Quantitative detection of pear-pathogenic Stemphylium vesicarium in orchards

Human vs. Machine, the Eyes Have It. Assessment of Stemphylium Leaf Blight on Onion Using Aerial Photographs from an NIR Camera

Aerial surveillance could be a useful tool for early detection and quantification of plant diseases, however, there are often confounding effects of other types of plant stress. Stemphylium leaf blight (SLB), caused by the fungus Stemphylium vesicarium, is a damaging foliar disease of onion. Studies were conducted to determine if near-infrared photographic images could be used to accurately assess SLB severity in onion research trials in the Holland Marsh in Ontario, Canada. The site was selected for its uniform soil and level topography. Aerial photographs were taken in 2015 and 2016 using an Xnite-Canon SX230NDVI with a near-infrared filter, mounted on a modified Cine Star—8 MK Heavy Lift RTF octocopter UAV. Images were taken at 15–20 m above the ground, providing an average of 0.5 cm/pixel and a field of view of 15 × 20 m. Photography and ground assessments of disease were carried out on the same day. NDVI (normalized difference vegetation index), green NDVI, chlorophyll index and plant senescence reflective index (PSRI) were calculated from the images. There were differences in SLB incidence and severity in the field plots and differences in the vegetative indices among the treatments, but there were no correlations between disease assessments and any of the indices.

The Role of Ascospores and Conidia, in Relation to Weather Variables, in the Epidemiology of Stemphylium Leaf Blight of Onion

Stemphylium leaf blight (SLB), caused by Stemphylium vesicarium, is an important foliar disease of onion in northeastern North America. The pathogen produces conidia and ascospores, but the relative contributions of these spore types to epidemics in onion is not known. Determining the abundance of ascospores and conidia during the growing season could contribute to a disease forecasting model. Airborne ascospores and conidia of S. vesicarium were trapped during the growing season of 2015 and 2016 at an onion trial in southern Ontario, Canada, using a Burkard 7-day volumetric sampler. Meteorological data were recorded hourly. Ascospore numbers peaked before the crop was planted and declined rapidly with time and at daily mean air temperatures >15°C. Conidia were present throughout the growing season and appear to be closely related to the development of SLB on onion. Daily spore concentrations were variable, but 59 to 73% of ascospores and approximately 60% of conidia were captured between 0600 and 1200 h. Spore concentrations increased 24 to 72 h after rainfall and precipitation and leaf wetness duration were consistently and positively associated with increases in numbers of conidia and subsequent SLB incidence. The first symptoms of SLB coincided with high numbers of conidia, rainfall, leaf wetness duration ≥8 h, and days with average daily temperature ≥18°C. The number of airborne ascospores was very low by the time SLB symptoms were observed. Ascospores may initiate infection on alternative hosts in early spring, while conidia are the most important inoculum for the epidemic on onions.

Specific detection of Lysobacter antibioticus strains in agricultural soil using PCR and real-time PCR

Lysobacter antibioticus is an important biocontrol bacteria against phytopathogens in soil, and with the ability to produce nonvolatile antimicrobial metabolites has been extensively characterised. It is important to establish applicable techniques to detect and monitor L. antibioticus directly and accurately in soil samples. We developed and tested 13 primer sets according to phenazine gene (phzA, phzB, phzD, phzF, phzS) and the cyclohexanone monooxygenase gene (phzNO1); a pair of primer phzNO1 F1/phzNO1 R1 based on the cyclohexanone monooxygenase (phzNO1) gene of L. antibioticus strain OH13 was selected and optimized polymerase chain reaction (PCR) amplification conditions for rapid and accurate detection. After screening eight strains of L. antibioticus, two strains of Lysobacter enzymogenes, one strain of Lysobacter capsici, Arthrobacterium, Bacillus, Microbacterium, Burkholderia, Pseudomonas and other bacterial strains isolated from different agricultural soils, the phzNO1 F1/phzNO1 R1 primers amplified a single PCR band of about 229 bp from L. antibioticus. The detection sensitivity with primers phzNO1 F1/phzNO1 R1 was 5.14 × 104 fg/25μL of genomic DNA and 2.254 × 1010 to 2.254 × 1011 colony-forming units/mL for the soil samples. Quantitative PCR assays were to develope as a specific method to monitor the L. antibioticus population in soil as well as guide soil micro-ecological management.

