Leaf Scald Research Articles

Molecular Detection and Quantification of Xanthomonas albilineans in Juice from Symptomless Sugarcane Stalks Using a Real-Time Quantitative PCR Assay.

Leaf scald, a bacterial disease caused by Xanthomonas albilineans (Ashby) Dowson, is a major limiting factor for sugarcane production worldwide. Accurate identification and quantification of X. albilineans is a prerequisite for successful management of this disease. A sensitive and robust quantitative PCR (qPCR) assay was developed in this study for detection and quantification of X. albilineans using TaqMan probe and primers targeting a putative adenosine triphosphate-binding cassette (ABC) transporter gene (abc). The novel qPCR assay was highly specific to the 43 tested X. albilineans strains belonging to different pulsed-field gel electrophoresis groups. The detection thresholds were 100 copies/µl of plasmid DNA, 100 fg/µl of bacterial genomic DNA, and 100 CFU/ml of bacterial suspension prepared from pure culture. This qPCR assay was 100 times more sensitive than a conventional PCR assay. The pathogen was detected by qPCR in 75.1% (410/546) of symptomless stalk samples, whereas only 28.4% (155/546) of samples tested positive by conventional PCR. Based on qPCR data, population densities of X. albilineans in symptomless stalks of the same varieties differed between two sugarcane production areas in China, Beihai (Guangxi Province) and Zhanjiang (Guangdong Province), and no significant correlation between these populations was identified. Furthermore, no relationship was found between these populations of the pathogen in asymptomatic stalks and the resistance level of the sugarcane varieties to leaf scald. The newly developed qPCR assay proved to be highly sensitive and reliable for the detection and quantification of X. albilineans in sugarcane stalks.

Registration of ‘CP 13‐1223’ sugarcane for Florida organic soils

Abstract‘CP 13‐1223’ (Reg. no. CV‐204, PI 698176) sugarcane (a complex hybrid of Saccharum spp.) was released to growers in June 2020 for producing on muck soils in Florida. It was developed through a decade‐long cooperative research program between the USDA‐ARS, the University of Florida Everglades Research and Education Center, and the Florida Sugar Cane League, Inc. This cultivar resulted from a poly‐cross made at the USDA‐ARS Canal Point (CP) Sugarcane Field Station. CP 13‐1223 was released because of its high commercial recoverable sucrose (CRS) and stalk weight, and demonstrated acceptable levels of cane and sucrose yields on muck soils. CP 13‐1223 has resistance to orange rust, leaf scald, Sugarcane mosaic virus strain E, ratoon stunt disease, and smut, and moderate resistance to brown rust. Based on results from 13 harvests across replicated yield trials at five locations, CP 13‐1223 performed 29.6 and 9.5% higher in CRS and stalk weight, respectively, than the check cultivar ‘CP 96‐1252’. It did not differ significantly from CP 96‐1252 in either cane yield, sucrose yield, or economic profitability. CP 13‐1223 is considered to have moderate tolerance to freezing in field tests.

Assessment of sugarcane cultivars with stable reaction to Xanthomonas albilineans under mechanical inoculation conditions

Leaf scald caused by Xanthomonas albilineans in sugarcane is one of the most important diseases around the world since it causes severe losses in both agricultural and industrial yields. In Cuba, resistance to this disease is considered a major selection criterion within the breeding program. The aim of this work was to detect sugarcane cultivars with stable reaction to X. albilineans by using both additive main effects and multiplicative interaction (AMMI) and linear general models. For this, 16 cultivars planted simultaneously in 2015, 2016, and 2017 in two locations: Jovellanos (Matanzas) and Florida (Camagüey) in Cuba were mechanically inoculated with a bacterial isolate previously characterized by sequencing its DNA fragment. Disease severity was recorded in plant cane and first ratoon, and results were consistent in both analyses and revealed that L55-5 and C323-68 were the most susceptible cultivars and C1051-73 was the most resistant; however, C1051-73, C275-80, C86-12, C88-382, C89-147, My5514, and Ty86-28 were the most stable across the years and localities evaluated. Results will allow adapting the methodology for the evaluation of the reaction to leaf scald of new sugarcane cultivars.

