars were also tested in a greenhouse experiment with mature plants. All tested cultivars/lines were found to be susceptible to the pathogen, and most of them showed different responses (relative tolerance vs. susceptibility) in the different assays; however, some consistent trends were found: cv. ADIR339 was relatively tolerant in all tested assays, and cv. 6407 and wild lines BLB-B and EAD-B were relatively tolerant in seed-transmission assays. We also provide evidence supporting a strong correlation between the level of susceptibility of a cultivar/line and the ability of the pathogen to adhere to or penetrate the seed. To the best of our knowledge, this is the first attempt to assess melon cultivars/lines for bacterial fruit blotch response.     

Evaluation of glyphosate-tolerant soybean cultivars for resistance to bacterial pustule

Xanthomonas axonopodis pv. glycines causes bacterial pustule of soybean, which is a common disease in many soybean-growing areas of the world and is controlled by a single recessive gene (rxp gene) commonly found in many conventional glyphosate-sensitive soybean cultivars. Since glyphosate-tolerant cultivars are commonly planted today, there has been no information about whether these new cultivars have bacterial pustule resistance. The goal of this study was to screen glyphosate-tolerant soybean cultivars for resistance to X. axonopodis pv. glycines. Three experiments were completed to evaluate resistance. In experiment 1, 525 commercial glyphosate-tolerant cultivars from 2001 were inoculated with X. axonopodis pv. glycines strain UIUC-1. Following inoculation, many of the cultivars were resistant (developed no detectable pustule symptoms) although 152 (~29%) developed bacterial pustule. In experiment 2, the aggressiveness of three strains (UIUC-1, UIUC-2, and ATCC 17915) of X. axonopodis pv. glycines were compared on three bacterial pustule-susceptible, glyphosate-tolerant cultivars. One strain (UIUC-1) was less aggressive than the other two (UIUC-2 and ATCC 17915) on all three cultivars examined. In experiment 3, 45 cultivars from 2005 (all different from 2001) were inoculated with X. axonopodis pv. glycines ATCC 17915. A range of disease severities developed with five cultivars (11%) having disease severity ratings as high as or higher than those on a susceptible check cultivar. Overall, these results suggested that resistance to bacterial pustule occurs in glyphosate-tolerant soybean cultivars, but not at 100% frequency, which means bacterial pustule outbreaks could occur when a susceptible cultivar is planted and conditions are conducive for bacterial pustule development.     

Determination of agronomic practices for the management of blight of chickpea caused by <Emphasis Type="Italic">Ascochyta rabiei</Emphasis> in Turkey: 1. Appropriate sowing date

In order to determine the most appropriate dates for planting chickpea in central Anatolia, Turkey, six cultivars were planted at three sites that differed in disease pressure. In two of the sites, disease pressure from Ascochyta rabiei was promoted by spreading infected chickpea debris on the soil surface at the time of planting and, at one of these, sprinkle irrigation was applied. In the third site, where conditions were dryer, no artificial inoculum was provided. Plants from seeds sown in early March had the most disease and in the sprinkle irrigated plots the disease severity ranged from 7.8 on the most susceptible cv. Canitez to 3.3 on the least susceptible Gokce as scored on the 1–9 scale where 1 = no disease and 9 represents a plant killed by the fungus. There was an inverse relationship between disease severity and yield, production from blight resistant cultivars of around 2,000 kg ha⁻¹ being more than twice that of susceptible ones. Delaying planting for 3–5 weeks reduced the severity of ascochyta blight but also reduced the yields in four of the six cultivars. In contrast, reduction in disease severity by delayed sowing resulted in yield increases for the susceptible cvs Canitez and Local, although yield level was not as much as those of the less susceptible cvs sown early. Delay of 6–9 weeks almost eliminated ascochyta blight but yields of all cultivars were seriously compromised by drought stress. In consequence, chickpea farmers are recommended to use resistant or tolerant cultivars and sow early in March. For less resistant cultivars, sowing in early April is recommended. Further delay is not recommended unless irrigation is provided and fungicide spraying is recommended where signs of infection are present under conditions conducive to the disease.     

Effect of plant genotype, virus isolate and temperature on the expression of the potato tuber necrotic ringspot disease (PTNRD)

The present study shows that a large range of potato cultivars (29/33 tested), widely grown in the world, are susceptible to potato tuber necrotic ringspot disease caused by potato virus Y. The three factors studied in this work, which proved to influence the level of tuber necrosis reaction, were, first, the plant genotype, since varietal behaviour exhibited large differences; second, the virus genotype, since variations of virulence occurred between the four isolates tested; and third, the environmental conditions, as shown by the different rates of tuber necrosis obtained under contrasting conditions of temperature as much during the growing period as during storage. Three of the cultivars tested, Spunta, Maris Piper and Thalassa, failed to produce necrotic tubers, although infected with a virulent tuber-necrosing isolate. This result, following observations on the inheritance of the tuber necrosis trait, suggests that at least a major dominant gene controls this reaction in non-sensitive cultivars. On the other hand, the extreme resistance genes ( Ry ) provide a good resistance to virus infection, thus, preventing tuber necrosis under field conditions.     

