Syama Chatterton

Identifying and Managing Root Rot of Pulses on the Northern Great Plains

Pulse crops (annual grain legumes such as field pea, lentil, dry bean, and chickpea) have become an important component of the cropping system in the northern Great Plains of North America over the last three decades. In many areas, the intensity of damping-off, seedling blight, root rot, and premature ripening of pulse crops is increasing, resulting in reduction in stand establishment and yield. This review provides a brief description of the important pathogens that make up the root rot complex and summarizes root rot management on pulses in the region. Initially, several specific Fusarium spp., a range of Pythium spp., and Rhizoctonia solani were identified as important components of the root rot disease complex. Molecular approaches have recently been used to identify the importance of Aphanomyces euteiches on pulses, and to demonstrate that year-to-year changes in precipitation and temperature have an important effect on pathogen prevalence. Progress has been made on management of root rot, but more IPM tools are required to provide effective disease management. Seed-treatment fungicides can reduce damping-off and seedling blight for many of the pathogens in this disease complex, but complex cocktails of active ingredients are required to protect seedlings from the pathogen complex present in most commercial fields. Partial resistance against many of the pathogens in the complex has been identified, but is not yet available in commercial cultivars. Cultural practices, especially diversified cropping rotations and early, shallow seeding, have been shown to have an important role in root rot management. Biocontrol agents may also have potential over the long term. Improved methods being developed to identify and quantify the pathogen inoculum in individual fields may help producers avoid high-risk fields and select IPM packages that enhance yield stability.

Identification of bacterial pathogens and races of Pseudomonas syringae pv. phaseolicola from dry bean fields in Western Canada

Abstract Several bacterial diseases can significantly impact dry bean production in western Canada. The objectives of this study were to assess the incidence of multiple bacterial diseases on dry bean in southern Alberta and to determine the prevalent races causing halo blight (HB) in western Canadian dry bean fields. Field surveys combined with a pathogen isolation component were conducted in 2012–2014 to determine relative frequency of bacterial pathogens. Bacterial pathogens causing HB and brown spot were most commonly isolated from symptomatic tissues, whereas the pathogens causing common blight (CBB) and bacterial wilt were infrequently detected. In order to screen bean breeding lines for resistance to HB, the races of Pseudomonas syringae pv. phaseolicola (Pph) present in western Canada needed to be determined. A total of 114 Pph isolates were recovered from dry bean samples from Alberta, Saskatchewan and Manitoba in 2010–2013. Virulence testing of these isolates on the halo blight dry bean differential set indicated that only races 2 and 6 were present. These races were equally predominant in Manitoba, race 2 comprised 81% of Alberta isolates, and only race 2 occurred in Saskatchewan. Screening of select CBB-resistant breeding lines indicated that all were susceptible to a race 2 isolate of Pph. However, several commercial cultivars either currently grown, or recently registered for commercial production in Alberta, showed reduced susceptibility to HB. These results should be applicable to dry bean breeding programmes attempting to incorporate halo blight resistance into Canadian-adapted dry bean lines.

A multiplex PCR assay for the detection and quantification of Sclerotinia sclerotiorum and Botrytis cinerea

Traditional culture methods for identifying the plant fungal pathogens Sclerotinia sclerotiorum (Lib.) de Bary and Botrytis cinerea Pers.:Fr. are slow and laborious. The goal of this study was to develop a multiplex real‐time PCR (qPCR) assay to detect and quantify DNA from S. sclerotiorum and B. cinerea. A primer set (SsIGS_5) for S. sclerotiorum was designed that targeted the intergenic spacer (IGS) regions of the ribosomal DNA. Addition of a probe to the assay increased its specificity: when the primer/probe set was tested against 21 fungal species (35 strains), amplification was detected from all S. sclerotiorum strains and no other species. For qPCR, the SsIGS_5 primer and probe set exhibited a linear range from 7·0 ng to 0·07 pg target DNA (R2 = 0·99). SsIGS_5 was then multiplexed with a previously published primer/probe set for B. cinerea to develop a high‐throughput method for the detection and quantification of DNA from both pathogens. When multiplexed, the sensitivity and specificity of both assays were not different from individual qPCR reactions. The multiplex assay is currently being used to detect and quantify S. sclerotiorum and B. cinerea DNA from aerosol samples collected in commercial seed alfalfa fields.

