Broad spectrum biological control of diseases caused by soilborne plant pathogens such as Pythium, Phytophthora, and Rhizoctonia solani requires the introduction into or presence of edaphic sources of organic nutrients in soil for sustenance of biocontrol agents. The decomposition level of organic matter critically affects the composition of bacterial taxa as well as the populations and activities of biocontrol agents. Competition, antibiosis, parasitism, and systemic induced resistance are all affected. Highly stabilized sources of Sphagnum peat consistently fail to support sustained biological control, even when inoculated with biocontrol agents. Composts, on the other hand, can serve as an ideal food base for biocontrol agents and offer an opportunity to introduce and establish specific biocontrol agents into soils, which in turn leads to sustained biological control based on the activities of microbial communities.

BIOCONTROL WITHIN THE CONTEXT OF SOIL MICROBIAL COMMUNITIES: A Substrate-Dependent Phenomenon.

Reliable, precise and accurate estimates of disease severity are important for predicting yield loss, monitoring and forecasting epidemics, for assessing crop germplasm for disease resistance, and for understanding fundamental biological processes including co-evolution. Disease assessments that are inaccurate and/or imprecise might lead to faulty conclusions being drawn from the data, which in turn can lead to incorrect actions being taken in disease management decisions. Plant disease can be quantified in several different ways. This review considers plant disease severity assessment at the scale of individual plant parts or plants, and describes our current understanding of the sources and causes of assessment error, a better understanding of which is required before improvements can be targeted. The review also considers how these can be identified using various statistical tools. Indeed, great strides have been made in the last thirty years in identifying the sources of assessment error inherent to visual rating, and this review highlights ways that assessment errors can be reduced—particularly by training raters or using assessment aids. Lesion number in relation to area infected is known to influence accuracy and precision of visual estimates—the greater the number of lesions for a given area infected results in more overestimation. Furthermore, there is a widespread tendency to overestimate disease severity at low severities (<10%). Both interrater and intrarater reliability can be variable, particularly if training or rating aids are not used. During the last eighty years acceptable accuracy and precision of visual disease assessments have often been achieved using disease scales, particularly because of the time they allegedly save, and the ease with which they can be learned, but recent work suggests there can be some disadvantages to their use. This review considers new technologies that offer opportunity to assess disease with greater objectivity (reliability, precision, and accuracy). One of these, visible light photography and digital image analysis has been increasingly used over the last thirty years, as software has become more sophisticated and user-friendly. Indeed, some studies have produced very accurate estimates of disease using image analysis. In contrast, hyperspectral imagery is relatively recent and has not been widely applied in plant pathology. Nonetheless, it offers interesting and potentially discerning opportunities to assess disease. As plant disease assessment becomes better understood, it is against the backdrop of concepts of reliability, precision and accuracy (and agreement) in plant pathology and measurement science. This review briefly describes these concepts in relation to plant disease assessment. Various advantages and disadvantages of the different approaches to disease assessment are described. For each assessment method some future research priorities are identified that would be of value in better understanding the theory of disease assessment, as it applies to improving and fully realizing the potential of image analysis and hyperspectral imagery.

Plant Disease Severity Estimated Visually, by Digital Photography and Image Analysis, and by Hyperspectral Imaging

Oomycetes form a deep lineage of eukaryotic organisms that includes a large number of plant pathogens which threaten natural and managed ecosystems. We undertook a survey to query the community for their ranking of plant-pathogenic oomycete species based on scientific and economic importance. In total, we received 263 votes from 62 scientists in 15 countries for a total of 33 species. The Top 10 species and their ranking are: (1) Phytophthora infestans; (2, tied) Hyaloperonospora arabidopsidis; (2, tied) Phytophthora ramorum; (4) Phytophthora sojae; (5) Phytophthora capsici; (6) Plasmopara viticola; (7) Phytophthora cinnamomi; (8, tied) Phytophthora parasitica; (8, tied) Pythium ultimum; and (10) Albugo candida. This article provides an introduction to these 10 taxa and a snapshot of current research. We hope that the list will serve as a benchmark for future trends in oomycete research.

/pdf/the-top-10-oomycete-pathogens-in-molecular-plant-pathology-3gxpkolrjj.pdf

The Top 10 oomycete pathogens in molecular plant pathology

Farmers are facing serious plant protection issues and phytosanitary risks, in particular in the tropics. Such issues are food insecurity, lower income in traditional low- input agroecosystems, adverse effects of pesticide use on human health and on the environment in intensive systems and export restrictions due to strict regulations on quarantine pests and limits on pesticide residues. To provide more and better food to populations in both the southern and northern hemispheres in a sustainable manner, there is a need for a drastic reduction in pesticide use while keeping crop pest and disease damage under control. This can be achieved by breaking with industrial agriculture and using an agro- ecological approach, whose main pillar is the conserva- tion or introduction of plant diversity in agroecosystems. Earlier literature suggest that increasing vegetational biodiversity in agroecosystems can reduce the impact of pests and diseases by the following mechanisms: (1) resource dilution and stimulo-deterrent diversion, (2)

https://link.springer.com/content/pdf/10.1007%2Fs13593-011-0022-4.pdf

Plant species diversity for sustainable management of crop pests and diseases in agroecosystems: a review

