Rice response to simultaneous bacterial blight and drought stress during compatible and incompatible interactions

Plant response to one type of stress can be affected by simultaneous exposure to a second stress, for example when abiotic and biotic stresses occur together. Ten rice genotypes comprising those with bacterial blight (BB) resistance (R) genes, drought quantitative trait loci (QTLs) plus a BB R gene, and BB susceptible genotypes, were subjected to mild and moderate drought stress and plants were inoculated with two Xoo strains (PXO99 and PXO145) to simulate the challenges rice crops face under simultaneous stress of drought and BB. Plant height and dry shoot biomass were significantly reduced by drought stress treatments. The BB disease lesion lengths varied according to rice genotypes and PXO99 Xoo multiplication and spread in planta was higher compared to that of PXO145, which generally decreased under mild drought stress. Rice genotype IRBB7 (Xa7) showed less Xoo spread and a reduced Xoo multiplication under drought stress compared to the well-watered control with PXO145. In contrast, in genotypes with a different BB R gene and/or drought QTLs [IRBB4 (Xa4), IR87705–6-9-B (Xa4 + qDYT _2.2 ), IR87707–445-B-B-B (Xa4 + qDYT _2.2  + qDYT _4.1 ) and IR87707–446-B-B-B (Xa4 + qDYT _2.2  + qDYT _4.1 )], Xoo multiplication and spread in planta was higher with drought stress. This study has shown that drought stress affected rice response to the BB pathogen and the response varied according to the rice genotype. It is concluded that evaluating rice varieties under combined abiotic and biotic stresses will be the best strategy to determine biotic stress resistance durability under climate change.     

Characterization of Pectobacterium carotovorum subsp. carotovorum and brasiliense from diseased potatoes in Kenya

Using a DNA-based typing method, 48 bacterial strains isolated from infected potato (Solanum tuberosum) tubers originating from Kenya were characterized. The pel gene specific primers showed that all the 48 bacterial strains were pectolytic. Subspecies-specific primers EXPCCF/EXPCCR and Br1f/L1r identified 66 % of the strains as Pectobacterium carotovorum subsp. carotovorum while 34 % were identified as Pectobacterium carotovorum subsp. brasiliense based on their characteristic band sizes of 550 and 322 bp, respectively. Amplification of the 16S-23S rDNA (ITS) region did not yield observable differences in banding patterns between the Kenyan strains. However, PCR-RFLP analysis together with partial nucleotide sequences of the housekeeping mdh and gapA genes confirmed the results obtained by the specific primers. Phylogenetic analysis of the concatenated partial gene sequences grouped Pectobacterium carotovorum subsp. carotovorum and Pectobacterium carotovorum subsp. brasiliense Kenyan strains together with those identified in other parts of the world with 90 % and 99 % bootstrap support values, respectively. Pathogenicity assays using representative Kenyan strains demonstrated varied levels of tuber maceration ability. The Pectobacterium carotovorum subsp. carotovorum and Pectobacterium carotovorum subsp. brasiliense Kenyan strains were shown to be less aggressive in causing soft rot when compared to type strains. This study describes for the first time the genetic diversity of pectolytic bacteria causing soft rot disease of potatoes in Kenya.     

Virulence profiling of <Emphasis Type="Italic">Xanthomonas oryzae</Emphasis> pv. <Emphasis Type="Italic">oryzae</Emphasis> isolates,causing bacterial blight of rice in India