Fusarium Diseases of Canadian Grain Crops: Impact and Disease Management Strategies

The genus Fusarium, first described in the early nineteenth century, is composed of a wide range of soil-borne saprophytic and pathogenic fungi. More than a few hundred different phylogenetic species of Fusarium have been identified to date. Plant species are the main target of Fusarium pathogenicity, although some species, including F. chlamydosporum, F. oxysporum and F. verticillioides, have been shown to infect immune-compromised humans. It is said that most plant species are susceptible to at least one disease caused by Fusarium fungi. Fusarium species can cause vascular wilt diseases, for which a broad range of host plants are susceptible, involving fungal colonization of the xylem via the roots and the growing mycelium eventually causes vessel obstruction, blocking transport of water to the aerial parts of the plant. In dicots, over 100 formae speciales of F. oxysporum have been identified as causative agents in vascular wilt, including F. oxysporum ff. spp. lycopersici, phaseoli and pisi, which infect tomato, beans and pea crops, respectively. Fusarium species also cause root rots and stem rots of various field crops worldwide, including peas and related pulse crops. In cereals and corn (maize) Fusarium crown rot (FCR) and Fusarium stalk (stem) rot, respectively, are caused by a different group of Fusarium pathogens from those responsible for diseases in dicots, and include F. graminearum, F. culmorum, F. avenaceum, F. verticillioides and F. pseudograminearum. In addition to root and stem rot diseases, Fusarium species also infect the inflorescence structures, causing Fusarium head blight (FHB; also known as scab) in cereals and Fusarium ear blight (sometimes referred to as FEB) in maize, and leads to damage and yield loss in developing kernels. There is an overlap of species responsible for Fusarium crown and stalk rots with those responsible for Fusarium head and ear blights. Many of these species produce harmful mycotoxins, including trichothecenes and fumonisins, which accumulate in the kernels of infected heads. In this chapter, we will start with an introduction to Fusarium species, their classification and genetics, provide a review of the Fusarium diseases of three groups of Canadian field crops (cereals, maize and pulses), followed by sections on disease management strategies, and Fusarium toxin quantification methods.

Temporal Dynamics of Botrytis cinerea and Sclerotinia sclerotiorum in Seed Alfalfa Fields of Southern Alberta, Canada

Blossom blight of seed alfalfa (Medicago sativa L.), caused by the fungal pathogens Botrytis cinerea and Sclerotinia sclerotiorum, is a potentially devastating disease on the Canadian Prairies in moist growing seasons. Monitoring the airborne spore concentrations of these pathogens could facilitate disease forecasting in the region. Nineteen seed alfalfa fields in southern Alberta, Canada were assessed throughout the growing seasons of 2014 and 2015. Trace levels of blossom blight symptoms were found in both years; however, plated floret and pod samples indicated that, overall, B. cinerea increased over the growing season whereas S. sclerotiorum decreased. In both seasons, Burkard 7-day volumetric spore samplers collected daily aerosol samples, and weather stations recorded environmental variables in three fields. Conidia and ascospores collected were quantified each day with a real-time polymerase chain reaction assay. Spore quantification indicated that both B. cinerea and S. sclerotiorum numbers remained low in July and increased in August. Both species had multiple days with high spore discharge, with seasonal maxima of 21,137 conidia and 2,265 ascospores. Exploratory model development indicated that spore discharge of both fungi is associated with environmental stressors such as large changes in relative humidity or high temperatures on preceding days.

Interactions of Root-Feeding Insects with Fungal and Oomycete Plant Pathogens

Soilborne fungal and oomycete pathogens are the causal agents of several important plant diseases. Infection frequently co-occurs with herbivory by root-feeding insects, facilitating tripartite interactions that modify plant performance and mortality. In an agricultural context, interactions between pathogens, herbivores, and plants can have important consequences for yield protection. However, belowground interactions are inherently difficult to observe and are often overlooked. Here, we review the impact of direct and indirect interactions between root-associated insects, fungi, and oomycetes on the development of plant disease. We explore the relationship between insect feeding injury and pathogen infection, as well as the role of insects as vectors of fungal and oomycete pathogens. Synergistic interactions between insects and phytopathogens may be important in weed suppression, and we highlight several promising candidates for biocontrol. Bridging the gap between entomological and pathological research is a critical step in understanding how interactions between insects and microorganisms modify the community structure of the rhizosphere, and how this impacts plant functioning. Furthermore, the identification of belowground interactions is required to develop effective pest monitoring and management strategies.

Potential role of pollen and pollinators in the spread of blossom blight of seed alfalfa caused by Botrytis cinerea

Abstract The fungal pathogen Botrytis cinerea Pers.: Fr. infects flowers of many plant species, including alfalfa (Medicago sativa L.). In southern Alberta, blossom blight, caused in part by B. cinerea, reduces yields of seed alfalfa in cool, wet growing seasons. Previous studies have suggested that B. cinerea uses pollen to begin infection, and that the fungal-infected pollen may be vectored by alfalfa leafcutter bees (Megachile rotundata Fab.). A longitudinal greenhouse study was performed to investigate the role of pollen in the infection process of alfalfa florets by B. cinerea. The effects of pollination (pollinated vs unpollinated) and inoculation method (dry vs aqueous suspension) on floret and pollen infection was tested. Florets were harvested at 0, 24, 48 and 94 h after inoculation. Pollinated florets that were dry inoculated had significantly greater levels of infection (84%) than similarly inoculated unpollinated florets (38%, P < 0.001); however no pollen infection was observed. In contrast, 90% of suspension-inoculated florets were infected and a small (<1%) but significant (P < 0.0001) level of infected pollen was observed in these samples. Leafcutter bees were also collected from seed alfalfa fields in 2013 and 2014 to estimate the prevalence of B. cinerea on pollen. Pollen removed from field bees showed no growth by B. cinerea; however, the bees that were directly plated on agar medium exhibited an increased load of B. cinerea as the growing season progressed. Thus, under typical field conditions, alfalfa pollen is not likely to be a significant factor contributing to the establishment and spread of B. cinerea.

