Infection and dispersal processes of Pseudomonas syringae pv. Coriandricola on coriander

An Australian isolate of Pseudomonas syringae pv. coriandricola (Psc) was used to study aspects of dispersal of the pathogen and infection of coriander. Needle inoculation of Psc in the stem of coriander resulted in up to 94% plant mortality. The total biomass of inoculated plants decreased significantly four weeks after inoculation as compared to the control plants. Systemic infection was rapid and one week after inoculation a bacterial population of about 104 to 108 cfu g-1 was isolated from different parts of the stems and roots. Surface contamination of coriander seed with Psc prior to planting resulted in low disease incidence (8%). However, spray inoculation of healthy coriander plants with Psc at concentrations equal to or greater than 103 CFU mL-1 caused disease on the leaves, suggesting infection was common through the stomata. Splash dispersal of Psc was demonstrated, with disease being observed on surrounding coriander seedlings. Psc bacteria were dispersed up to 70 cm from the inoculum sourc

Genetic diversity of Xanthomonas citri subsp. citri in citrus orchards in northwest Paraná state, Brazil

Xanthomonas citri subsp. citri, which causes Asiatic citrus canker (ACC), is an important pathogen of citrus in Brazil and elsewhere. The genetic diversity of X. citri subsp. citri pathotype ‘A’ has not been studied in Brazil at a local scale (up to 300 km). Forty isolates were sampled from lesions of ACC on citrus in three orchards in Paraná state, Brazil. Twelve minisatellite markers were used to characterize the genetic diversity of the isolates. An Unweighted Paired Group Method of Arithmetic Averages tree was used for identifying unique multilocus haplotypes but there was no association between haplotypes and source locations. An analysis of molecular variance among populations showed that 98% of the variance was accounted for within the populations, and only 2% was accounted for among populations. Differences among populations was not significant (Φ=0.018, P=0.2). The relatively high, yet uniform, genetic diversity among isolates and low degree of spatial differentiation between populations of X. citri subsp. citri suggests that the populations in Paraná state have a common origin and strong historical epidemiological links

Disease Incidence-Severity Relationships on Leaflets, Leaves, and Fruit in the Pecan-Venturia effuse Pathosystem

The most destructive disease of pecan in the southeastern United States is scab, caused by Venturia effusa. Incidence (I)-severity (S) relationships have not previously been characterized in this pathosystem, but incidence measures can save time and should have higher accuracy compared with estimates of severity. Ten scab-susceptible cultivars and seedling trees were assessed for I and S of scab on fruit (1,972 trees) and foliage (compound leaves and leaflets, 1,129 trees) between 2010 and 2014. Samples were assessed on a tree basis, and sample size ranged from 10 to 100 specimens per tree. The range in mean I and S was different depending on the organ (fruit I = 0 to 100%, S = 0 to 100%; compound leaves and leaflets, I = 0 to 100%, S = 0-10.1%, respectively). However, mean I could be 100% at a mean S 80%. The observation that low severity persists until a high incidence of scab is achieved may limit the ability of incidence data to clearly differentiate treatment effects, even when based on the CLL transformation. But if found to be effective, and if used, it would reduce the labor requirements and result in more accurate data being obtained, as incidence estimates do not tend to suffer from the same subjective biases as do visual estimates of severity

Supplemental Material for Brungardt and Bock, 2023

Tab delimited annotation file for the Fraxinus pennsylvanica 609 de novo transcriptome including blast, Pfam, Kegg, GO and SignalP information.</p

Infection and dispersal processes of Pseudomonas syringae pv. coriandricola on coriander

An Australian isolate of Pseudomonas syringae pv. coriandricola (Psc) was used to study aspects of dispersal of the pathogen and infection of coriander. Needle inoculation of Psc in the stem of coriander resulted in up to 94% plant mortality. The total biomass of inoculated plants decreased significantly four weeks after inoculation as compared to the control plants. Systemic infection was rapid and one week after inoculation a bacterial population of about 104 to 108 cfu g-1 was isolated from different parts of the stems and roots. Surface contamination of coriander seed with Psc prior to planting resulted in low disease incidence (8%). However, spray inoculation of healthy coriander plants with Psc at concentrations equal to or greater than 103 CFU mL-1 caused disease on the leaves, suggesting infection was common through the stomata. Splash dispersal of Psc was demonstrated, with disease being observed on surrounding coriander seedlings. Psc bacteria were dispersed up to 70 cm from the inoculum source

Improved prediction of leaf emergence for efficacious crop protection: assessing field variability in phyllotherms for upper leaves in winter wheat and winter barley

The choice of the phyllotherm value for predicting leaf emergence under field conditions is pivotal to the success of fungicide-based disease risk management in temperate cereals. In this study, we investigated phyllotherm variability for predicting the emergence of the three uppermost leaves (i.e., three last leaves to emerge) in winter wheat and winter barley fields. Data from four sites representative of wheat and barley growing regions in Luxembourg were used within the PROCULTURE model to predict the emergence of F-2, F-1 and F (F being the flag leaf) during the 2014-2019 cropping seasons. The phyllotherms tested ranged between 100°Cd and 160°Cd, in 15°Cd steps, including the current default value of 130°Cd. The comparisons between the observed and predicted emerged leaf area were qualitatively evaluated using the mean absolute error (MAE), the root mean square error (RMSE) and Willmott's index (WI). A phyllotherm of 100°Cd accurately and reliably predicted the emergence of all three upper leaves under the various environmental conditions and crop cultivars of winter wheat and winter barley over the study period. MAE and RMSE were generally &lt;5% and the WI values were most often greater or equal to 0.90 for F-1 and F. For phyllotherm values greater or equal to 115°Cd, the prediction errors generally increased for F-1 and F, with MAE and RMSE exceeding 20% in most cases. F-2 agreement between observed and predicted values was generally similar when using 100°Cd or 115°Cd. These results tie in valuable, complementary information regarding the variability of phyllotherms within leaf layers in winter wheat and winter barley in Luxembourg. Accurate and reliable leaf emergence prediction from F-2 to F allows for timely fungicide application, which ensures lasting protection against infections by foliar fungal disease pathogens. Hence, understanding phyllotherms can help ensure timely, environmentally sound, and efficacious fungicide application while increasing the likelihood of improved yields of winter wheat and winter barley

What Interval Characteristics Make a Good Categorical Disease Assessment Scale?

