Plant disease resistance test

Plant Disease Resistance Testing Service – Comprehensive Evaluation of Plant Health, Pathogen Resistance and Biocontrol Efficacy for Bulgarian Agriculture and Horticulture

As an ISO/IEC 17025 accredited contract research laboratory, we provide comprehensive plant disease resistance testing services to Bulgarian agricultural producers, seed companies, plant breeders, and biocontrol product developers. Plant disease resistance is a critical trait that determines crop yield, quality, and sustainability in the face of bacterial, fungal, viral, and nematode pathogens. Our test protocols evaluate the resistance of plant varieties, cultivars, and breeding lines to key pathogens, as well as the efficacy of biocontrol agents, fungicides, and induced resistance elicitors. We employ controlled-environment and field-based assays using standardised inoculation, scoring, and statistical methods. All procedures are aligned with ISTA, EPPO, and OECD guidelines, as well as BDS (Bulgarian Institute for Standardisation) requirements, and our reports are recognised by the Bulgarian Food Safety Agency (BFSA), the Ministry of Agriculture and Food (MAF), and the Bulgarian agricultural research and advisory community.

Plant Material, Pathogens and Biocontrol Agents We Test

Our experimental protocols cover a wide range of plant species and diseases relevant to Bulgarian agriculture, horticulture, and forestry.

Arabidopsis thaliana – model plant for disease resistance mechanism studies
Cereal crops – wheat, barley, oats, maize – evaluated against Fusarium spp., Puccinia spp., Blumeria graminis (powdery mildew)
Vegetable crops – tomato, pepper, cucumber, lettuce – evaluated against Phytophthora infestans, Fusarium oxysporum, Pseudomonas syringae, Xanthomonas spp.
Oilseed crops – sunflower, rapeseed – evaluated against Sclerotinia sclerotiorum, Plasmopara halstedii, Verticillium spp.
Fruit and vine crops – grapevine, apple, strawberry – evaluated against Botrytis cinerea, Plasmopara viticola, Venturia inaequalis
Ornamental and pot plants – geranium, petunia, cyclamen – evaluated against Rhizoctonia solani, Pythium ultimum, Botrytis spp.
Legume crops – soybean, bean, pea – evaluated against Fusarium spp., Rhizoctonia solani, Pseudomonas spp.
Biocontrol agents and inducers – including Bacillus, Trichoderma, Pseudomonas strains, chitosan, and salicylic acid analogues

Disease Resistance Assessment – Experimental Design and Inoculation

We use a range of standardised inoculation methods and experimental designs to evaluate disease resistance and the efficacy of disease management products.

Inoculum preparation – fungal, bacterial, and oomycete pathogens are cultured on standard media (e.g., PDA, NA, V8 juice agar) and inoculum is standardised to a specific concentration (spores/mL, CFU/mL, or mycelial fragments) using spectrophotometry (OD600) or haemocytometer counting.
Inoculation methods – we employ a variety of inoculation techniques depending on the pathogen and plant part: (a) spraying of spore suspension onto leaves (foliar pathogens), (b) soil drench (root and soil-borne pathogens), (c) wound inoculation (for stem and fruit pathogens), (d) seed coating (for seed-borne pathogens), and (e) detached leaf assays (for rapid screening).
Controlled environment – plants are grown in growth chambers or greenhouses at a temperature of 20-26 °C (depending on the pathosystem), with 16 h light/8 h dark photoperiod and 70-90 % relative humidity to optimise disease development.
Experimental design – we use a completely randomised design (CRD) or randomised complete block design (RCBD) with a minimum of 5-10 replicate plants per treatment, repeated over 2-3 independent experiments to account for environmental variability.
Controls – the design includes: (a) a mock-inoculated (water or buffer only) control, (b) a susceptible control cultivar or line, (c) a resistant control cultivar (for comparison), and (d) a positive biocontrol or fungicide standard, where appropriate.

Disease Scoring and Severity Assessment

We use quantitative and qualitative scoring systems to capture disease progression and severity. The most common approaches include:

Disease incidence (%) – the percentage of plants showing any visible disease symptom at a given time point; useful for measuring the speed of disease spread.
Disease severity index (DSI) – a 0-5 or 0-9 scale (0 = no symptoms, 5/9 = severe necrosis or wilting) is applied to each plant or leaf; the average score per treatment is calculated.
Area under disease progress curve (AUDPC) – calculated from repeated disease severity readings (e.g., at 3-day intervals) to capture the rate of disease progression over time; a lower AUDPC indicates higher resistance.
Leaf area damaged (%) – for foliar diseases, we use digital image analysis (ImageJ) to quantify the percentage of leaf area covered by lesions or sporulation.
Root and stem rating – for root and vascular diseases, we uproot plants at the end of the experiment and rate root necrosis, stem discolouration, and vascular browning on a 0-4 scale.
Molecular markers (qPCR) – for latent or symptomless infections, we extract DNA from plant tissues and quantify pathogen DNA using species-specific primers; the relative pathogen biomass is expressed as ng pathogen DNA / mg plant tissue.

Resistance Mechanism Studies – Defence Enzyme Assays and Gene Expression

To help clients understand the mode of action, we offer several mechanistic assays to measure host defence responses.

