How Breeders and Pathologists Assess Disease Resistance

Breeders and plant pathologists use methods that range from field-based visual checks to high-throughput genetic analysis in the lab.

1. Phenotypic Screening (Visual Assessment)

   This is the most common and traditional approach.

   • Process: Grow multiple cultivars in field/greenhouse conditions and deliberately inoculate them with the target pathogen.
   • Quantification: Visually score plants using a disease severity scale (e.g., 0–9, where 0 = no symptoms and 9 = plant death). Compare scores with known resistant and susceptible checks.
   • Metrics: Record yield impact, lesion size/number, and % leaf area affected.

2. Molecular Marker-Assisted Selection (MAS)

   Once a specific resistance (R) gene is identified, plants can be screened without disease inoculation.

   • Process: Collect a small DNA sample (e.g., from seedlings).
   • Quantification: Use PCR to detect genetic markers (DNA sequences tightly linked to the target R gene).
   • Benefit: Faster and more economical—enables early-stage screening of thousands of plants and selection of only those carrying the desired resistance allele.

3. Genetic and Genomic Analysis

   Modern approaches that survey large portions—or all—of the genome.

   • GWAS (Genome-Wide Association Study): Scan genomes of diverse lines and correlate variants (e.g., SNPs) with observed resistance to discover novel resistance loci/genes.
   • Transcriptomics (RNA-Seq): Profile gene expression during infection to identify defense pathways. Resistant lines typically show rapid, strong induction of defense-related genes compared with susceptible lines.