Development of a high throughput optical density assay to determine fungicide sensitivity of oomycetes

Evidence of Phosphite Tolerance in Phytophthora cinnamomi from New Zealand Avocado Orchards

There is a limited number of chemical control agents for managing Phytophthora root and collar rot diseases of avocado internationally; of these, phosphite is one of the most effective. To determine whether prolonged phosphite use in New Zealand avocado orchards has led to decreased sensitivity of Phytophthora cinnamomi to phosphite, 57 isolates were collected from phosphite-treated and -untreated avocado orchards and screened for tolerance using a mycelial growth inhibition assay. The inhibitory effect of phosphite on mycelial growth was tested in vitro using six concentrations of phosphite. Based on changes in mycelial growth using optical density measurements to calculate the effective concentration to reduce growth by 50% (EC₅₀) estimates, three phosphite-susceptible (EC₅₀ range = 18.71 to 29.26 µg/ml) and three tolerant (EC₅₀ range = 81.85 to 123.89 µg/ml) isolates were selected. The effects of phosphite on the colonization of lupin (Lupinus angustifolius) seedling roots and sporangia and zoospore production of three susceptible and three tolerant isolates were determined. The three tolerant isolates colonized lupin roots more extensively than the three susceptible isolates in the presence of phosphite at 5 and 10 g/liter. The tolerant isolates were able to asymptomatically colonize further above the lesion margin in the lupin treated with phosphite at 5 g/liter relative to the phosphite-susceptible isolates but no isolates were completely resistant to phosphite. The tolerant isolates produced more sporangia and, consequently, zoospores in the presence of phosphite than the susceptible isolates. The detection of phosphite tolerance by P. cinnamomi in planta and in vivo is concerning for the future efficacy of phosphite to manage Phytophthora diseases.

Methods for Fungicide Efficacy Screenings: Multiwell Testing Procedures for the Oomycetes Phytophthora infestans and Pythium ultimum

Oomycetes-borne diseases represent a serious problem for agriculture sustainability due to the high use of chemical products employed for their control. In recent years, increasing concerns on side effects associated with fungicide utilization have led to the reduction of the permissible modes of action, with the remaining ones continuously threatened by the increase of resistant strains in the pathogen populations. In this context, it is mandatory to develop new generation fungicides characterized by high specificity towards the target species and low environmental impact to guarantee the sustainability, productivity, and quality of food production. Fungicide discovery is a lengthy and costly process, and despite these urgent needs, poor description and formalization of high-throughput methodologies for screening the efficacy of active compounds are commonly reported for these kinds of organisms. In this study, a comprehensive picture of two high-throughput practices for efficient fungicide screening against plant-pathogenic oomycetes has been provided. Different protocols using multiwell plates were validated on approved crop protection products using Phytophthora infestans and Pythium ultimum as the model species. In addition, detailed statistical inputs useful for the analysis of data related to the efficacy of screenings are included.

Discovery of γ-Lactam Alkaloid Derivatives as Potential Fungicidal Agents Targeting Steroid Biosynthesis

Biological control of plant pathogens is considered as one of the green and effective technologies using beneficial microorganisms or microbial secondary metabolites against plant diseases, and so microbial natural products have played important roles in the research and development of new and green agrochemicals. To explore the potential applications for natural γ-lactam alkaloids and their derivatives, 26 γ-lactams that have flexible substituent patterns were synthesized and characterized, and their in vitro antifungal activities against eight kinds of plant pathogens belonging to oomycetes, basidiomycetes, and deuteromycetes were fully evaluated. In addition, the high potential compounds were further tested using an in vivo assay against Phytophthora blight of pepper to verify a practical application for controlling oomycete diseases. The potential modes of action for compound D1 against Phytophthora capsici were also investigated using microscopic technology (optical microscopy, scanning electron microscopy, and transmission electron microscopy) and label-free quantitative proteomics analysis. The results demonstrated that compound D1 may be a potential novel fungicidal agent against oomycete diseases (EC₅₀ = 4.9748 μg·mL^-1 for P. capsici and EC₅₀ = 5.1602 μg·mL^-1 for Pythium aphanidermatum) that can act on steroid biosynthesis, which can provide a certain theoretical basis for the development of natural lactam derivatives as potential antifungal agents.

A High-Throughput Microtiter-Based Fungicide Sensitivity Assay for Oomycetes Using Z'-Factor Statistic

Current methods to quantitatively assess fungicide sensitivity for a diverse range of oomycetes are slow and labor intensive. Microtiter-based assays can be used to increase throughput. However, many factors can affect their quality and reproducibility. Therefore, efficient and reliable methods for detection of assay quality are desirable. The objective of this study was to develop and validate a robust high-throughput fungicide phenotyping assay based on spectrophotometric quantification of mycelial growth in liquid culture and implementation of quality control with Z' factor and growth curves. Z' factor was used to ensure that each isolate grew enough in the absence of fungicides compared with the negative control, and growth curves were used to ensure active growth at the time of concentration of a fungicide that reduces growth by 50% (EC₅₀) estimation. EC₅₀ and relative growth values were correlated in a side-by-side comparison with values obtained using the amended medium (gold standard) assay. Concordance correlation indicated that the high-throughput assay is accurate but may not be as precise as the amended medium assay. To demonstrate the utility of the high-throughput assay, the sensitivity of 216 oomycete isolates representing four genera and 81 species to mefenoxam and ethaboxam was tested. The assay developed herein will enable high-throughput fungicide phenotyping at a population or community level.