Novel metrics to classify fire blight resistance of 94 apple cultivars

Fire blight (Erwinia amylovora), a potentially devastating disease in apple, can cause floral, fruit and structural damage and even tree death. Most commercial apple cultivars are susceptible and the resistance/susceptibility of many modern cultivars has not been evaluated. Fire blight resistance/susceptibility is difficult to phenotype due to quantitative resistance, impacts of tree vigour and environment on susceptibility, and the erratic nature of the disease. Resistance/susceptibility levels were determined for 94 apple cultivars and important breeding parents. In 2016 and 2017, multiple actively growing shoots per tree (about three trees per cultivar) were challenged with E. amylovora Ea153n via a cut-leaf inoculation method. Proportion of current season's shoot length blighted (SLB) was calculated for each shoot. To classify cultivar responses, estimated marginal SLB means were compared to four controls, representing highly susceptible (HS) to highly resistant (HR), via Dunnett's tests. Cultivar responses ranged from HS to HR with estimated marginal SLB means of 0.001–0.995 in 2016 and 0.000–0.885 in 2017. Most cultivars demonstrated similar resistance/susceptibility levels in both years (ρ = 0.657, P < 0.0001). K-means clustering was used to classify cultivars into three resistance/susceptibility groups based on incidence, average severity (SLB), and maximum severity values (maximum SLB and age of wood infected). Sixteen cultivars were consistently moderately resistant (MR) to HR while the remainder ranged from HS to MR. An updated comparison of susceptibility of important cultivars is provided. Resistance/susceptibility information gained could be used to identify genetic loci associated with resistance/susceptibility and/or inform parental selection in apple scion breeding programmes.     

Evaluation of apricot (Prunus armeniaca L.) resistance to Apple chlorotic leaf spot virus in controlled greenhouse conditions

Apple chlorotic leaf spot virus (ACLSV) seems to be the causal agent of apricot viruela disease. This disease has become an important problem for apricot production in Spain, mainly affecting the ‘Búlida’ cultivar, although no information is available about the behaviour of other cultivars with regards to ACLSV. In this study, the behaviour of 29 apricot cultivars against ACLSV (Apr 62 isolate) was evaluated under controlled conditions in an insect-proof greenhouse. Three different rootstocks, ‘GF305’ peach, ‘Real Fino’ apricot and ‘Adesoto’ plum, were first inoculated by grafting ACLSV-infected bark and were later grafted with the apricot cultivar to be evaluated. Apricot cultivars were evaluated during three cycles of study. ACLSV was asymptomatic on the leaves of all cultivars and rootstocks, so level of susceptibility or resistance was determined by virus detection through RT-PCR. ‘GF305’ rootstock showed a greater susceptibility level than ‘Real Fino’ and ‘Adesoto’. Most of the cultivars were susceptible to ACLSV with different levels of susceptibility, and only ‘Bergeron’ and ‘Mauricio’ were resistant.     

Ralstonia solanacearum causing potato bacterial wilt: host range and cultivars’ susceptibility in Rwanda

Members of the Ralstonia solanacearum species complex (RSSC), causing potato bacterial wilt or brown rot, are highly contagious and there are no known cultivars with durable resistance to the pathogen. This study hypothesized (a) that crops intercropped or rotated with potato, plants in the same family, and plants grown in the neighbouring fields can host the pathogen and they can be potential sources of primary inoculum, and (b) that potato cultivars currently multiplied by the public tissue culture laboratory in Rwanda are less susceptible to the pathogen. Fourteen plant species and potato, and nine potato cultivars were tested for susceptibility to an RSSC phylotype II strain under greenhouse conditions. The bacteria induced symptoms on potato, tomato, tree tomato, sweet pepper, and eggplant only. Among the plant species with symptoms, potato, tomato, and tree tomato wilted completely. There was a significant difference in days to symptom expression and to complete wilting (p < .0001). While all tested potato cultivars were found to be susceptible, the number of days to first symptom expression, days to complete wilting, area under the disease progress curve (AUDPC), and the number and weight of harvested tubers varied considerably. Cultivars Cruza, Kinigi, and CIP-58 were less susceptible whereas the cultivars Gikungu, Kirundo, and Victoria were highly susceptible. There is a strong need to search for other sources of resistance. The results indicate that some plant species that might serve as a reservoir of the bacterium should be avoided in the vicinity of potato crops.     