Lack of efficacy of transgenic pea (Pisum sativum L.) stably expressing antifungal genes against Fusarium spp. in three years of confined field trials

Fusarium root rot is a major pea disease in Canada and only partial tolerance exists in germplasm. Transgenic technologies may hold promise but the economic benefits of genetically modified (GM) pea will need to surpass the regulatory costs, time and labor involved in bringing a GM crop to market. European pea (Pisum sativum L.) cultivars expressing four antifungal genes, 1-3 β glucanase (G), endochitinase (C) (belonging to PR proteins family), polygalacturonase inhibiting proteins (PGIPs) (P) and stilbene synthase (V) have been transformed for disease tolerance and showed disease tolerance under laboratory conditions. Transgenic lines with four antifungal genes inserted either individually or stacked through crossing were tested for their efficacy against Fusarium root rot (Fusarium avenaceum) in confined trials over three years (2013 to 2015) in comparison with two parental German lines and three Canadian lines. Superior emergence, higher fresh weight or lower disease ratings above and below ground, of transgenic lines in presence of disease inoculum were not observed consistently in the three years of field experiments when compared to the parental and Canadian lines in the presence of disease inoculum. No indication of an advantage of stacked genes over single genes was observed. Most transgenic lines had lower relative gene expression in the roots than in the leaves in greenhouse trials suggesting a possible explanation for poor tolerance to Fusarium root rot. Field trials are necessary to verify the agronomic performance and ecological relevance of the promising effects detected under laboratory conditions.

Effect of Lygus spp and Botrytis spp on faba bean (Vicia faba L.) seed quality – are there insect-pathogen interactions?

Abstract: Lygus bugs and Botrytis fungal pathogen, the causal agent of chocolate spot in faba bean, can cause necrotic spots on faba bean seeds, thereby reducing market value. The mid-pod stage is the most susceptible stage for chocolate spot development and Lygus infestation in faba beans. Therefore, we hypothesised that the concomitant presence of Lygus spp. and Botrytis spp. might increase seed necrosis. Hence, the study was conducted to determine (i) the spatial and local distribution of chocolate spot and Lygus spp. in central and southern Alberta, and (ii) the association of chocolate spot disease severity and Lygus abundance. Chocolate spot and Lygus were present in all the counties surveyed. Chocolate spot had a negative association with Lygus abundance, but only the latter was significantly associated with seed necrosis. Botrytis spp. were frequently isolated from seeds despite the lack of expression of chocolate spot on the foliage. No significant effect of Lygus abundance on Botrytis isolation from seeds was found. Therefore, seed quality losses can occur both due to the fungal pathogen and the insect, which likely occupy different niches influenced by microclimate. Economic thresholds and management strategies will be required to keep insect populations and disease progression under check.

Microwave radiation effect with and without chemical seed treatments on halo blight infected seed, seed germination, plant vigour, and yield of dry beans

Abstract: Halo blight, caused by Pseudomonas syringae pv. phaseolicola, is a seed-borne disease of dry bean (Phaseolus vulgaris L.) that lowers seed quality and yield. Over 2 yr, laboratory and field studies were conducted to evaluate the effect of microwave radiation on two market classes: navy (‘Envoy’) and white kidney (‘GTS 402’) bean. In the laboratory, seed germination and vigour decreased up to 15% after 40 s of microwave exposure, where <7% decrease was observed during 0–30 s. Disease plating showed no correlation between pathogen colonization of the seed and microwave radiation, as incidence of pathogen colonization was low across all exposure times. In field trials in Morden and Winkler, MB, microwave radiation was tested alone and in combination with copper hydroxide 53.8% and pyraclostrobin + fluxapyroxad + metalaxyl. Seed treatment with copper hydroxide slightly decreased the incidence of halo blight but had little impact on seed pick, hundred-seed weight, yield, or return on investment. Pyraclostrobin + fluxapyroxad + metalaxyl seed treatment had no effect on any of these parameters. Microwave radiation lowered seed emergence by up to 9% but did not reduce disease incidence and severity or increase yield or return of investment when applied alone or in combination with a chemical treatment.