Characterization of a new mitovirus infecting the phytopathogenic fungus Microdochium albescens.

A novel mycovirus was identified infecting the phytopathogenic fungus Microdochium albescens. The characterized dsRNA segment, corresponding to the replicative intermediate of the mitovirus genome, is 2,562 base pairs (bp) in length, with an A+U content of 62.3%. A single open reading frame (ORF) was identified, encoding a putative RNA-dependent RNA polymerase (RdRp) of 706 amino acids. Phylogenetic analysis showed that this virus should be classified as a member of a new species in the genus Mitovirus, family Mitoviridae, for which we propose the name "Mitovirus gaucho". This is the first report of a mycovirus infecting the phytopathogenic fungus M. albescens, the causative agent of leaf scald on rice in Brazil.

Registration of ‘CP 13‐4100’ sugarcane

Abstract‘CP 13‐4100’ (Reg. no. CV‐202, PI 698546) sugarcane cultivar (a complex hybrid of Saccharum spp.) was developed through cooperative research conducted by the USDA‐ARS, the University of Florida, and the Florida Sugar Cane League, Inc. The Florida Sugarcane Variety Committee (FSVC) released CP 13‐4100 in June 2020 to growers for commercial cultivation on mineral (sand) soils. It originated from a polycross made at Canal Point (CP) on 3 Dec. 2008 where CP 96‐1252 was the female parent and the male parent was unknown and could be any one of genotypes used in the polycross. CP 13‐4100 was released by FSVC for its high cane and sucrose yields, acceptable commercial recoverable sucrose, and resistances to orange rust, leaf scald, Sugarcane mosaic virus strain E (mosaic), and smut, and acceptable levels of susceptibility to brown rust. CP 13‐4100 and commercial reference checks (i.e., CL 88‐4730, CP 96‐1252, CPCL 97‐2730) yield data were collected from 12 harvests (i.e. three crop cycles—plant cane, first, and second ratoon—at four sand‐soil locations) in final‐stage replicated yield trials. Compared with CL 88‐4730 and CPCL 97‐2730, CP 13‐4100 had significantly (P ≤ .0001) higher cane yield (tonnes of cane per hectare: 18.7 and 29.5%), sucrose yield (tonnes of sugar per hectare: 26.3 and 30.7%), and economic index (EI; 10.9–16.9%) and no significant differences from CP 96‐1252 in these traits. CP 13‐4100 tested negative for the Bru1 marker, which is linked to a quantitative major locus of brown rust resistance. CP 13‐4100 could be considered moderately tolerant to freezing among 23 tested genotypes.

Distribution, phylogeny, and pathogenicity of Xanthomonas albilineans causing sugarcane leaf scald in Mexico

Disease outbreaks attributed to leaf scald of sugarcane caused by Xanthomonas albilineans were recently reported in southern Mexico. In this study, the distribution of leaf scald was investigated in the main sugarcane producing-states in Mexico. The disease was found in eight of 14 sugarcane-growing states and in 12 of 31 fields. In contaminated fields, incidence of leaf scald based on symptomatic plants ranged from 3 to 15%. The causal agent was identified by phylogenetic reconstruction using a single concatenated sequence of four genes (16S rRNA, fusA, leuS, and rpiA). Twelve X. albilineans isolates from different locations in Mexico produced disease symptoms after inoculation into stalks of susceptible variety Mex 69–290. Microscopic examination of infected stalks revealed structural changes within vascular and non-vascular tissues. These alterations reflected acute disease development and highlighted the importance of selecting varieties resistant to X. albilineans in breeding programs in Mexico.

