The correct combination of statements regarding disease resistance in plants in the given context is: (3) A, B, and D.

Explanation of each option:

A. Qualitative resistance follows Mendelian ratio: True. Qualitative resistance is typically controlled by one or a few major genes showing clear dominant-recessive allelic relationships, thus following Mendelian inheritance patterns with discrete phenotypic classes in F2 populations.

B. In the F2 individuals demonstrating qualitative resistance, “resistance” is dominant: True. Usually, in qualitative resistance, the resistant allele is dominant over the susceptible allele, resulting in resistant F1 being crossed and generating resistant phenotypes following dominant Mendelian ratios in offspring.

C. Quantitative resistance is always monogenic: False. Quantitative resistance is polygenic and controlled by multiple genes with minor effects each, leading to continuous variation in resistance levels rather than simple Mendelian ratios.

D. Qualitative resistance can be polygenic: True. Though qualitative resistance is often monogenic, it can sometimes involve multiple major genes (polygenic), especially when multiple resistance genes interact epistatically or additively to produce discrete resistant phenotypes.

Introduction:
Plant disease resistance is a vital trait for crop sustainability, controlled by complex genetic mechanisms. It is broadly classified into qualitative resistance, which follows Mendelian inheritance with major gene effects, and quantitative resistance, which is polygenic with minor gene contributions. This article explains the differences and genetic control underlying these resistance types in F1 and F2 progenies.

Detailed Explanation of the Statements:

• Statement A: Qualitative resistance is based on major genes that segregate in offspring according to Mendel’s laws, producing distinct categories of resistant and susceptible plants consistent with Mendelian ratios (e.g., 3:1 for a single gene).

• Statement B: In qualitative resistance, the resistance trait is typically dominant. F1 progeny from crosses between resistant and susceptible parents show resistance, indicating the dominance of resistance alleles. This pattern continues into the F2 generation.

• Statement C: Quantitative resistance is not monogenic; rather, it involves many genes (polygenic) each contributing small effects, resulting in continuous variation in resistance rather than discrete classes, hence not following Mendelian ratios.

• Statement D: While qualitative resistance is often monogenic, it can sometimes be polygenic when multiple resistance genes contribute to the phenotype, interacting epistatically or additively. This can still produce qualitative Mendelian-like ratios in F2 populations due to major gene effects.

This clarifies that statements A, B, and D are correct regarding the qualitative and quantitative disease resistance traits analyzed in the F2 generation.