Disturbance in biosynthesis of arachidonic acid impairs the sexual development of the onion blight pathogen Stemphylium eturmiunum

The formation of sexual fruiting bodies for plant pathogenic fungi is a key strategy to propagate their progenies upon environmental stresses. Stemphylium eturmiunum is an opportunistic plant pathogen fungus causing blight in onion. This self-fertilizing filamentous ascomycete persists in the soil by forming pseudothecia, the sexual fruiting body which helps the fungus survive in harsh environments. However, the regulatory mechanism of pseudothecial formation remains unknown. To uncover the mechanism for pseudothecial formation so as to find a practical measure to control the propagation of this onion pathogen, we tentatively used DNA methyltransferase inhibitor 5-azacytidine (5-AC) to treat S. eturmiunum. 5-AC treatment silenced the gene-encoding monoacylglycerol lipase (magl) concomitant with the presence of the inheritable fluffy phenotype and defectiveness in pseudothecial development. Moreover, the silence of magl also resulted in a reduction of arachidonic acid (AA) formation from 27 ± 3.1 µg/g to 9.5 ± 1.5 µg/g. To correlate the biosynthesis of AA and pseudothecial formation, we created magl knockdown and overexpression strains. Knockdown of magl reduced AA to 11 ± 2.4 µg/g, which subsequently disabled pseudothecial formation. In parallel, overexpression of magl increased AA to 37 ± 3.4 µg/g, which also impaired pseudothecial formation. Furthermore, exogenous addition of AA to the culture of magl-silenced or magl knockdown strains rescued the pseudothecial formation but failed in the gpr1 knockdown strain of S. eturmiunum, which implicates the involvement of AA in signal transduction via a putative G protein-coupled receptor 1. Thus, AA at a cellular level of 27 ± 3.1 µg/g is essential for sexual development of S. eturmiunum. Disturbance in the biosynthesis of AA by up- and down-regulating the expression of magl disables the pseudothecial development. The specific requirement for AA in pseudothecial development by S. eturmiunum provides a hint to curb this onion pathogen: to impede pseudothecial formation by application of AA.

Epidemiological Features and Trends of Brown Spot of Pear Disease Based on the Diversity of Pathogen Populations and Climate Change Effects

Brown spot of pear, caused by the fungus Stemphylium vesicarium, is an emerging disease of economic importance in several pear-growing areas in Europe. In recent years, new control strategies combining sanitation practices and fungicide applications according to developed forecasting models have been introduced to manage the disease. However, the pathogenic and saprophytic behavior of this pathogen makes it difficult to manage the disease. In addition, climate change can also result in variations in the severity and geographical distribution of the disease. In this study, ecological and epidemiological aspects of brown spot of pear disease related to inoculum characterization and climate change impact were elucidated. The pathogenic variation in S. vesicarium populations from pear orchards and its relationship to inoculum sources (air samples, leaf debris, and infected host and nonhost tissues) was determined using multivariate analysis. In total, six variables related to infection and disease development on cultivar Conference pear detached leaves of 110 S. vesicarium isolates were analyzed. A high proportion of isolates (42%) were nonpathogenic to pear; 85% of these nonpathogenic isolates were recovered from air samples. Most isolates recovered from lesions (93%) and pseudothecia (83%) were pathogenic to pear. A group of pathogenic isolates rapidly infected cultivar Conference pear leaves resulted in disease increase that followed a monomolecular model, whereas some S. vesicarium isolates required a period of time after inoculation to initiate infection and resulted in disease increase that followed a logistic model. The latter group was mainly composed of isolates recovered from pseudothecia on leaf debris, whereas the former group was mainly composed of isolates recovered from lesions on pear fruit and leaves. The relationship between the source of inoculum and pathogenic/aggressiveness profile was confirmed by principal component analysis. The effect of climate change on disease risk was analyzed in two pear-growing areas of Spain under two scenarios (A2 and B1) and for three periods (2005 to 2009, 2041 to 2060, and 2081 to 2100). Simulations showed that the level of risk predicted by BSPcast model increased to high or very high under the two scenarios and was differentially distributed in the two regions. This study is an example of how epidemiological models can be used to predict not only the onset of infections but also how climate change could affect brown spot of pear. [Formula: see text] Copyright © 2018 The Author(s). This is an open-access article distributed under the CC BY-NC-ND 4.0 International license .