Assessment and modeling using machine learning of resistance to scald (Rhynchosporium commune) in two specific barley genetic resources subsets

Barley production worldwide is limited by several abiotic and biotic stresses and breeding of highly productive and adapted varieties is key to overcome these challenges. Leaf scald, caused by Rhynchosporium commune is a major disease of barley that requires the identification of novel sources of resistance. In this study two subsets of genebank accessions were used: one extracted from the Reference set developed within the Generation Challenge Program (GCP) with 191 accessions, and the other with 101 accessions selected using the filtering approach of the Focused Identification of Germplasm Strategy (FIGS). These subsets were evaluated for resistance to scald at the seedling stage under controlled conditions using two Moroccan isolates, and at the adult plant stage in Ethiopia and Morocco. The results showed that both GCP and FIGS subsets were able to identify sources of resistance to leaf scald at both plant growth stages. In addition, the test of independence and goodness of fit showed that FIGS filtering approach was able to capture higher percentages of resistant accessions compared to GCP subset at the seedling stage against two Moroccan scald isolates, and at the adult plant stage against four field populations of Morocco and Ethiopia, with the exception of Holetta nursery 2017. Furthermore, four machine learning models were tuned on training sets to predict scald reactions on the test sets based on diverse metrics (accuracy, specificity, and Kappa). All models efficiently identified resistant accessions with specificities higher than 0.88 but showed different performances between isolates at the seedling and to field populations at the adult plant stage. The findings of our study will help in fine-tuning FIGS approach using machine learning for the selection of best-bet subsets for resistance to scald disease from the large number of genebank accessions.

Elite rice variety ‘VL Dhan 159’ for cultivation under organic condition of Uttarakhand hills

VL Dhan 159 is an early duration rice variety suitable for cultivation under rainfed upland organic condition of Uttarakhand hills. This variety has been developed by ICAR-Vivekananda Parvatiya Krishi Anusandhan Sansthan(VPKAS) Almora, Uttarakhand for the kharif season and released by the State Varietal Release Committee of Uttarakhand in 2020, Subsequently it was notified by the Central Seed Committee vide notification number S.O.500(E) dated the 29th January, 2021. VL Dhan 159 is derived from the cross VL 66/HPR 2143 with a maturity duration of 110-115 days. Under organic conditions, this variety has recorded an average yield of 2,054 kg/ha and out yielded the check varieties of Vivek Dhan 154 by 42.34 per cent, VL Dhan 221 by 27.34 per cent and VL Dhan 156 by 33.64 per cent. It was found resistant against leaf and neck blast (score 0-3), brown leaf spot (score 3-4), sheath rot (score 1), leaf scald (score 1-3), false smut (score 0), stem borer and leaf folder (score 0-1) under natural condition. It has more number of panicles/m2 (229) compared to the checks of Vivek Dhan 154 (200) and VL Dhan 156 (172). This variety has non lodging intermediate plant height, semi-erect with well exerted panicle. It has exhibited a better quality characteristics like high hulling (82.8%) and milling (72.1%), intermediate Alkali Spreading Value (4.0), Gel consistency (50) and amylose content (25.93%). Grain Chalkiness is very occasionally present. It has short bold grain with L/B ratio of 2.00 Key wordsVL Dhan 159, organic condition, Rainfed upland, hill rice, early duration, high yielding variety

Volatile compounds from plum genotypes with different levels of resistance to leaf scald disease

AbstractPlum leaf scald (PLS) is the main obstacle to plum expansion in Brazil. This disease is caused by Xylella fastidiosa, which colonizes the foregut of xylem‐sap feeding insects and the xylem of the plants. Breeding programmes identified new plum genotypes that showed no leaf scald symptoms in the field and remained negative for X. fastidiosa for at least 10 years. However, the mechanisms that confer this resistance are not fully elucidated. One hypothesis is the emission of volatile organic compounds by the host. Therefore, we investigated the volatile compound profile from six plum genotypes with different resistance levels to PLS. Thirty‐nine compounds were identified. The major constituents in the plum volatile extract were phytol (46.76%) and limonene (2.48%). Cultivar Laetitia presented the highest total alkane content, Simka showed the highest alcohol content, while Fortune and SC13 exhibited the highest concentration of ketones. Cedrol was not detected in susceptible genotypes but was found in the resistant ones, SC7 and SC15. (E,E)‐α‐farnesene was detected in high concentrations in Fortune but was not found in SC13 and SC15. According to principal components analysis, genotypes were divided into four groups based on the similarity of the volatile profile: 1: Fortune and Laetitia; 2: Simka and SC13; 3: SC7; and 4: SC15. This is the first description of leaf volatile chemical components of Prunus salicina, and the putative factor related to resistance in SC7 and SC15 is discussed. We also propose a terpene pathway of the main volatile compounds found.

