Plant pathology

In short

Plant pathology is the science of plant disease — its cause, the mechanism by which a pathogen damages a host, how disease spreads in a population, and how it is managed. It matters because India loses roughly 15–25% of potential crop output to pests and diseases (estimates vary widely by crop and source), and single epidemics have changed history: the 1943 Bengal famine was worsened by brown spot of rice (*Cochliobolus miyabeanus*), and wheat rusts drove the whole Indian rust-resistance breeding programme. The topic is examined in B.Sc Botany mycology and plant pathology papers, and in competitive exams such as ICAR-AIEEA (PG), ICAR-NET, ARS/NET for Plant Pathology, GATE Life Sciences and state agriculture-officer exams. Employers include ICAR institutes (IARI, IIRR, ICRISAT-linked programmes), State Agricultural Universities, KVKs, seed companies (Nuziveedu, Rasi, Mahyco), agrochemical firms (UPL, Bayer, Syngenta, Coromandel), biocontrol producers and plant-quarantine stations.

Explained

What a disease is, and what causes it

• Disease: a sustained, injurious deviation from normal physiology caused by a continuous irritant — contrast with injury (a sudden, single event, e.g. hail, lightning).
• Pathogen: the organism causing disease; pathogenicity is its ability to cause disease; virulence is the degree of that ability; aggressiveness is the quantitative capacity to colonise and reproduce.
• Symptom = host's visible response (wilt, chlorosis, necrosis, galls); sign = the pathogen's own structures (mycelium, pycnidia, uredinia, bacterial ooze).
• Biotic causes: fungi and oomycetes (largest group, ~70–80% of plant diseases), bacteria, phytoplasmas, viruses, viroids, nematodes, protozoa (phytomonads), parasitic flowering plants (*Cuscuta*, *Striga*, *Orobanche*), algae (*Cephaleuros* red rust of tea).
• Abiotic (non-parasitic) disorders: nutrient deficiency (khaira disease of rice = Zn deficiency), waterlogging, salinity, air pollutants (ozone fleck), herbicide injury — not transmissible, often uniformly distributed in a field, which is itself a diagnostic clue.
• Koch's postulates (proof of pathogenicity): (1) associate pathogen with diseased tissue, (2) isolate in pure culture, (3) inoculate on a healthy susceptible host and reproduce the same symptoms, (4) re-isolate the same organism. Fail for obligate parasites that will not grow in culture (rusts, powdery mildews, viruses, phytoplasmas) — then molecular Koch's postulates or electron microscopy/PCR evidence are used.

Nutritional relationships and parasitism

• Biotroph (obligate parasite): needs living host tissue — rusts, powdery mildews, downy mildews, *Albugo*; typically forms haustoria, causes little immediate necrosis, cannot be cultured on ordinary media.
• Necrotroph (perthotroph): kills tissue with enzymes and toxins, then feeds — *Alternaria*, *Pythium*, *Sclerotium*, *Rhizoctonia*; usually culturable, often facultative parasites.
• Hemibiotroph: biotrophic early, necrotrophic later — *Magnaporthe oryzae*, *Phytophthora infestans*, *Colletotrichum* (which forms a quiescent appressorium and resumes at ripening — latent anthracnose in mango and chilli).
• Obligate saprophyte / facultative parasite / facultative saprophyte is the classical four-way ladder: pure saprophyte → facultative parasite (*Fusarium*) → facultative saprophyte (*Ustilago*) → obligate parasite (*Puccinia*).
• Host range: monophagous (narrow, e.g. *Puccinia graminis tritici*) to polyphagous (*Sclerotium rolfsii*, *Rhizoctonia solani*, >500 hosts).
• Formae speciales, physiologic races, pathotypes: host-specialised populations within one species, identified on differential host sets — e.g. wheat rust races identified at ICAR-IIWBR Regional Station, Flowerdale, Shimla.

