Diagnosis of turfgrass diseases

Probe-Based Loop-Mediated Isothermal Amplification Assay for Rapid Detection of Two Clarireedia spp., the Causal Agent of Dollar Spot of Turfgrass

Dollar spot is a major fungal disease affecting turfgrass worldwide and can quickly destroy turfgrass swards. An assimilating probe-based loop-mediated isothermal amplification (LAMP) assay was developed to detect Clarireedia monteithiana and C. jacksonii, the causal agents of dollar spot within the continental United States. Five LAMP primers were designed to target the calmodulin gene with the addition of a 6-carboxyl-fluorescein florescent assimilating probe, and the temperature amplification was optimized for C. jacksonii and C. monteithiana identification. The minimum amount of purified DNA needed for detection was 0.05 ng μl-1. Specificity assays against host DNA and other turfgrass pathogens were negative. Successful LAMP amplification was also observed for dollar spot-infected turfgrass field samples. Further, a DNA extraction technique via rapid heat-chill cycles and visualization of LAMP results via a florescent flashlight was developed and adapted for fast, simple, and reliable detection in 1.25 h. This assimilating probe-based LAMP assay has proved successful as a rapid, sensitive, and specific method for the detection of C. monteithiana and C. jacksonii in pure cultures and from symptomatic turfgrass leaves blades. The assay represents a promising technology to be used in the field for on-site, point-of-care pathogen detection.

Distribution and Prevalence of Plant-Parasitic Nematodes of Turfgrass at Golf Courses in China

We sampled 127 turfgrass soil samples from 33 golf courses in NC, EC, and SC for plant-parasitic nematodes (PPNs). PPNs were extracted from soil samples using the shallow dish method and were identified at the genus or species levels with a combination of morphological and molecular methods. The results revealed 41 species of nematode belonging to 20 genera and 10 families. Nine genera are new records of PPNs associated with turfgrass in China. The PPNs show strong geographical distributions. Of the 20 genera, Helicotylenchus, Paratrichodorus, Hoplolaimus, Meloidogyne, Hemicriconemoides, and Mesocriconema showed higher infestation and frequency, and most of these genera had numbers in soil samples above established damage thresholds. Four golf courses had soil samples with PPNs > 30%, indicating the potential for nematode damage. The biodiversity indices H’, SR, J’, λ, and H2 showed significant differences among different regions and turfgrass species; H’, SR, J’, and H2 were significantly higher in EC than in NC and SC, while λ was lowest in EC. Creeping bentgrass had the highest H’, SR, J’, and H2 and the lowest λ in comparison with seashore paspalum and hybrid bermudagrass. These findings provide baseline information on the occurrence of turfgrass-associated PPNs in China, and have important implications for the effective management of PPNs causing damage on turfgrass.

Development of a Co-Dominant Cleaved Amplified Polymorphic Sequences Assay for the Rapid Detection and Differentiation of Two Pathogenic Clarireedia spp. Associated with Dollar Spot in Turfgrass

Dollar spot is one of the most destructive diseases in turfgrass. The causal agents belong to the genus Clarireedia, which are known for causing necrotic, sunken spots in turfgrass that coalesce into large damaged areas. In low tolerance settings like turfgrass, it is of vital importance to rapidly detect and identify the pathogens. There are a few methods available to identify the genus Clarireedia, but none of those are rapid enough and characterize down to the species level. This study produced a co-dominant cleaved amplified polymorphic sequences (CAPS) test that differentiates between C. jacksonii and C. monteithiana, the two species that cause dollar spot disease within the United States. The calmodulin gene (CaM) was targeted to generate Clarireedia spp. specific PCR primers. The CAPS assay was optimized and tested for specificity and sensitivity using DNA extracted from pure cultures of two Clarireedia spp. and other closely related fungal species. The results showed that the newly developed primer set could amplify both species and was highly sensitive as it detected DNA concentrations as low as 0.005 ng/µL. The assay was further validated using direct PCR to speed up the diagnosis process. This drastically reduces the time needed to identify the dollar spot pathogens. The resulting assay could be used throughout turfgrass settings for a rapid and precise identification method in the US.