Defence enzyme activity – we measure the activity of key defence enzymes: (a) peroxidase (POD), (b) polyphenol oxidase (PPO), (c) phenylalanine ammonia-lyase (PAL), (d) β-1,3-glucanase, and (e) chitinase. Enzyme activity is measured spectrophotometrically and expressed as U/mg protein.
Phytoalexin and phenolic content – we quantify total phenolic and flavonoid content (Folin-Ciocalteu method) and specific phytoalexins (e.g., resveratrol in grapevine, glyceollin in soybean) by HPLC-MS, as markers of induced resistance.
Gene expression analysis (RT-qPCR) – we extract RNA from treated and control plant tissues and quantify the expression of defence-related genes (e.g., PR1, PDF1.2, LOX, NPR1) using gene-specific primers and standard curve methods. Relative expression is normalised to a housekeeping gene (e.g., Actin or 18S rRNA).
Reactive oxygen species (ROS) burst measurement – we measure the production of hydrogen peroxide (H₂O₂) in leaf discs using the Amplex Red assay, and superoxide (O₂⁻) using the NBT (nitroblue tetrazolium) staining method, as an indicator of early defence signalling.

Biocontrol Agent Efficacy – Screening and Dose-Response Testing

In addition to host resistance, we offer specialised testing of biocontrol agents and plant defence inducers, designed to support product development and label claims.

- Antagonism and co-culture assays – we test the direct antagonism of candidate biocontrol strains (e.g., Bacillus, Pseudomonas, Trichoderma) against target pathogens in dual culture (PDA or NA). Inhibition zone diameters (mm) are measured and the percentage of mycelial growth inhibition is calculated.
- Seed treatment and root dipping – we apply the biocontrol agent (or inducer) to seeds or roots before transplanting, and then challenge plants with the target pathogen. The reduction in disease severity compared to untreated controls is calculated as the biocontrol efficacy (%).
- Foliar spray and root drench efficacy – we evaluate the efficacy of liquid or wettable powder formulations by spray application or drenching, followed by pathogen inoculation. The optimal dose and timing are determined in preliminary experiments.
- Dose-response testing – for product optimisation, we test the biocontrol agent at 3-5 dose levels (e.g., 10⁶ to 10¹⁰ CFU/mL) and determine the minimum effective dose (ED₅₀) and the maximum safe dose (no phytotoxicity). Data are fitted to a sigmoidal dose-response curve (logistic model).
- Stability and persistence – we assess the survival of the biocontrol agent on the phyllosphere or rhizosphere over time (up to 14 days) using selective plating or qPCR; a population of >10⁵ CFU/g of tissue is typically required for effective biocontrol.

Phytotoxicity and Crop Safety – Ensuring Product Compatibility

Before recommending any treatment, we evaluate its safety to the target plant species, particularly when the active agent is a novel biocontrol strain or a chemical inducer.

Phytotoxicity assessment – we apply the product at 1×, 2×, and 4× the recommended rate to healthy plants and score for visible symptoms (chlorosis, necrosis, stunting, malformation) after 7 and 14 days. A score ≤ 1 (mild, transient) is considered safe; any persistent severe damage (> 3) is reported as phytotoxic.
Growth inhibition test – we measure the fresh and dry biomass of treated plants compared to untreated controls; a reduction of > 20 % is considered a negative effect.
Seed emergence inhibition – we treat seeds with the product and count germination and emergence over 10 days in a germination chamber; a > 10 % reduction compared to the untreated control is flagged.
Phytotoxicity under stress conditions – where relevant, we repeat the phytotoxicity tests under combined stress (drought, salinity, or high temperature) to ensure safety under suboptimal growing conditions.

Data Analysis and Interpretation – Statistical Reporting

We provide a comprehensive statistical analysis of all disease resistance and biocontrol efficacy data, using accepted statistical methods and software.

Descriptive statistics – mean, standard deviation, standard error, and coefficient of variation for each treatment group.
Inferential statistics – one-way or two-way ANOVA (with Tukey’s HSD or Dunnett’s post-hoc test) to compare multiple treatments; or Student’s t-test for pairwise comparisons.
Non-parametric methods – for ordinal disease scores, we use the Kruskal-Wallis test (with Dunn’s multiple comparison) or the Mann-Whitney U test.
Area under disease progress curve (AUDPC) – calculated using the trapezoidal rule and compared between treatments.
Dose-response modelling – for biocontrol and inducer products, we fit a four-parameter logistic model (Hill equation) to calculate ED₅₀ and confidence intervals.
Principal component analysis (PCA) – for multi-parameter datasets (enzymes, gene expression, disease scores), we perform PCA to identify correlations between resistance mechanisms and disease outcome.

Report Acceptance & Compliance with Bulgarian and European Agricultural Standards

All plant disease resistance experiments are performed under our ISO/IEC 17025 accreditation and in accordance with Good Experimental Practice (GEP) and EPPO guidelines. Our final reports provide a complete description of the plant material, pathogen strains, growth conditions, inoculation methods, and experimental design; all raw data for disease scores, biomass, enzyme activities, and gene expression; statistical summaries and graphical representations (bar charts, growth curves); photographic documentation of disease symptoms; and a clear conclusion on the resistance level of the tested varieties, or the efficacy and safety of the tested biocontrol agents. The reports also include a compliance statement with relevant BDS standards and the Bulgarian Plant Protection Act. These documents are accepted by the Bulgarian Food Safety Agency (BFSA), the Ministry of Agriculture and Food, and Bulgarian agricultural input registrars for product dossiers, label claims, and quality assurance. Bilingual (Bulgarian/English) versions are available to facilitate submissions to national and EU authorities.