Bacterial blight (BB) of rice caused by Xanthomonas oryzae pv. oryzae (Xoo), remains a major production constraint in rice cultivation especially in irrigated and rainfed lowland ecosystems in India. The pathogen is highly dynamic in nature and knowledge on pathotype composition among the Xoo population is imperative for designing a scientific resistance breeding program. In this study, four hundred isolates of Xoo collected from diverse rice growing regions of India were analyzed for their virulence and genetic composition. Virulence profiling was carried out on a set of differentials consisting of 22 near isogenic lines (NILs) of IR24 possessing different BB resistance genes and their combinations along with the checks. It was observed that different NILs possessing single BB resistance gene were susceptible to about 59–94% of the Xoo isolates except IRBB 13 (containing BB resistance gene xa13), which showed susceptibility to about 35% of the isolates. Based on the reaction of the Xoo isolates on the differentials, they were categorized into 22 pathotypes. Among the 22 pathotypes, IXoPt-1 and IXoPt-2 were least virulent and IXoPt # 18–22 were highly virulent. Pathotype IXoPt-19 which was virulent on all single BB resistance genes except xa13 constituted the major pathotype (22.5% isolates) and was widely distributed throughout India (16 states). This was followed by pathotype IXoPt-22 (17.25%) which was virulent on all the NILs possessing single BB resistance genes. Molecular analysis was carried out using two outwardly directed primers complementary to sequence of IS1112, a repetitive element of Xoo. A high level of genetic polymorphism was detected among these isolates and the isolates were grouped into 12 major clusters. The data indicated complex nature of evolution of the Xoo pathotypes and there was no strong correlation between pathotypes and genetic clusters as each genetic cluster was composed of Xoo isolates belonging to different pathotypes. The study indicated that none of the single BB resistance genes can provide broad spectrum resistance in India. However, two-gene combinations like xa5 + xa13 and different 3 or 4 genes combination like Xa4 + xa5 + xa13, Xa4 + xa13 + Xa21, xa5 + xa13 + Xa21 and Xa4 + xa5 + xa13 + Xa21 are broadly effective throughout India.     

Proteomic analysis of Colletotrichum kahawae-resistant and susceptible coffee fruit pericarps

Coffee berry disease (CBD) is caused by the fungus Colletotrichum kahawae and is restricted to the African continent, where it generates losses of up to 80 % of coffee production. Weather conditions in certain growing areas at high altitudes in Colombia appear to be very favourable for the development of this disease. Certain genotypes of Coffee arabica are resistant to this pathogen, such as the Timor Hybrid and some Ethiopian accessions. It is important to identify the proteins in these coffee genotypes that are associated with resistance to this fungus. Therefore, we compared the proteomes of two genotypes that are resistant to different isolates of C. kahawae with the proteome of the susceptible coffee genotype Caturra. We optimized the methodology applied for the extraction, cleaning and purification of proteins from the green fruit pericarp at 150 to 170 days after flowering. Through two-dimensional differential gel electrophoresis, proteomic map images were obtained for the resistant and susceptible genotypes. Fifty-two protein spots that were significantly different between the resistant and susceptible genotypes were detected. These protein spots were isolated and sequenced via mass spectrometry. The sequence analysis identified 14 proteins in the Timor Hybrid and 14 in CCC1147 that were associated with resistance and pathogen defence.     

Critical disease components of common bacterial blight to effectively evaluate resistant genotypes of snap bean

Common bacterial blight (CBB) caused by Xanthomonas axonopodis pv. phaseoli (Xap) is the main bacterial disease of snap bean. The present work aimed to select snap bean genotypes that are resistant to CBB based on three components of resistance: area under the disease progress curve, latent period, and lesion diameter on pods (DL). A completely randomized two-factor factorial design with six replications was used to evaluate leaves and pods of 14 snap bean and three common bean genotypes (PI 207262, BAC-6 and A-794) with high resistance to CBB. Two Xap isolates, 1394-98 and 775-90, were used to inoculate leaves and pods. Data were analyzed using nonparametric statistics. Significant differences were observed among the evaluated genotypes for all of the components of resistance, except for DL. The snap bean UENF 1482 demonstrated good performance in two disease resistance components. For this reason, this genotype can be recommended for use in breeding programs that aim to generate snap bean genotypes resistant to Xanthomonas axonopodis pv. phaseoli.     

Resistance in field pea (Pisum sativum) to the black spot disease complex in Western Australia