Plant pathologists most often obtain quantitative information on disease severity using visual assessments. Category scales have been used for assessing plant disease severity in field experiments, epidemiological studies, and for screening germplasm. The most widely used category scale is the Horsfall-Barratt (H-B) scale, but reports show that estimates of disease severity using the H-B scale are less precise compared with nearest percent estimates (NPEs) using the 0 to 100% ratio scale. Few studies have compared different category scales. The objective of this study was to compare NPEs, the H-B midpoint converted data, and four different linear category scales (5 and 10% increments, with and without additional grades at low severity [0.1, 0.5, 1.0, 2.0, 5.0, 10.0, 15.0, 20.0…100%, and 0.1, 0.5, 1.0, 2.0, 5.0, 10.0, 20.0, 30.0…100%, respectively]). Results of simulations based on known distributions of disease estimation using the type II error rate (the risk of failing to reject H0 when H0 is false) showed that at disease severity ≤ 5%, a 10% category scale had a greater probability of failing to reject H0 when H0 is false compared with all other methods, while the H-B scale performed least well at 20 to 50% severity. The 5% category scale performed as well as NPEs except when disease severity was ≤ 1%. Both the 5 and 10% category scales with the additional grades included performed as well as NPEs. These results were confirmed with a mixed model analysis and bootstrap analysis of the original rater assessment data. A better knowledge of the advantages and disadvantages of category scale types will provide a basis for plant pathologists and plant breeders seeking to maximize accuracy and reliability of assessments to make an informed decision when choosing a disease assessment method

'Candidatus Liberibacter' Pathosystems at the Forefront of Agricultural and Biological Research Challenges

4 Pág.The “yellow branch” symptom was one of the first recorded symptoms of citrus greening in South Africa later identified to be caused by ‘Candidatus Liberibacter africanus’. Depicted image is a typical example of this symptom on Citrus reticulata observed in winter 2021 (da Graça et al.).Peer reviewe

Multiple Mutations and Overexpression in the <i>CYP51A</i> and <i>B</i> Genes Lead to Decreased Sensitivity of <i>Venturia effusa</i> to Tebuconazole

Multiple demethylation-inhibiting (DMI) fungicides are used to control pecan scab, caused by Venturia effusa. To compare the efficacy of various DMI fungicides on V. effusa, field trials were conducted at multiple locations applying fungicides to individual pecan terminals. In vitro assays were conducted to test the sensitivity of V. effusa isolates from multiple locations to various concentrations of tebuconazole. Both studies confirmed high levels of resistance to tebuconazole. To investigate the mechanism of resistance, two copies of the CYP51 gene, CYP51A and CYP51B, of resistant and sensitive isolates were sequenced and scanned for mutations. In the CYP51A gene, mutation at codon 444 (G444D), and in the CYP51B gene, mutations at codon 357 (G357H) and 177 (I77T/I77L) were found in resistant isolates. Expression analysis of CYP51A and CYP51B revealed enhanced expression in the resistant isolates compared to the sensitive isolates. There were 3.0- and 1.9-fold increases in gene expression in the resistant isolates compared to the sensitive isolates for the CYP51A and CYP51B genes, respectively. Therefore, two potential mechanisms—multiple point mutations and gene over expression in the CYP51 gene of V. effusa isolates—were revealed as likely reasons for the observed resistance in isolates of V. effusa to tebuconazole

Disease severity assessment in epidemiological studies: accuracy and reliability of visual estimates of Septoria leaf blotch (SLB) in winter wheat

Several methods are available to measure the severity of Septoria leaf blotch (SLB) of winter wheat but differ in accuracy, reliability, ease of use and rapidity. The accuracy and reliability of visual assessments of SLB severity by raters was determined by comparison with assumed actual values obtained by digital image analysis. Raters included one plant pathologist with extensive experience of visual disease assessment, and three other raters who were trained prior to field observations using standard area diagrams and the software DISTRAIN. Initially analyses were performed using SLB severity over the full 0-100% range; subsequently, to explore error over short ranges of the 0-100% scale, the scale was divided into sequential 10%-increments (i.e., 0-10%, 10-20%,…90-100%) based on the actual values. Lin’s concordance correlation (LCC) analysis demonstrated that all raters were accurate when compared over the whole severity range (LCC coefficient (ρc) = 0.92-0.99). However, agreement between actual SLB severities and the estimates by raters was less good when compared over the short intervals of the 10×10% classes (ρc = -0.12-0.99, depending on the percentage class and the experiment), demonstrating that agreement will vary depending on the actual disease range over which it is compared. Inter-rater reliability over the full 0-100% range measured using correlation analysis was high between each pair of raters (r = 0.970 to 0.992, P<0.0001), which was confirmed by the inter-class correlation coefficient (ρ ≥ 0.927). This study provides new insight into using a full range of actual disease severity versus limited ranges to ensure a realistic measure of rater accuracy and reliability, in addition to contributing to the ongoing debate on the use of visual disease estimates based on the 0-100% ratio scale for epidemiological research