Combined morphological and molecular approach for identification of Stemphylium vesicarium inoculum in pear orchards

Stemphylium vesicarium is the causal agent of brown spot of pear, an important disease reported in pear-growing areas of Europe. The pathogen is able to colonize pear leaf debris and dead tissues of herbaceous plants and produce abundant ascospores and conidia that are capable of infecting pear trees. Inoculum monitoring in pear orchards is mainly achieved through spore traps and species identification is based on conidial morphology, but the similarities on conidial traits among species of Stemphylium make correct identification difficult. In this work a total of thirty-seven Stemphylium isolates from pear orchards were characterized at the morphological, pathogenic, and molecular level. Correspondence among ITS and gpd sequences and morphological traits were evaluated. Species identification based exclusively on morphological data was not feasible. Combined morphological and molecular data were necessary for unambiguous identification of isolates in the S. vesicarium species group. Only isolates identified as S. vesicarium were pathogenic on pear. The study revealed that several species of Stemphylium coexist in pear orchards with S. vesicarium, the causal agent of BSP, and that combined morphological and molecular data are needed to differentiate them. Consequently, direct measurements of the airborne inoculum using volumetric spore traps may overestimate the actual pathogen population.

Indigenous populations of three closely related Lysobacter spp. in agricultural soils using real-time PCR

Previous research had shown that three closely related species of Lysobacter, i.e., Lysobacter antibioticus, Lysobacter capsici, and Lysobacter gummosus, were present in different Rhizoctonia-suppressive soils. However, the population dynamics of these three Lysobacter spp. in different habitats remains unknown. Therefore, a specific primer-probe combination was designed for the combined quantification of these three Lysobacter spp. using TaqMan. Strains of the three target species were efficiently detected with TaqMan, whereas related non-target strains of Lysobacter enzymogenes and Xanthomonas campestris were not or only weakly amplified. Indigenous Lysobacter populations were analyzed in soils of 10 organic farms in the Netherlands during three subsequent years with TaqMan. These soils differed in soil characteristics and crop rotation. Additionally, Lysobacter populations in rhizosphere and bulk soil of different crops on one of these farms were studied. In acid sandy soils low Lysobacter populations were present, whereas pH neutral clay soils contained high populations (respectively, <4.0-5.87 and 6.22-6.95 log gene copy numbers g(-1) soil). Clay content, pH and C/N ratio, but not organic matter content in soil, correlated with higher Lysobacter populations. Unexpectedly, different crops did not significantly influence population size of the three Lysobacter spp. and their populations were barely higher in rhizosphere than in bulk soil.

An update on control of brown spot of pear

Brown spot of pear is a fungal disease producing high economical losses in several pear-growing areas in Europe. Fungicide applications during the growing period either at fixed schedule or delivered according to the BSPcast forecasting system are not enough to control the disease under favorable conditions. New strategies have been introduced to control the inoculum production using sanitation methods. These methods are based on combinations of leaf litter removal during winter and biological control agent applications during late winter, spring and summer. These practices reduce both the inoculum pressure and disease levels. Therefore, the resulting optimized disease management consists of a combination of sanitation methods applied during the whole year with chemical fungicides scheduled according to the BSPcast forecasting model during the vegetative period. It is expected that the control of brown spot could be further refined upon availability of rapid methods for inoculum potential analysis. However, this analysis is difficult due to the variability in pathogenicity within the pathogen population.