Black spot (also referred as Ascochyta blight, Ascochyta foot rot and black stem; Ascochyta leaf and pod spot) is a devastating disease of field pea (Pisum sativum) caused by one or more pathogenic fungi including Didymella pinodes, Ascochyta pisi, Phoma pinodella and P. koolunga. Development of resistant germplasm has been slow because of the low level of resistance found in the available germplasm, poor reliability of screening methods and the polygenic nature of inheritance. Field studies, undertaken to assess F8 and F9 derived lines for resistance against the black spot complex, confirmed that some lines developed in the Australian breeding program show improvement in resistance over commercial cultivars. Disease scores across test lines ranged from 5.33 to 7.82 (0–9 scale where 0?=?no disease symptoms, and 9?≥?90 % leaf area affected) and from 5.37 to 8 in 2012 and 2013, respectively. In 2012, the eight most resistant lines had scores 5.33 to 6, with OZP1207 the most resistant line. In 2012, forty three lines were significantly more resistant (disease score?≤?6.67) than the susceptible standard, Helena (7.82), 14 lines were not significantly different to the most resistant commercial cultivar, Parafield (6.33), and 27 lines were significantly more resistant than PBA Percy (7.67). In 2013, WAPEA2211 was clearly the most resistant line (5.37) followed by a group of seven lines with slightly less resistance scored at 6. Use of these lines in breeding will further enhance resistance in commercial cultivars, particularly by inter-crossing among the more genetically diverse lines to accumulate minor genes for resistance. While there was no overall relationship between disease scores in 2012 and 2013 (R ²?=?0.029), presumably due to the highly variable pathogen composition of the black spot complex at the screening site and across seasons in Western Australia, a few lines, such as WAPEA2211, 04H349P-05HO2005, 06H109P-9 and 06H459P-1, showed significant resistance in both years, appear to have resistance to multiple pathogens in the black spot complex, and are of particular significance.     

Detection of streptomycin resistance in Erwinia amylovora strains isolated from apple orchards in Chihuahua, Mexico

Fire blight, one of the most severe diseases of apple and pear, is caused by the bacterium Erwinia amylovora. One control method is the use of antibiotics like streptomycin; however, streptomycin is the only antibiotic registered to control fire blight. A total of 107 E. amylovora strains were isolated from apple orchards located in Cuauhtémoc and Guerrero, Chihuahua, two major apple-producing areas in Mexico, showing 40 and 24 % streptomycin-resistant strains, respectively. The identification of E. amylovora strains was performed by polymerase chain reaction (PCR) amplification of a 900-bp region located within the non-transferable pEA29 plasmid and by amplification of a specific 1,269-bp region located on the E. amylovora chromosome. The 107 isolates tested carried the pEA29 plasmid, and 36 % of the isolates from both locations showed high resistance to streptomycin at levels that ranged from 200 to ≥1,000 μg ml^?1 streptomycin. The strA-strB and aadA genes, which encode enzymes that inactivate streptomycin, and a mutation in codon 43 of the rpsL gene that confers high resistance to the antibiotic were examined to determine the mechanism of streptomycin resistance. In total, 95 % of the resistant strains showed a single base pair mutation in codon 43 of the rpsL gene, causing an amino acid substitution in ribosomal protein S12. The presence of strA-strB and aadA genes or the rpsL mutation was not identified in the other 5 % of resistant strains, suggesting the existence of a new streptomycin resistance mechanism in E. amylovora.     

Genetic variability and virulence of Meloidogyne incognita populations from Brazil to resistant cotton genotypes

The root-knot nematode Meloidogyne incognita is widely distributed and a major pathogen of cotton (Gossypium spp.) worldwide. The objectives of this study were to assess the genetic variability and aggressiveness of Brazilian populations of M. incognita in cotton. Five populations of M. incognita and one isolate of M. enterolobii (outgroup) were used in the molecular analysis. Our results showed that only 2.7 % of the RAPD and AFLP fragments were polymorphic. Despite the existence of two races (races 3 and 4) and two esterase phenotypes (I1 and I2), a low genetic variability among populations was observed, which might be due to the mitotic parthenogenetic mode of reproduction of this pathogen. The aggressiveness/virulence among populations towards different cotton genotypes was also studied. None of the populations was virulent to the resistant cotton genotypes M-315 RNR, TX-25, CIR1343, Wild Mexican Jack Jones and CIR1348 (reproduction factor <1). Two populations of M. incognita from the states of Mato Grosso do Sul and Parana (Umuarama) (races 4 and 3, respectively) were highly aggressive to the susceptible control FM966 and virulent to the accessions LA-887 and Clevewilt-6 that showed moderate resistance to other populations tested.     