Building the Embrapa rice breeding dataset for efficient data reuse

AbstractEmbrapa has led breeding programs for irrigated and upland rice (Oryza sativa L.) since 1977, generating a large amount of pedigree and phenotypic data. However, there were no systematic standards for data recording nor long‐term data preservation and reuse strategies. With the new aim of making data reuse practical, we recovered all data available and structured it into the Embrapa Rice Breeding Dataset (ERBD). In its current version, the ERBD includes 20,504 crosses involving 9,974 parents, the pedigrees of most of the 4,532 inbred lines that took part in advanced field trials, and phenotypic data from 2,711 field trials (1,118 irrigated, 1,593 upland trials), representing 226,458 field plots. Those trials were conducted over 38 years (1982–2019), in 247 locations, in latitudes ranging from 3°N to 33°S. Phenotypic traits included grain yield, days to flowering, plant height, canopy lodging, and five important fungal diseases: leaf blast, panicle blast, brown spot, leaf scald, and grain discoloration. The total number of data points surpasses 1.27 million. Descriptive statistics were computed over the dataset, split by cropping systems (irrigated or upland). The mean heritability of grain yield was high for both systems, at around .7, whereas the mean coefficient of variation was 13.9% for irrigated trials and 18.7% for upland trials. The ERBD offers the possibility of conducting studies on different aspects of rice breeding and genetics, including genetic gain, G×E analysis, genome‐wide association studies and genomic prediction.

Index selection can improve the selection efficiency in a rice recurrent selection population

To attain success with selection process, the breeders need to understand the genetic variability into each population developed by breeding program. In this purpose, breeders need to estimate the variance components and genetic and phenotypic parameters. Besides, they need to have the ability to select simultaneously genetically top progenies for multiple traits. This study used a field trial, with a panel of 198 S0:2 lowland rice progenies, evaluated in three environments during two years in order to: (i) estimate genetic parameters for different traits in a rice breeding population, (ii) evaluate the impact of direct selection and selection performed through three selection indices in a rice population developed by recurrent selection, and (iii) compare three selection indices using different economic weights with direct and indirect selection. Significant genetic variation was found among progenies for: grain yield, plant height, days to flowering, panicle blast, leaf scald and grain discoloration. The direct selection provided high gains for single trait selection, but this method may results in undesirable changes in related traits. Thus, between the evaluated indices and in comparison, to indirect selection the Mulamba and Mock index provided simultaneously greater gains for all the evaluated traits. Furthermore, we found that when the random weights and $$b$$ -variation index were used as economic weight, it was found lower gains or no gains, showing that the gains attained with Smith and Hazel and Tai indices were linked to economic weights used.

First sharpshooter species proven as vectors of Xylella fastidiosa subsp. multiplex in Prunus salicina trees in Brazil

Plum leaf scald (PLS), caused by the vector-borne bacterium Xylella fastidiosa, is a major obstacle to the expansion of plum crops in Brazil. In other affected crops, this pathogen is naturally transmitted mainly by sharpshooter leafhoppers (Hemiptera: Cicadellidae: Cicadellinae). Despite the importance of PLS, the vector species responsible for bacterial spread in plums remain unknown. This study, therefore, aimed to determine the X. fastidiosa transmission ability of three sharpshooter species commonly found in plum orchards in southern and southeastern Brazil: Macugonalia cavifrons (Stål), Macugonalia leucomelas (Walker), and Sibovia sagata (Signoret). After a 72-h acquisition access period to plum trees infected with X. fastidiosa subsp. multiplex (sequence type 67), the insects were transferred for an inoculation access period of 96 h to 10–20 healthy Prunus salicina plants for each sharpshooter species, using four insects per plant. Bacterial infection in the plants was verified 7 months after inoculation by polymerase chain reaction. Results showed that the three sharpshooter species were able to transmit X. fastidiosa to plums, with mean transmission efficiencies by single insects of 16%, 14%, and 18% for M. cavifrons, M. leucomelas, and S. sagata, respectively. To our knowledge, this study is the first to identify vector species and prove the transmission of a PLS strain of X. fastidiosa subsp. multiplex by sharpshooters in Brazil.