An Improved Grading System For Measuring Plant Diseases Vol-36

Influence of temperature and wetness duration on infection of strawberry flowers by Botrytis cinerea and disease incidence of fruit originating from infected flowers

Six different individuals (raters) assessed the severity of Phomopsis leaf blight on strawberry leaflets in five experimental repetitions over 2 years by making a direct visual estimation of the percentage of diseased area of each leaflet or by using the Horsfall-Barratt (H-B) disease scale. Intra-rater and inter-rater reliability and accuracy were determined, and then the relationship between visually estimated severity values and actual severity values was evaluated. Agreement in estimated disease severity values between assessment times by the same raters (i.e., intra-rater reliability), and agreement in disease severity values among raters at a single assessment time (i.e., inter-rater reliability), were both high, with most correlation coefficients being greater than 0.85. The intra-class correlation for overall agreement among raters ranged from 0.80 to 0.96 for the five repetitions. Based on the concordance coefficient calculated for each rater in each repetition, agreement between estimated and actual severity (i.e., accuracy) was somewhat lower than reliability. The relationship between estimated and actual severity was linear, and there was a slight trend to overestimate disease severity. The H-B scale was not more reliable or accurate than direct estimation of severity, and the linear relationship between estimated and actual severity did not support the principles underling the H-B scale. Both size of leaflets and number of lesions per leaflet slightly affected the error in estimate of disease severity.

https://apsjournals.apsnet.org/doi/pdfplus/10.1094/PHYTO.2003.93.8.995

Reliability and accuracy of visual estimation of phomopsis leaf blight of strawberry.

(...) Generally, fruit disease incidence increased with increased wetness durations, but on mature fruit at 35 C, disease incidence decreased over time from a maximum of 39%. Optimum temperature for infection on both immature and mature fruit was between 25 and 30 C, with greater than 80% disease incidence after 13 hr of wetness. A regression model using the logit of disease incidence as the dependent variable accurately described infection level as a function of wetness duration (W) and temperature (T). Terms in the model were W, WT, WT 2 , and WT 3 , and all estimated parameters were significant. Coefficients of determination for combined data from three repetitions of the experiment were 0.71 and 0.83 for immature and mature fruit, respectively

Influence of temperature and wetness duration on infection of immature and mature strawberry fruit by Colletotrichum acutatum

Heterogeneity of the incidence of downy mildew of grape, caused by Plasmopara viticola, was quantified in an experimental Ohio vineyard. The proportion of diseased leaves on each of 15 shoots (sampling units) per plot was determined for 18 separate plots at two times (generally August and September) during each of 3 yr. The binary data analogue of Taylor's power law, in which the logarithm of the observed variance is regressed on the logarithm of the theoretical variance for a binomial (random) distribution, provided strong and consistent evidence that diseased leaves were aggregated. Year and assessment time did not affect power law parameters. The estimate of the regression slope (b), an overall measure of heterogeneity, was 1.30 (SE = 0.04). Heterogeneity in individual plots was measured with variance ratio and C(α) tests and with the aggregation parameter (θ) of the beta-binomial distribution fitted to the data. Except for mean incidence less than 0.05, the majority of the plots had significant heterogeneity, and the data were better described by the beta-binomial than by the binomial distribution. Estimates of θ were variable but were highest in the middle range of disease incidence. Using the modified power law, sampling curves were generated to precisely estimate disease incidence

Spatial heterogeneity of the incidence of grape downy mildew

An electronic warning system for grape downy mildew- based on models for the infection of leaves of Vitis lambrusca, production of sporangia by Plasmopara viticola in lesions, and sporangial survival-was tested over 7 years in Ohio. Grapevines were sprayed with metalaxyl plus mancozeb (Ridomil MZ58) when the warning system indicated that environmental conditions were favorable for sporulation and subsequent infection. Over the 7 years, plots were sprayed from one to four times according to the warning system, and from four to 10 times according to the standard calendar-based schedule (depending on the date of the initiation of the experiment). The warning system resulted in yearly reductions of one to six sprays (with median of three sprays). Disease incidence (i.e., proportion of leaves with symptoms) in unsprayed plots at the end of the season ranged from 0 to 86%, with a median of 68%. Incidence generally was very similar for the warning-system and standard-schedule treatments (median of 7% of the leaves with symptoms), and both of these incidence values were significantly lower (P < 0.05) than that found for the unsprayed control, based on a generalized-linear-model analysis. Simplifications of the disease warning system, where sprays were applied based only on the infection or sporulation components of the system, were also effective in controlling the disease, although more fungicide applications sometimes were applied. Effective control of downy mildew, therefore, can be achieved with the use of the warning system with fewer sprays than a with a standard schedule.

https://apsjournals.apsnet.org/doi/pdfplus/10.1094/PDIS.2000.84.5.549

Michael A. Ellis

Papers

Influence of temperature and wetness duration on infection of strawberry flowers by Botrytis cinerea and disease incidence of fruit originating from infected flowers

Reliability and accuracy of visual estimation of phomopsis leaf blight of strawberry.

Influence of temperature and wetness duration on infection of immature and mature strawberry fruit by Colletotrichum acutatum

Spatial heterogeneity of the incidence of grape downy mildew

Evaluation of a Disease Warning System for Downy Mildew of Grapes.