Evaluation of different methods for the characterization of carrot resistance to the alternaria leaf blight pathogen (Alternaria dauci) revealed two qualitatively different resistances

Alternaria leaf blight (ALB), caused by Alternaria dauci, is one of the most damaging foliar diseases of carrot worldwide. The aim of this study was to compare different methods for evaluating levels of carrot resistance to ALB. Three techniques were investigated by comparison with a visual disease assessment control: in vivo conidial germination, a bioassay based on a drop‐inoculation method, and in planta quantification of fungal biomass by quantitative PCR (Q‐PCR). Three carrot cultivars showing different degrees of resistance to A. dauci were used, i.e. a susceptible cultivar (Presto) and two partially resistant genotypes (Texto and Bolero), challenged with an aggressive or a very aggressive isolate of A. dauci. Both partially resistant genotypes produced a higher mean number of germ tubes per conidium (up to 3·42±0·35) than the susceptible one (1·26±0·18). The drop‐inoculation results allowed one of the partially resistant genotypes (Bolero, log₁₀(S+1) = 1·34±0·13) to be distinguished from the susceptible one (1·90±0·13). By contrast, fungal growth measured by Q‐PCR clearly differentiated the two partially resistant genotypes with log₁₀(I) values of 2·77±0·13 compared to the susceptible cultivar (3·65±0·13) at 15 days post‐inoculation. This result was strongly correlated (r² = 0·91) with the disease severity index scored at the same date. Data obtained with the different assessment methods strongly suggest that the Texto and Bolero genotypes have different genetic resistance sources.     

Identification of new resistant sources for bacterial blight in pomegranate

Bacterial blight is a highly devastating disease caused by Xanthomonas axonopodis pv. punicae, recording 60 to 80 percent yield-loss of pomegranate in India. In the present investigation, a total of 209 genotypes including 105 exotic types from USDA, 66 wild types and 38 cultivated types from India were screened and categorized into fifteen clusters using cluster and principal component analysis. Genotypes of cluster 15, viz. 108 B and 99 A from USDA and 318734, Daru-18 and IIHR-30 from India, were found to be resistant to bacterial blight while genotypes of cluster 9 were highly susceptible. Two genotypes, each from cluster 15 (318734) and 9 (Ruby), were compared for biochemical and histological parameters to understand the defense mechanism. Significantly, higher accumulation of defense related metabolites, viz. total phenol, flavonoid and antioxidant contents, were observed in resistant genotype (318734). Fewer numbers of stomatal pores that served as portals of entry for plant pathogens were recorded in this genotype. Resistance observed in genotype 318734 might be due to an incompatible interaction between host and pathogen compared to other genotypes. This is the first report of putative resistance sources in pomegranate against Xanthomonas axonopodis pv. punicae.     

Evaluation of resistance of Spanish olive cultivars to Verticillium dahliae in inoculations conducted in greenhouse

The resistance of 28 Spanish olive cultivars to Verticillium dahliae was evaluated in an experiment conducted under greenhouse conditions, by impregnating plant roots with a semisolid fluid mass of a mixture of culture medium and the conidia and mycelium of the fungus. Five-month-old olive plants were inoculated with a cotton defoliating isolate of V. dahliae. ‘Frantoio’ and ‘Picual’ were used as resistant and susceptible reference cultivars, respectively. Cultivars were assessed on the basis of final values of the area under the disease progress curve, mean severity of symptoms, and mortality at 26 weeks following inoculation. Verticillium wilt disease developed more slowly and reached lower values of these parameters than those normally recorded in previous studies conducted in growth chambers, using root-dip inoculation in a conidial suspension of the pathogen. However, most of the evaluated cultivars exhibited susceptible or moderately susceptible reactions to the infections caused by V. dahliae. In particular, a group of eight cultivars, from the same group as ‘Picual’, such as ‘Manzanilla de Abla’, ‘Manzanilla del Centro’ and ‘Negrillo de Iznalloz’, were significantly more susceptible than ‘Frantoio’. Conversely, ‘Escarabajillo’, ‘Menya’ and ‘Sevillana de Abla’ exhibited a high level of resistance to the disease, no dead plants, and vegetative recovery. Field experiments are currently being carried out to confirm the level of resistance assigned to these last genotypes. If confirmed, these genotypes will act as potential resistant genitors for inclusion in current olive breeding programs or for use as resistant rootstocks.     