Registration of ‘CP 11‐1640’ sugarcane

Abstract‘CP 11‐1640’ (Reg. no. CV‐199, PI 695085) sugarcane (a complex hybrid of Saccharum spp.) was developed through cooperative research conducted by the USDA–ARS, the University of Florida, and the Florida Sugar Cane League, Inc., and released by the Florida Sugarcane Variety Committee (FSVC) to growers for commercial cultivation on mineral (sand) soils in Florida in June 2019. It was selected from a polycross made at Canal Point on 2 Jan. 2009 with Canal Point sugarcane breeding line CP 01‐2390 (female parent). Its male parent was unknown and could be any one of the clones used in the polycross. CP 11‐1640 was released by FSVC because of its high cane and sucrose yields, acceptable commercial recoverable sucrose, and resistance to brown rust, orange rust, leaf scald, Sugarcane mosaic virus strain E (mosaic), and smut. Based on the results of 12 harvests with three crops (plant cane, first ratoon, and second ratoon) at four locations in final‐stage replicated yield trials on sand soils and compared with reference cultivars (CL 88‐4730, CP 96‐1252, CPCL 97‐2730), CP 11‐1640 had significantly higher cane yield (13.4–16.9%), sucrose yield (12.0–12.8%), and economic index (10.9–16.9%) than CL 88‐4730 and CPCL 97‐2730 and did not differ from CP 96‐1252 in any of these traits. Cane fiber content of CP 11‐1640 (138.9 kg Mg−1) was equivalent to that of CL 88‐4730 but slightly higher than that of CP 96‐1252 and CPCL 97‐2730. CP 11‐1640 was considered to be moderately tolerant to freezing among 18 genotypes tested.

Registration of ‘CP 11‐1314’ sugarcane

Abstract‘CP 11‐1314’ (Reg. no. CV‐196, PI 693071) sugarcane, a complex hybrid of Saccharum spp., was developed through cooperative research conducted by the USDA‐ARS, the University of Florida, and the Florida Sugar Cane League, Inc.; it was released to Florida growers in June 2018 for cultivation on organic soils and further recommended in June 2019 for sand soils. This cultivar was derived from a polycross between CP 05‐1451 and a mixture of potential pollen donors. CP 11‐1314 was selected for release based on high cane yield and acceptable sucrose content. In addition, CP 11‐1314 displays good ratooning ability (particularly on muck soils), minimal sucrose decrease following freezing temperatures, and overall disease resistance. CP 11‐1314 is positive for the Bru1 locus, which confers resistance to brown rust, and is resistant or moderately resistant to all other sugarcane diseases of concern in Florida, including ratoon stunt disease, leaf scald, mosaic, and orange rust.

Naked eye evaluation and quantitative detection of the sugarcane leaf scald pathogen,

Sugarcane leaf scald caused by the bacterium Xanthomonas albilineans is a major disease of sugarcane worldwide. Whereas erratic symptoms make phenotypic detection challenging, molecular methods require expensive instruments and labour, and longer sample-to-answer times. We report a novel method for detection of X. albilineans DNA in sugarcane xylem sap. The method involves (i) boiling lysis-based DNA extraction from sugarcane sap; (ii) magnetic purification of target sequences directly from the lysate through use of magnetic bead-bound capture probes; and (iii) DNA sandwich hybridisation platform for HRP/TMB/H2O2 reaction-based naked eye visualisation and electrochemical detection of the target. The method is sensitive (limit of detection 100 fM) and reproducible (relative standard deviation <7%) with linear dynamic range 100 fM–1 nM (R2 = 0.99). The method was tested on a range of sugarcane cultivars of known resistance ratings (susceptible, intermediate resistant, and resistant) for leaf scald disease from an inoculated field trial. Detection levels agreed with the resistance rating of cultivars tested. In addition, qPCR results strongly correlated with our assay (r = 0.91–0.99, P < 0.01) and cultivar resistance rating. We believe that our assay could be useful for rapid screening as well as sensitive quantification of target pathogen DNA in infected sugarcane plants.

