The Influence of Irrigation Frequency and Cultivar Blends on the Severity of Multiple Root Diseases in Sugar Beets

Understanding Root Rot Disease in Agricultural Crops

Root rot diseases remain a major global threat to the productivity of agricultural crops. They are usually caused by more than one type of pathogen and are thus often referred to as a root rot complex. Fungal and oomycete species are the predominant participants in the complex, while bacteria and viruses are also known to cause root rot. Incorporating genetic resistance in cultivated crops is considered the most efficient and sustainable solution to counter root rot, however, resistance is often quantitative in nature. Several genetics studies in various crops have identified the quantitative trait loci associated with resistance. With access to whole genome sequences, the identity of the genes within the reported loci is becoming available. Several of the identified genes have been implicated in pathogen responses. However, it is becoming apparent that at the molecular level, each pathogen engages a unique set of proteins to either infest the host successfully or be defeated or contained in attempting so. In this review, a comprehensive summary of the genes and the potential mechanisms underlying resistance or susceptibility against the most investigated root rots of important agricultural crops is presented.

Managing Crop Diseases Under Water Scarcity

The significance of water scarcity to crop production and food security has been globally recognized as a pivotal sustainability challenge in the UN Sustainable Development Goals (86). The critical link between water scarcity and sustainability is adaptation. Various changes in water use practices have been employed to alleviate production constraints. However, the potential for these changes to influence crop diseases has received relatively little attention, despite the circumglobal importance of diseases to agricultural sustainability. This article reviews what is known about the realized effects of scarcity-driven alterations in water use practices on diseases in the field in order to raise awareness of the potential for both increased disease risk and possible beneficial effects on crop disease management. This is followed by consideration of the primary mechanistic drivers underlying disease outcomes under various water use adaptation scenarios, concluding with a vision for disease-water co-management options and future research needs.

Factors Influencing Rust (Melampsora apocyni) Intensity on Cultivated and Wild Apocynum venetum in Altay Prefecture, China

Rust (Melampsora apocyni) on Apocynum venetum is the major constraint to the commercial development of this medicinal herb. To determine the factors influencing rust intensity (maximum disease index [DI_max]), rust was investigated from 2011 to 2015 in both cultivated and wild A. venetum plants. Partial least squares path modeling (PLS-PM) was used to analyze the paths and extent of the factors related to pathogen, environment, and host that affect rust intensity. DI_max exhibited considerable variations across years and study sites, with variations linked to various factors fostering disease development. PLS-PM explained 80.0 and 70.1% of variations in DI_max in cultivated and wild plants, respectively. Precipitation was the key factor determining DI_max in both cultivated and wild plants (path coefficient [PC] = 0.313 and 0.544, respectively). In addition, the topsoil water content in cultivated plants and the total vegetation coverage in wild plants were also critical determinants of DI_max via their effects on the microclimatic factor (contribution coefficients [CC] = 0.681 and 0.989, respectively; PC = 0.831 and 0.231, respectively). In both cultivated and wild plants, host factors were mainly dominated by A. venetum density (CC = 0.989 and 0.894, respectively), and their effect on DI_max via the microclimatic factor (PC = 0.841 and 0.862, respectively) exceeded that via the inoculum factor (PC = 0.705 and 0.130, respectively). However, the indirect effects led to DI_max variation, while the dilution effect on host (CC = 0.154) from weed in wild plants led to the indirect effect size in wild plants of 0.200, which was lower than -0.699 in cultivated plants.

Potential of Epicoccum purpurascens Strain 5615 AUMC as a Biocontrol Agent of Pythium irregulare Root Rot in Three Leguminous Plants

Epicoccum purpurascens stain 5615 AUMC was investigated for its biocontrol activity against root rot disease caused by Pythium irregulare. E. purpurascens greenhouse pathogenicity tests using three leguminous plants indicated that the fungus was nonpathogenic under the test conditions. The germination rate of the three species of legume seeds treated with a E. purpurascens homogenate increased significantly compared with the seeds infested with P. irregulare. No root rot symptoms were observed on seeds treated with E. purpurascens, and seedlings appeared more vigorous when compared with the non-treated control. A significant increase in seedling growth parameters (seedling length and fresh and dry weights) was observed in seedlings treated with E. purpurascens compared to pathogen-treated seedlings. Pre-treating the seeds with the bioagent fungus was more efficient for protecting seeds against the root rot disease caused by P. irregulare than waiting for disease dispersal before intervention. To determine whether E. purpurascens produced known anti-fungal compounds, an acetone extract of the fungus was analyzed by gas chromatography mass spectrometry. The extract revealed a high percentage of the cinnamic acid derivative (trimethylsiloxy) cinnamic acid methyl ester. The E. purpurascens isolate grew more rapidly than the P. irregulare pathogen in a dual culture on potato dextrose agar nutrient medium, although the two fungi grew similarly when cultured separately. This result may indicate antagonism via antibiosis or competition.