How pathogens attack and how plants defend

• Disease cycle steps: inoculation → prepenetration (germination, appressorium) → penetration → infection → incubation → colonisation → reproduction → dissemination → survival (overwintering/oversummering).
• Penetration routes: direct through cuticle (appressorium + infection peg, turgor-driven, as in *Magnaporthe*), through natural openings (stomata for rust urediniospores and *Xanthomonas*; hydathodes in bacterial blight of rice; lenticels; nectaries in fire blight), or through wounds (common for bacteria, *Agrobacterium*, nematode-made holes).
• Weapons of the pathogen: cell-wall-degrading enzymes (cutinases, pectinases, cellulases, proteases); toxins — host-specific (HC-toxin, victorin, *Alternaria* AAL-toxin) and non-host-specific (tabtoxin, fusaric acid, cercosporin); growth regulators (IAA in crown gall, cytokinins in fasciation, gibberellins in bakanae of rice causing elongation); polysaccharides (EPS of *Ralstonia* blocking xylem → wilt).
• Pre-formed (passive) defence: thick cuticle, wax, lignified cell walls, closed stomata, pre-formed phenolics, saponins, raphides.
• Induced (active) defence: recognition of PAMP/MAMP by surface PRRs → PTI (PAMP-triggered immunity); pathogen effectors suppress PTI; intracellular R proteins (NB-LRR) recognise effectors → ETI, usually the hypersensitive response (HR) — rapid programmed cell death with ROS burst.
• Structural responses: cork layer, abscission layer, gum deposition, tylose formation in xylem, callose papillae at the penetration point.
• Biochemical responses: phytoalexins (pisatin, phaseollin, rishitin, capsidiol — synthesised *de novo*), PR proteins (chitinases, β-1,3-glucanases), phenol oxidation, lignification.
• SAR (systemic acquired resistance) — broad, long-lasting, salicylic-acid/NPR1-mediated, marked by PR-gene expression; ISR (induced systemic resistance) — triggered by rhizobacteria (*Pseudomonas fluorescens*), jasmonate/ethylene-mediated. Chemical activators: acibenzolar-S-methyl, β-aminobutyric acid.
• Flor's gene-for-gene hypothesis (from flax rust): for every resistance gene *R* in the host there is a corresponding avirulence gene *Avr* in the pathogen; disease is incompatible (resistant) only when dominant *R* and dominant *Avr* are both present — all other three combinations give disease.
• Resistance types: vertical/race-specific (single R gene, complete, easily broken) vs horizontal/general/polygenic (partial, durable); slow rusting (Lr34, Sr2 complexes) and tolerance (yields despite infection) are distinct from true resistance; escape is not resistance at all.

Classification, epidemiology and forecasting

• Classification bases: by symptom (wilt, rot, blight, mildew, rust, smut, canker, gall, mosaic), by part affected (foliar, root, vascular, fruit, post-harvest), by host crop (the usual exam arrangement), by pathogen taxon, or by extent (local vs systemic).
• Disease triangle: susceptible host × virulent pathogen × favourable environment — remove any side and there is no disease. Add time → disease tetrahedron; add human interference → disease pyramid.
• Epidemic (epiphytotic): sharp increase in disease in a population in time and space; endemic: constantly present at a low level in an area; sporadic: irregular; pandemic: across countries. Epidemiology studies the population dynamics.
• Monocyclic (one infection cycle per season — smuts, many soil-borne wilts, loose smut of wheat) vs polycyclic (many cycles — rusts, late blight, blast); polycyclic diseases explode, so forecasting and spray schedules matter most there.
• Van der Plank's terms: *x* = proportion of disease, *r* = apparent infection rate, *p* = latent period, *i* = infectious period; compound interest disease (polycyclic) vs simple interest disease (monocyclic).
• Disease forecasting systems in India: JHULSACAST (potato late blight, ICAR-CPRI Shimla), BLITECAST/Smith periods logic, Dutt's rule and Beaumont period for late blight, and advisory issues from ICAR-NCIPM (National Research Centre for Integrated Pest Management) and the Crop Pest Surveillance and Advisory Project (CROPSAP, Maharashtra).
• Survival of inoculum: sclerotia (*Sclerotium*, *Rhizoctonia*), chlamydospores (*Fusarium*), oospores (*Phytophthora*, *Pythium*), teliospores (smuts), infected seed (loose smut of wheat, bacterial blight of rice), volunteer plants and alternate/collateral hosts (*Berberis* for *P. graminis*; wild grasses for blast).

Major diseases of Indian crops (exam-standard set)

Diagnosis and laboratory methods

• Field diagnosis: pattern (random = biotic, uniform/patterned = abiotic or spray damage), progression over time, host range, presence of signs, soil and weather history.
• Lab: moist chamber incubation; isolation on PDA/nutrient agar; tissue-sectioning and staining (cotton blue in lactophenol, trypan blue, KOH mounts); Koch's postulates.
• Bacteria: gram stain, ooze test, hypersensitivity on tobacco, biochemical (LOPAT, oxidase, levan), Biolog.
• Viruses: sap transmission with abrasive on indicator hosts (*Chenopodium amaranticolor* local lesions), graft/dodder transmission, vector transmission tests, electron microscopy, ELISA (DAS-ELISA for routine indexing), RT-PCR.
• Molecular: PCR with ITS/rDNA primers, qPCR, LAMP (field-usable), nested PCR for phytoplasma 16S, DNA barcoding, LFIA/immunostrips used in Indian quarantine stations.
• Nematodes: Cobb's sieving and decanting, Baermann funnel, staining roots with acid fuchsin.
• Disease assessment: per cent disease incidence (PDI as incidence) and per cent disease index/severity using standard 0–9 scales (SES scale of IRRI for rice), the modified Cobb scale for rust severity, Horsfall–Barratt scale, and AUDPC for progress over time.