Plum leaf scald: characteristics of the causative agent,symptoms, dissemination, control and prevention

Abstract The very slow-growing gram-negative bacterium, Xylella fastidiosa, inhabits the xylem and is vector-borne. It causes several diseases in plants, including plum leaf scald (PLS), which is the main limiting factor over the years for the crop expansion, both in productivity and in cultivation areas in Brazil. There is little information about the management and control of the disease. Brazilian breeding programs of plum trees have launched few resistant cultivars. The aim of this work was to carry out a systematic review with current information regarding PLS, characteristics of the causative agent, symptoms, transmission/dissemination, control and prevention. The use of certified propagation material, free from bacteria and the elimination of infected plants are the main measures used in Brazil. The vast majority of commercial cultivars are highly susceptible to bacteria and only few cultivars of interest have any resistance. In 2017 the Agricultural Research and Rural Extension Company of Santa Catarina launched the ‘Zafira’ plum cultivar, the first cultivar for commercial purposes that is not naturally infected; however, it was observed that the transmission of the bacteria occurs by grafting. The Institute of Rural Development of Paraná IAPAR-EMATER evaluated different plum crosses and concluded that the ‘PR-1095’ genotype was the most resistant and did not show any foliar symptoms of the disease. However, the PCR test revealed the presence of the bacteria, indicating that the genotype is probably tolerant. In contrast, ‘PR 1142’, ‘PR 1149’ and ‘PR 1260’ genotypes do not show symptoms or the presence of the bacteria by PCR, resulting in resistance to the disease. These genotypes have not been released, and further studies are still needed.

Registration of ‘CP 10‐1716’ sugarcane

Abstract‘CP 10‐1716’ (Reg. no. CV‐195, PI 691536) sugarcane (a complex hybrid of Saccharum spp.) was released (7 June 2018) through cooperative research by the USDA–ARS Sugarcane Field Station at Canal Point, the University of Florida, and the Florida Sugar Cane League, Inc. CP 10‐1716 is recommended for use on mineral (sand) soils in Florida. CP 10‐1716 resulted from polycross X07‐1257 (CP 00‐2180 × Mix07Q; male parent is unknown). Replicated trials at four sand soil locations, with yield data from 12 harvests and three crop cycles (i.e., plant cane, first ratoon, and second ratoon) showed that CP 10‐1716 had cane and sugar yields, and profitability (economic index, US$ ha−1), that were similar to check cultivars (‘CL 88‐4730’ and ‘CP 96‐1252’). Commercial recoverable sucrose (CRS) in CP 10‐1716 was significantly lower than the check averages, but the early CRS was not significantly different from the mean of the checks. The economic index for CP 10‐1716 vs. the mean of the checks was $2490 vs. $2445 ha−1, respectively, with CP 10‐1716 showing no significant difference from the mean of the checks. CP 10‐1716 was rated as moderately resistant to ratoon stunt disease and resistant to brown and orange rust, mosaic, smut, leaf scald, and yellow leaf disease on sand soils. CP 10‐1716 is positive for the Bru1 molecular marker that is one of the indicators for brown rust resistance. CP 10‐1716 has low to moderate freeze tolerance as it relates to sucrose degradation.

