Plant Breeding for Disease Resistance

NCERT grounding

This subtopic sits in the plant-breeding section of Strategies for Enhancement in Food Production. NCERT opens the discussion plainly: a wide range of fungal, bacterial and viral pathogens affect the yield of cultivated crop species, especially in tropical climates, and crop losses can often be significant — up to 20–30 per cent, or sometimes even total. In this situation, breeding and developing cultivars resistant to disease enhances food production and reduces the dependence on fungicides and bacteriocides.

The textbook then fixes a definition that NEET tests repeatedly. Resistance of the host plant is the ability to prevent the pathogen from causing disease, and it is determined by the genetic constitution of the host plant. Because resistance is a heritable, gene-controlled trait, it can be moved between plants by ordinary sexual crossing — which is exactly why plant breeding can build resistant varieties at all. NCERT also stresses a practical prerequisite: before breeding is undertaken, it is important to know the causative organism and the mode of transmission of the disease.

NCERT (Strategies for Enhancement in Food Production): "Resistance of the host plant is the ability to prevent the pathogen from causing disease and is determined by the genetic constitution of the host plant."

Breeding crops for disease resistance

Disease-resistance breeding is not a separate science from ordinary crop improvement — it is the same toolkit aimed at a particular trait. NCERT is explicit that breeding for disease resistance is carried out either by the conventional breeding techniques described for any agronomic character, or by mutation breeding. Both routes end with a cultivar whose genome carries genes that block a named pathogen, but they differ in where the resistance gene comes from.

The first idea to lock in is the distinction between the pathogen groups. NCERT names representative diseases for each: among the fungal diseases are rusts — such as brown rust of wheat, red rot of sugarcane and late blight of potato; among the bacterial diseases is black rot of crucifers; and among the viral diseases are tobacco mosaic and turnip mosaic. A resistant cultivar is bred against a specific disease, so the variety name, the crop and the pathogen always travel together as a triplet in exam questions.

The two methods answer one question: does a resistance gene already exist somewhere we can reach it? If a resistant plant exists — within the crop itself, in an old landrace, or in a wild relative — conventional hybridisation can pull that gene into a high-yielding background. If no such gene can be found in the available germplasm, the breeder must create new variation, and that is where mutation breeding takes over. NCERT lists the same logic when it notes that conventional breeding is "often constrained by the availability of limited number of disease resistance genes" already present in crop varieties or wild relatives.

Conventional breeding: hybridisation and selection

The conventional method of breeding for disease resistance is hybridisation and selection. NCERT states directly that its steps are essentially identical to those for breeding any other agronomic character such as high yield — the same five-step pipeline used across the chapter, simply pointed at a resistance trait. The textbook gives the sequence as four sequential operations specific to resistance work.

Step 1 is the heart of resistance breeding. The breeder does not invent resistance; it must already exist somewhere in the germplasm collection — the entire collection of plants and seeds carrying all the diverse alleles for a crop. Wild relatives are especially valuable here. NCERT notes that several wild relatives of cultivated species carry useful resistance characters but have very low yield, so the resistance gene must be introduced into a high-yielding cultivated variety rather than the wild plant being grown directly.

Steps 2 and 3 carry out that introduction. Because high-yielding crops and resistant wild relatives are usually different plants, the desired characters have to be combined from two different parents by cross-hybridisation. The pollen of the chosen male parent is placed on the stigma of the female parent; only a small fraction of crosses — often one in a few hundred to a thousand — actually yields the wanted combination, so careful selection of superior recombinants among the progeny is essential. The selected plants are then self-pollinated for several generations until they reach uniformity, so the resistance does not segregate out in later crops.

Step 4 is the testing-and-release stage common to every crop-improvement programme. The new resistant lines are evaluated for yield and other agronomic traits in research fields under controlled fertiliser, irrigation and crop-management conditions, then tested in farmers' fields for at least three growing seasons across the agroclimatic zones where the crop is grown, always compared against a reference or check cultivar. Only after this is the cultivar released and commercialised. NCERT's worked example of gene transfer is bhindi: resistance to yellow mosaic virus in Abelmoschus esculentus was transferred from a wild species, producing the new variety Parbhani Kranti.

Mutation breeding

Conventional breeding has one hard limit: it can only move resistance genes that already exist. NCERT states the constraint precisely — conventional breeding is often limited by the small number of disease-resistance genes present and identified in crop varieties or wild relatives. When screening the entire available germplasm turns up no usable resistance gene, the breeder must create new variation, and that is the role of mutation breeding.

Mutation is the process by which genetic variation is created through changes in the base sequence within genes, resulting in a new character or trait not found in the parental type. It is possible to induce mutations artificially through the use of chemicals (chemical mutagens) or radiations such as gamma radiation. Plant material is treated with the mutagen, then screened for resistant variants; plants that show the desirable resistant character are selected and used as a source in breeding. This whole process — inducing mutations and selecting the useful ones — is what NCERT calls mutation breeding.

NCERT's worked example of mutation breeding is mung bean: resistance to yellow mosaic virus and powdery mildew was induced by mutations. This single example covers both a viral disease (yellow mosaic virus) and a fungal disease (powdery mildew), making it an efficient one to memorise. Note carefully that the bhindi case — Parbhani Kranti — is not mutation breeding; there the resistance gene already existed in a wild species and was transferred by hybridisation. NCERT also mentions that beyond mutation breeding, other methods used are selection among somaclonal variants and genetic engineering, but the syllabus core for this subtopic is the conventional-versus-mutation pair.

Why "same crop or related wild species" matters

A subtle point NCERT makes is that all its disease-resistance examples involve resistance genes that lie either in the same crop species being bred, or in a related wild species. Transfer of the resistance gene is achieved by sexual hybridisation between the target plant and the source plant, followed by selection. This is precisely why conventional breeding cannot reach beyond the crossing barrier — and why mutation breeding becomes necessary when even the wild relatives lack a usable gene.

Resistant cultivars to memorise

The single most exam-relevant block of this subtopic is NCERT Table 9.1 — the list of crop varieties bred by hybridisation and selection for resistance to fungal, bacterial and viral diseases. NEET frequently asks students to match a variety to its crop or to its disease, so the full triplet of crop, variety and pathogen must be memorised exactly.

Two cross-checks help avoid confusion. First, Pusa Swarnim is the same variety as Karan rai — NCERT lists them together — and it is a Brassica resistant to white rust, a fungal disease. Second, cauliflower carries two named varieties, Pusa Shubhra and Pusa Snowball K-1, both bred against black rot, a bacterial disease of crucifers. Chilli's Pusa Sadabahar is the only purely viral example in the table, resisting three viral problems at once: chilli (chilly) mosaic virus, tobacco mosaic virus and leaf curl.

One mnemonic anchor helps: every variety in the table except Himgiri begins with "Pusa", reflecting its development at the Indian Agricultural Research Institute (Pusa, New Delhi). Himgiri — the wheat variety against leaf rust, stripe rust and hill bunt — is the lone non-Pusa name, which makes it the most commonly tested single entry.

Worked examples

Common confusion & NEET traps

Disease-resistance breeding generates a cluster of look-alike facts: two breeding methods, three pathogen groups, and five named cultivars that all share the "Pusa" prefix except one. NEET exploits exactly these overlaps, so a few distinctions are worth fixing firmly.