Identification of genetic loci conferring partial resistance to southern corn rust through a genome-wide association study

Southern corn rust (SCR), a fungal disease caused by Puccinia polysora Underw., can result in severe yield loss in maize (Zea mays L.). Identification of resistant germplasms and understanding the genetic basis of resistance would aid maize disease-resistant breeding. In this study, 253 maize inbred lines were used to evaluate the resistance to SCR at two locations in China from 2015 to 2016. A genome-wide association study (GWAS) was conducted using the Maize SNP3K Beadchip. Based on mixed linear model, seven single nucleotide polymorphisms (SNPs) associated with partial resistance to SCR were identified at P < 1.77 × 10^?5, including three SNPs aligned with previously reported genomic regions. A significant correlation (R² = 0.76) between partial resistance to SCR and the number of resistant alleles was observed. Several highly resistant germplasms harboring resistant alleles were identified, such as, ‘43.7’, ‘DH02’, ‘Zheng39’, ‘T2’ and ‘JH3372’. This research will increase our understanding of the genetic basis of partial resistance to SCR and provide important guidance for SCR resistance breeding.     

Disease severity,incidence and races of <Emphasis Type="Italic">Setosphaeria turcica</Emphasis> on sorghum in Uganda

In order to understand the underlaying causes of new severe turcicum leaf blight outbreaks in East Africa, a survey was undertaken in Uganda to examine the sorghum—Setosphaeria turcica interaction in terms of disease severity and incidence, the overall fungal population structure, and new resistant resources. Highest disease severities were recorded on caudatum accessions, whereas kafir genotypes were most resistant. The disease was more severe in the most humid farmlands compared to moderately dry agro-ecologies. In districts with wide adoption of the Epuripur variety a very high incidence (100%) of turcicum leaf blight was found. The two S. turcica mating type genes MAT1-1 and MAT1-2 assessed on fungal isolates deriving from both sorghum and maize diseased leaves were found in 20 of 23 districts sampled and in equal proportions. Upon cross inoculation on maize differential lines, four S. turcica isolates were identified as race 1, two as race 2, and one isolate corresponded to race 0 and race 3, respectively. The remaining 10 S. turcica isolates did not cause any disease symptoms on the maize lines assessed. Highly resistant accessions originating from a regional collection were found among the five sorghum races (kafir, guinea, caudatum, bicolor and durra), and are now implemented in new sorghum disease resistance programs.     

Identification of Rhizoctonia solani AG1-IB as a causal agent of leaf blight in Cynanchum paniculatum in China

A severe disease causing leaf blight of Cynanchum paniculatum was observed in China. From the diseased plants, 24 fungal isolates were obtained. Morphological observations and nuclear conditions of all the isolates indicated characteristics of Rhizoctonia solani. The ribosomal DNA internal transcribed spacer sequences of the isolates showed more than 99 % sequence homology with published sequences of R. solani anastomosis group 1 subgroup IB (AG1-IB). All tested isolates were pathogenic and showed significant symptoms as observed in the fields. This is the first report of leaf blight of C. paniculatum caused by R. solani AG1-IB worldwide.     

Levels of fire blight (<Emphasis Type="Italic">Erwinia amylovora</Emphasis>) susceptibility of native apple,pear and quince germplasm from Lake Van Basin,Turkey

Fire blight resistance of apple, pear and quince genetic resources from Lake Van Basin (eastern Turkey) was tested using Erwinia amylovora strain Ea Van. Shoot tips of 92 native accessions (48 accessions for apple, 38 accessions for pear and 6 accessions for quince) were wounded for inoculation, and artificially inoculated with pathogenic bacteria under greenhouse conditions. The levels of resistance of accessions were classified in comparison with control varieties according to the genotype susceptibility index (GSI%) scores based on the lesion length on shoots of each genotype. Fire blight resistance of accessions consisted of five classes: resistant (R), moderately resistant (MR), moderately susceptible (MS), susceptible (S) and highly susceptible (HS). GSI% scores differed significantly among accessions from each fruit species (p < 0.01). GSI values ranged from 12.4% to 64.1% for apple genotypes, from 17.2% to 55.1% for pear genotypes, and from 17.8% to 43.4% for quince genotypes. No resistant genotypes of apple, pear and quince were observed. Seven accessions of apple, two accessions of pear and one accession of quince were MR. 25 accessions of apple, 14 accessions of pear and one accession of quince were MS. These findings indicate a considerable variation in fire blight resistance and could contribute to breeding efforts regarding fire blight resistance in apple, pear and quince.     