Selection of plum genotypes for resistance to leaf scald

ABSTRACT Plum (Prunus salicina Lindl.) cultivation is seriously compromised in Brazil due to the occurrence of leaf scald, a disease caused by the bacterium Xylella fastidiosa subsp. multiplex. The objective of this study was to evaluate plum genotypes agronomically promising for resistance to leaf scald, under natural conditions of occurrence of the disease. Ten out of 50 plum genotypes belonging to the germplasm bank of “Instituto Agronômico do Paraná (IAPAR)” were selected. Five evaluations of leaf scald severity were carried out from January to February (2017 and 2018). Polymerase chain reaction (PCR) was also performed for samples of these genotypes to determine the presence of the bacterium X. fastidiosa subsp. multiplex. The genotypes differed in relation to their susceptibility to leaf scald. PR-1095 was the most resistant genotype and did not show any leaf symptom of the disease. In contrast, genotypes PR-1126 and PR-1137 had the greatest susceptibility to the disease, showing severe scald symptoms. However, PCR revealed the presence of the bacterium even in the most resistant genotypes, indicating, for example, that PR-1095, which had no symptoms of the disease, is probably tolerant to leaf scald.

Pyrenophora teres and Rhynchosporium secalis Establishment in a Mediterranean Malt Barley Field: Assessing Spatial, Temporal and Management Effects

Malt barley is one of the promising crops in Greece, mainly due to high yields and contract farming, which have led to an increase in malt barley acreage. Net form net blotch (NFNB), caused by Pyrenophora teres f. teres, and barley leaf scald, caused by Rhynchosporium secalis, are among the most important barley diseases worldwide and particularly in Greece. Their occurrence in malt barley can exert a significant negative effect on malt barley grain yield and quality. An experimental trial across two growing seasons was implemented in Greece in order (i) to estimate the epidemiology of NFNB and leaf scald in a barley disease-free area when the initial inoculation of the field occurs through infected seeds, (ii) to explore the spatial dynamics of disease spread under the interaction of the nitrogen rate and genotype when there are limited sources of infected host residues in the soil and (iii) to assess the relationship among the nitrogen rate, grain yield, quality variables (i.e., grain protein content and grain size) and disease severity. It was confirmed that both NFNB and leaf scald can be carried over from one season to the next on infected seed under Mediterranean conditions. However, the disease severity was more pronounced after the barley tillering phase when the soil had been successfully inoculated, which supports the hypothesis that the most important source of primary inoculum for NFNB comes from infected host residue. Increasing the rate of nitrogen application, when malt barley was cultivated in the same field for a second year in a row, caused a non-significant increase in disease severity for both pathogens from anthesis onwards. However, hotspot and commonality analyses revealed that spatial and genotypic effects were mainly responsible for hiding this effect. In addition, it was found that the effect of disease infections on yield, grain size and grain protein content varied in relation to the genotype, pathogen and stage of crop development. The importance of crop residues in the evolution of both diseases was also highlighted.

Settling and feeding behavior of sharpshooter vectors of Xylella fastidiosa on plum genotypes resistant to leaf scald disease

Plum leaf scald (PLS) is a serious disease caused by Xylella fastidiosa subsp. multiplex, a pathogenic bacterium transmitted by xylem-sap feeding leafhoppers, commonly named sharpshooters (Hemiptera: Cicadellidae: Cicadellinae). In Brazil, the development of plum genotypes that apparently are not infected under field conditions has opened new perspectives for disease control based on host plant resistance. For example, PLS incidence on ‘SC13’ and ‘SC7’ genotypes is almost null under field conditions, although ‘SC13’ is infected by graft inoculation. Thus, we hypothesized that the performance of these genotypes in the field may be related to vector behavior. To verify this hypothesis, we investigated the settling preference and probing and feeding behavior of the sharpshooters Bucephalogonia xanthophis (Berg) and Sibovia sagata (Signoret) on the PLS-resistant plum genotypes, ‘SC7’ and ‘SC13’, and on the naturally infected cultivar, ‘Laetitia’. In the settling experiments, ‘SC7’ was less preferred by both vector species. For S. sagata held on ‘SC13’, reduced sap ingestion rates were observed. For B. xanthophis, probing and feeding activities differed between individuals held on ‘SC13’ and ‘Laetitia’. Specifically, duration of the stylet pathway phase, duration of xylem sap ingestion events, and number of interruptions of the xylem phase differed for B. xanthophis held on ‘SC13’ and ‘Laetitia’. The results show that the resistant genotypes affected host plant selection by the sharpshooters and highlight the importance of incorporating vector behavior into studies aimed at identifying or developing X. fastidiosa-resistant genotypes.