Turfgrass Disease Diagnosis: Past, Present, and Future

Turfgrass is a multibillion-dollar industry severely affected by plant pathogens including fungi, bacteria, viruses, and nematodes. Many of the diseases in turfgrass have similar signs and symptoms, making it difficult to diagnose the specific problem pathogen. Incorrect diagnosis leads to the delay of treatment and excessive use of chemicals. To effectively control these diseases, it is important to have rapid and accurate detection systems in the early stages of infection that harbor relatively low pathogen populations. There are many methods for diagnosing pathogens on turfgrass. Traditional methods include symptoms, morphology, and microscopy identification. These have been followed by nucleic acid detection and onsite detection techniques. Many of these methods allow for rapid diagnosis, some even within the field without much expertise. There are several methods that have great potential, such as high-throughput sequencing and remote sensing. Utilization of these techniques for disease diagnosis allows for faster and accurate disease diagnosis and a reduction in damage and cost of control. Understanding of each of these techniques can allow researchers to select which method is best suited for their pathogen of interest. The objective of this article is to provide an overview of the turfgrass diagnostics efforts used and highlight prospects for disease detection.

Detection of root-infecting fungi on cool-season turfgrasses using loop-mediated isothermal amplification and recombinase polymerase amplification

Root-infecting fungal pathogens such as Gaeumannomyces avenae, Ophiosphaerella korrae, and Magnaporthiopsis poae cause extensive damage to amenity turfgrasses in temperate climates. The diseases they cause are difficult to diagnose by visual symptoms or microscopic inspection, and traditional polymerase chain reaction-based assays require large financial investments in equipment such as thermal cyclers and highly trained staff. The primary objective of this research was to develop fast and accurate detection assays for the three pathogens listed above that did not require the use of thermal cycling equipment. Loop-mediated isothermal amplification (LAMP) and recombinase polymerase amplification (RPA) assays were developed for each pathogen based on known fungal cultures. The assays were tested on 27 samples received at the University of Wisconsin's Turfgrass Diagnostic Laboratory in 2016 and 2017 and both methods provided accurate diagnoses within about 30 min with minimal sample preparation. However, the RPA assays had lower levels of false positive contamination relative to the LAMP assays and are more likely to be effective in a field or diagnostic laboratory for improved turf root-pathogen detection.

Approaches to Training Practitioners in the Art and Science of Plant Disease Diagnosis

Allowing plant pathology students to tackle fictitious or real crop problems during the course of their formal training not only teaches them the diagnostic process, but also provides for a better understanding of disease etiology. Such a problem-solving approach can also engage, motivate, and enthuse students about plant pathology in general. This paper presents examples of three problem-based approaches to diagnostic training utilizing freely available software. The first provides an "adventure-game" simulation where students are asked to provide a diagnosis and recommendation after exploring a hypothetical scenario or "case". Guidance is given on how to create these scenarios. The second approach involves students creating their own scenarios. The third uses a diagnostic template combined with reporting software to both guide and capture students' results and reflections during a real diagnostic assignment.

Avenacin Production in Creeping Bentgrass (Agrostis stolonifera) and Its Influence on the Host Range of Gaeumannomyces graminis

Avenacinase activity has been shown to be a key factor determining the host range of Gaeumannomyces graminis on oats (Avena sativa). G. graminis var. avenae produces avenacinase, which detoxifies the oat root saponin avenacin, enabling it to infect oats. G. graminis var. tritici does not produce avenacinase and is unable to infect oats. G. graminis var. avenae is also reported to incite take-all patch on creeping bentgrass (Agrostis stolonifera). It is unknown whether creeping bentgrass produces avenacin and if the avenacin-avenacinase interaction influences G. graminis pathogenicity on creeping bentgrass. The root extracts of six creeping bentgrass cultivars were analyzed by fluorimetry, thin-layer chromatography, and high performance liquid chromatography for avenacin content. Avenacin was not detected in any creeping bentgrass cultivars, and pathogenicity assays confirmed that both G. graminis var. avenae and G. graminis var. tritici can infect creeping bentgrass and wheat (Triticum aestivum), but only G. graminis var. avenae incited disease on oats. These results are consistent with the root analyses and confirm that avenacinase activity is not required for creeping bentgrass infection by G. graminis.

Integrated approaches for detection of plant pathogenic bacteria and diagnosis of bacterial diseases

Disease diagnosis is based on a number of factors, including laboratory tests for pathogen identification. Rapid development of genomic techniques for characterization of bacteria over the past decade has greatly simplified and improved pathogen detection and identification, but DNA-based methods have not yet entirely replaced traditional culture and phenotypic tests in the plant industry. The first section of this review focuses on rapid immunodiagnostic and DNA-based detection methods for known bacterial pathogens in plants or plant products, which often manifest no symptoms of disease. The second section covers the broader topic of disease diagnosis and new methods for identifying and characterizing bacteria.