Seed transmission of verticillium wilt of cotton

Twenty-nine cotton genotypes with varying levels of susceptibility to Verticillium dahliae were grown in infested plots at Nazilli, Ayd?n, in 2008–2009. The highest level of disease incidence was recorded in cultivars ‘BA-151’, ‘Celia’, ‘Çukurova-1518’, ‘Fla?’ and ‘Mara? 92’, and averaged 85–95% for all genotypes in both years. The incidence of V. dahliae in seed averaged 29.8% for cv. Çukurova-1518, 27.6% for Fla?, 24.6% for cv. BA-151, 19.0% for cv. Celia and 16.2% for Mara? 92. Two hundred seeds from each genotype were planted, two seeds per pot, in a steam-pasteurized mixture of soil, peat, and sand. Pots were placed close to each other on a greenhouse bench to obtain a thick canopy. Typical disease symptoms appeared about 12–13 weeks after sowing. Maximum disease incidence values averaged 3.3% for Celia, 4.5% for Mara? 92, 8% for BA-151, 9% for Fla? and 9.5% for Çukurova-1518.     

Impact of <Emphasis Type="Italic">Dysdercus koenigii</Emphasis> Fabricius (Hemiptera: Pyrrhcoridae) density-dependent population on agronomic and qualitative characteristics of different transgenic cotton varieties

The cotton stainer, Dysdercus koenigii Fabricius (Hemiptera: Pyrrhcoridae), has become a major threat to transgenic cotton as it causes warts on the internal carpel wall of cotton boll, severe lint staining, lint locks, and lint lesions. Thus, keeping in view the importance of this pest on cotton, in the present study, screening of 13 transgenic cotton genotypes was performed and the population of D. koenigii was determined on these genotypes during 2012–13. Furthermore, among these genotypes, a high yielding advanced cultivar (FH-114) was selected for further experiments. A number of five different densities of D. koenigii (5, 10, 15, 20, and 25) at adult stage with well-developed proboscis were released in the cages along with a control treatment. Cotton genotypes FH-312, FH-2073, FH-Lalazar, FH-142, and MNH-886 possessed minimum population (1.33 D. koenigii per plant) compared to FH-324 and FH-444 (6.0 D. koenigii per plant). The number of bolls per plant and boll weight decreased as a result of increased densities. The increased density of D. koenigii resulted in fewer bolls per plant (12.1 bolls), reduced boll weight (2.1 g), germination (39.1%), ginning out turn (38.3%), staple length (27.8 mm), and staple fineness (4.0 μg/in.) when compared with un-infested plants (30.6, 3.2 g, 77.3%, 41.5%, 28.8 mm, and 4.4 μg/in.), respectively. Correlation analysis revealed that bolls per plant, boll weight, germination, ginning out turn, and staple fineness resulted in negative and significant correlation with density of D. koenigii with r-values of ?0.95, ?0.98, ?0.98, ?0.8, and ?0.85, respectively. Coefficient of determination (R²) demonstrated that bolls per plant, boll weight, germination, ginning out turn, staple length, and staple fineness contributed with 91, 96, 96, 79, 43 and 73%, respectively, to the total variability at different densities of D. koenigii. Our findings demonstrated that D. koenigii has become potential major pest and causes quantitative and qualitative losses to transgenic cotton, hence, there is a need to develop appropriate control measures for controlling its population to avoid further losses.     