The role of seedling infection in epiphytotics of ascochyta blight on chickpea

Didymella rabiei, the causal agent of ascochyta blight, survives on infected seeds and seedlings. Diseased seedlings originating from infected seeds occasionally serve as the source for primary infection in chickpea crops. Experiments carried out independently in Australia and in Israel provided quantitative information on the temporal and spatial distribution of ascochyta blight from initial infections and on the relationship between the amount of initial infection and the intensity of subsequent epiphytotics for cultivars differing in susceptibility to the pathogen. Disease spread over short distances (<10 m) from individual primary infections, was governed by rain and wind, and was up to five times greater down-wind than up-wind. Cultivar response to D. rabiei significantly affected the distance and area over which disease spread and the intensity of the disease on infected plants. At onset of the epiphytotic, the relationship between disease spread and time was exponential (P < 0.05; R ² > 0.95) and the area of the resulting foci was over 10 times greater in susceptible cultivars than in resistant cultivars. Regression equations showed the relationship between disease severity and the distance from the focus-plants was inverse-linear for all cultivars tested (P < 0.05). A simulation model based on the experimental data revealed that even if primary infection is infrequent (less than 1% of plants), the consequences are potentially devastating when susceptible cultivars are used. The epidemiological information and simulation model generated by this study provide an increased understanding of the development of an epiphytotic in which the primary foci of disease originate from infected chickpea seedlings.     

Traditional and commercial apple and pear cultivars as sources of resistance to fireblight1

In Hungary, fireblight research programmes were initiated on pear in 1999 and on apple in 2000, with the aim of evaluating the susceptibility or resistance of commercial cultivars. Sources of resistance for future breeding were also sought among traditional apple cultivars collected from Ukraine and pear cultivars in the Hungarian gene bank (Szigetcsép). Experiments were done under secure conditions. Inocula were mixtures of characteristic Erwinia amylovora isolates from pear and apple in Hungary. Host responses (symptom development, disease severity and multiplication rate of bacterial cells in host tissues) were assessed on shoots, flowers and fruits. About 30 pear and 30 apple cultivars, and 35 apple hybrids, were tested and grouped into four categories for pear and three for apple. Of the pear cultivars tested, 50% were susceptible, 30% moderately susceptible and only 10% of low susceptibility. Different plant organs occasionally displayed different responses. Members of the last two groups might serve as useful candidates for growing under IPM conditions. Among the traditional Hungarian varieties tested, we found high resistance in ‘Sikulai’ and ‘Szemes alma’, which could be used as sources of fireblight resistance in breeding programmes and also grown in organic orchards. Furthermore, among the offspring of the apple ‘Prima’ (scab‐resistant), we have found highly resistant lines.     

Aleppo pine provenances vary in susceptibility and secondary chemical response to Gremmeniella abietina infection

Aleppo pine is a thermophilic species that has become a popular tree species in plantings even beyond its natural habitat. In 1999 the pathogenic fungus Gremmeniella abietina was isolated for the first time in Aleppo pine plantations in Spain. The main objective of this study was to analyse the variation in susceptibility to G. abietina (European race, biotype A) infections among trees from five Aleppo pine provenances. Artificial inoculations were performed at two different times in January 2012. The susceptibility of the trees was evaluated by recording disease severity and the length of internal stem necrosis produced by the pathogen. In addition, the concentrations of two flavanone compounds were quantified as putative indicators of resistance. Provenances and G. abietina isolates both significantly affected levels of necrosis although the results were dependent on the time of inoculation. There was a relationship between provenance and resistance, with trees of higher elevation provenances being more susceptible to disease than trees of lower elevation provenances. The amount of naringenin flavanone from trees was significantly different between provenances, suggesting that it is a possible indicator value for resistance. Conversely, no differences were found in the concentration of eriodictyol in trees between provenances. The results show that the provenance‐dependent variation in the susceptibility of Aleppo pine to G. abietina should be considered in selection of seed sources for restoration.     

Effect of leaf scald (<Emphasis Type="Italic">Xanthomonas albilineans</Emphasis>) on polyamine and phenolic acid metabolism of two sugarcane cultivars

Polyamine and phenolic acid levels as well as activities of some enzymes of their biosynthetic metabolism were examined in two sugarcane (Saccharum officinarum) cultivars differing in susceptibility to leaf scald, a disease caused by the bacterium, Xanthomonas albilineans. Juice obtained from both infected cultivars showed significantly increased levels of free putrescine and ornithine decarboxylase activity. However, the pathogen induced different changes in the two cultivars in subsequent metabolic steps. Whereas acid insoluble conjugated spermidine completely disappeared from the highly susceptible cv. C 439-52, an increase in acid insoluble conjugated polyamines was observed in the moderately susceptible cv. L 55-5. Phenolic acid metabolism also differed in the two cultivars. Since total phenolic acid content and phenylalanine ammonium lyase activity was greater in both cultivars after infection, distribution of phenolic acids between free or conjugated forms diverted into different pathways. The level of susceptibility of the two cultivars is discussed in terms of changes in these compounds.     

Disease levels in winter wheat,rye and triticale grown on soil artificially inoculated withCephalosporium gramineum

Field experiments with winter cereals grown on soil inoculated withC. gramineum showed that wheat and rye cultivars possess some resistance to the pathogen, while the triticale cultivars were the most susceptible. Higher tolerance of the tested wheat cultivars was connected mainly with slow development of disease symptoms; rye cultivars had, on average, lower percentages of plants infected byC. gramineum. The greatest variation in susceptibility toC. gramineum occurred among the selected cultivars of triticale.