Improving Root Health and Yield of Dry Beans in the Nebraska Panhandle with a New Technique for Reducing Soil Compaction

A field study conducted during the 2001 and 2002 growing seasons investigated the integration of fungicide applications and tillage methods for reducing root health problems in dry bean (Phaseolus vulgaris) plants by alleviating soil compaction and its potential exacerbation of root disease. Several cultural practices were combined with applications of the strobilurin fungicide azoxystrobin. Soil compaction was created artificially throughout the entire plot area. Six treatments, consisting of four tillage treatments and two combinations of tillage or applications of azoxystrobin, were tested to alleviate the compaction and enhance root health. Tillage treatments included a compacted control with no additional tillage, formation of beds approximately 10 cm above soil surface, zone tillage with an implement using in-row shanks, and both zone tillage and bedding combined. Fungicide treatments utilized the combination of both zone tillage and bedding with fungicide applications, and a fungicide treatment singly. Effects of compaction on plant vigor and disease development and severity were evaluated 67 and 83 days after planting in 2001 and 2002, respectively, by a visual estimation of plot vigor and by destructively sampling and making root and hypocotyl disease ratings on dry bean plants from nonharvest rows. Soil resistance and moisture were measured in plots 80 and 104 days after planting in 2001 and 2002, respectively, to estimate degree of compaction. In both years, Fusarium root rot, caused by Fusarium solani f. sp. phaseoli, was determined to be the main root disease impacting plant health in studies. All measured variables (root disease index, plant vigor ratings, total seed yield, seed size, and soil resistance) were significantly improved by any treatment that included zone tillage prior to planting. No added advantages were observed for decreasing disease or improving root health and plant performance with the use of azoxystrobin or by planting on raised beds. This is the first study to evaluate zone tillage as a method of reducing plant stress and root disease in dry bean plants.

Nonparametric analysis of ordinal data in designed factorial experiments

ABSTRACT Plant disease severity often is assessed using an ordinal rating scale rather than a continuous scale of measurement. Although such data usually should be analyzed with nonparametric methods, and not with the typical parametric techniques (such as analysis of variance), limitations in the statistical methodology available had meant that experimental designs generally could not be more complicated than a one-way layout. Very recent advancements in the theoretical formulation of hypotheses and associated test statistics within a nonparametric framework, together with development of software for implementing the methods, have made it possible for plant pathologists to analyze properly ordinal data from more complicated designs using nonparametric techniques. In this paper, we illustrate the nonparametric analysis of ordinal data obtained from two-way factorial designs, including a repeated measures design, and show how to quantify the effects of experimental factors on ratings through estimated relative marginal effects.

Ecology and epidemiology of benyviruses and plasmodiophorid vectors

Beet necrotic yellow vein virus (BNYVV) and Beet soilborne mosaic virus (BSBMV) are members of the genus Benyvirus, and Burdock mottle virus (BdMV) is a tentative member. BNYVV and BSBMV are vectored by the plasmodiophorid Polymyxa betae, which has a worldwide distribution. Polymyxa betae is morphologically indistinguishable from P. graminis, but recent molecular studies support separation of the two species. The geographic distribution of BNYVV is also worldwide, but BSBMV has been identified only in the United States. In Europe and Japan, several genotypic strains of BNYVV have been identified, and those with a fifth RNA appear to be more aggressive. No thorough survey of genotypic variability of BNYVV or BSBMV has been conducted in the United States. However, both viruses are widespread and frequently found in the same field, infecting the same beet plant. The implications of this close proximity, with regard to disease incidence and severity, and for recombination, are uncertain. Recent technological advances that permit improved detection and quantification of these viruses and their vector offer tremendous research opportunities.