Pathogenic variation and virulence related responses of <Emphasis Type="Italic">Ascochyta lentis</Emphasis> on lentil

Ascochyta blight of lentil (Lens culinaris ssp. culinaris) is caused by Ascochyta lentis. The disease causes severe damage to all aerial parts of the plant and may lead to total crop loss during extremely severe epidemics. To identify qualitative differences in resistance within Australian lentil crops, variation in virulence was examined among 17 isolates of A. lentis on six differential lentil genotypes (ILL7537, ILL5588 (cv. Northfield), ILL6002, ILL5722 (cv. Digger), ILL481 (cv. Indianhead) and CIPA203 (cv. Nipper)). Six distinct virulence patterns were identified, with Pathotype I (AL4) being highly virulent, causing disease on all genotypes except ILL7537 and pathotype VI (Kewell) exhibiting low virulence on all genotypes. Histopathology studies were carried out to further understand interaction differences between isolate-host combinations and add to the knowledge of possible resistance mechanisms underlying lentil’s defence to the pathogen. The infection process was compared between lentil genotypes with different levels of resistance and isolates with different levels of virulence. Microscopic and biochemical differences were observed between compatible and incompatible interactions, which were related to time-after-inoculation, with slower responses noted in susceptible lentil genotypes. Relatively fast release of reactive oxygen species (ROS) and a subsequent hypersensitive response (HR) was central to initial defence at the point of penetration in the most resistant lentil genotypes.     

Genetic variability of tolerance to Verticillium albo-atrum and Verticillium dahliae in Medicago truncatula

Verticillium wilt caused by Verticillium albo-atrum and Verticillium dahliae is responsible for yield losses in many economically important crops. The capacity of pathogenic fungi to adapt to new hosts is a well-known threat to the durability of resistant crop varieties. In the present work, 25 Medicago truncatula genotypes from a core collection and six V. albo-atrum and V. dahliae strains were used to study the potential of non-host Verticillium strains isolated from different plant species to infect this legume plant and the plant’s susceptibility to the pathogens. The experiment was designed as factorial with randomised complete blocks and with three repetitions. The wilt symptoms caused by V. albo-atrum and V. dahliae were scored on a disease index scale from 0 to 4, during 30 days after inoculation of 10-day-old plantlets. Disease severity was quantified by maximum symptom scores (MSS) and areas under the disease progress curves (AUDPC), which integrate the time course of symptom development. Highly significant differences were observed among plant genotypes and fungal strains, and their interaction was also significant. The correlation between MSS and AUDPC was 0.86. The most severe symptoms were caused by the alfalfa strain V. albo-atrum V31-2 and the least severe by V. dahliae JR2, as shown by mean values obtained on the 25 genotypes. M. truncatula genotype TN8.3 was the most susceptible genotype by mean values obtained with the six fungal strains, whereas F11013-3, F83005.9 and DZA45.6 were highly resistant to all strains studied. The results can be used to choose parents for studying the genetics of resistance in Medicago truncatula to Verticillium strains with different levels of aggressiveness.     

Characterization of European Plasmopara viticola isolates with reduced sensitivity to cymoxanil

Cymoxanil has been used for over 30 years to control grape downy mildew (Plasmopara viticola) in European vineyards, prevalently in mixture with other fungicides active on this disease. In the 1990’s cases of P. viticola resistant to cymoxanil were detected using a leaf disc assay. In this study, we establish that the presence of only 1 % of resistant isolates in a P. viticola population will allow the detection of cymoxanil resistance in the leaf disc assay. A poor correlation (R?=?0.194) was observed between the leaf disc assay and a whole- plant test for 38 P. viticola field populations collected in 2004. Over 60 % of these populations were characterized as fully sensitive in a whole-plant assay compared to 10 % in the leaf disc assay. Five P. viticola field isolates resistant to cymoxanil reverted to full sensitivity after six to nine transfers to untreated vines, indicating that cymoxanil resistance in P. viticola is unstable. Two European P. viticola populations sensitive to cymoxanil became resistant when transferred 12–14 times on vines treated with cymoxanil. In contrast, two populations originating from the USA and three monozoospore isolates from France retained full sensitivity to the fungicide after 13 cycles on cymoxanil-treated plants. Whole-plant experiments were conducted in the laboratory to compare the efficiency of spray programs to delay the development of cymoxanil resistance. Whereas the continuous use of cymoxanil alone quickly selected for resistance, the mixture of cymoxanil and folpet applied either continuously or in strict or block alternation effectively prevented the development of resistance over 10 generations of the fungus. These results demonstrate that resistance to cymoxanil in P. viticola can be managed with appropriate spray programs.