Management — principles, chemicals and Indian regulation

• Classical principles (Whetzel): exclusion, eradication, protection, resistance; therapy and avoidance/regulation are added in modern lists.
• Cultural: crop rotation, summer deep ploughing, soil solarisation, adjusting sowing date, roguing, removal of collateral hosts, balanced N, raised beds and drainage, trap/border crops, clean seed and certified planting material.
• Physical: hot-water seed treatment (52 °C/10 min for loose smut historically; solar heat therapy of Luthra for wheat), hot-air treatment of sugarcane setts (54 °C moist hot air), soil steaming in nurseries, pruning and burning.
• Host resistance: backcross breeding, gene pyramiding, marker-assisted selection (Improved Pusa Basmati 1 with *xa13* + *Xa21*), multilines, varietal diversification and rotation of resistance genes to avoid boom-and-bust cycles.
• Biological control: *Trichoderma viride*/*T. harzianum* (seed treatment 4–10 g/kg seed; mycoparasitism, antibiosis, competition), *Pseudomonas fluorescens* (ISR, siderophores), *Bacillus subtilis*, *Ampelomyces quisqualis* for powdery mildew, *Pochonia*/*Purpureocillium* for nematodes, neem cake and FYM amendments.
• Chemical groups: protectants — Bordeaux mixture (CuSO₄ + lime), copper oxychloride, mancozeb, chlorothalonil, sulphur; systemics — carbendazim and thiophanate-methyl (MBC, tubulin), triazoles/DMI (propiconazole, tebuconazole — ergosterol biosynthesis), strobilurins/QoI (azoxystrobin — complex III), metalaxyl/mefenoxam (oomycete RNA polymerase), SDHI (fluxapyroxad); antibiotics — streptocycline, validamycin. Resistance management: rotate FRAC groups, use mixtures, avoid repeated solo use of single-site fungicides.
• IDM (integrated disease management): combine resistant variety + clean seed + cultural measures + biocontrol + need-based chemicals guided by surveillance and ETL thinking; the approach promoted through ICAR-NCIPM and state agriculture departments.
• Indian regulation and institutions: the Insecticides Act, 1968 and Insecticides Rules, 1971 govern registration (Central Insecticides Board & Registration Committee, CIB&RC, Faridabad), licensing, labelling and banned/restricted lists — fungicides are covered because "insecticide" is defined broadly; the Destructive Insects and Pests Act, 1914 and the Plant Quarantine (Regulation of Import into India) Order, 2003 (amended frequently) govern import, phytosanitary certificates and the NBPGR/DPPQS quarantine network; India is a signatory to the IPPC and follows ISPM standards. The Seeds Act, 1966 and Seed Certification Standards set disease tolerance limits for certified seed. A Pesticide Management Bill has been under consideration to replace the 1968 Act — not enacted as of now, so quote the 1968 Act. Maximum Residue Limits for pesticides on food are notified under the Food Safety and Standards Act, 2006 by FSSAI. Always verify the current label claim and MRL, as registrations and banned lists change by notification.

Key points

• Disease vs injury — disease is a continuous irritation and a process; injury is a single, sudden event.
• Disease triangle — host + pathogen + environment; time and human action extend it to tetrahedron and pyramid.
• Koch's postulates — four steps to prove pathogenicity; fail for obligate parasites and viruses.
• Biotroph vs necrotroph — biotrophs need living cells and form haustoria; necrotrophs kill with enzymes and toxins first.
• Inoculum — any pathogen part that can infect; inoculum potential = quantity × ability relative to host resistance.
• Incubation vs latent period — incubation ends when symptoms appear; latent period ends when the pathogen becomes infectious.
• Penetration routes — direct, natural openings (stomata, hydathodes, lenticels, nectaries), or wounds.
• Hypersensitive response — rapid local cell death that confines a biotroph; the hallmark of R-gene (ETI) resistance.
• Phytoalexins — low-molecular-weight antimicrobials made *de novo* after infection (rishitin, pisatin, capsidiol).
• Gene-for-gene — resistance only when dominant *R* meets dominant *Avr*; the other three combinations give disease.
• Vertical vs horizontal resistance — race-specific and complete but fragile, versus polygenic, partial and durable.
• Monocyclic vs polycyclic — one cycle per season (simple interest) versus many (compound interest, epidemic-prone).
• Heteroecious macrocyclic rust — *P. graminis* has five spore stages on two hosts (wheat and *Berberis*).
• Soil-borne survival structures — sclerotia, oospores, chlamydospores, teliospores; the reason rotation and solarisation work.
• Vectors — aphids (non-persistent potyviruses), whitefly *Bemisia tabaci* (begomoviruses, e.g. cotton leaf curl), leafhoppers (phytoplasmas, tungro), thrips (tospoviruses, bud necrosis of groundnut).
• Management principles — exclusion, eradication, protection, resistance, therapy; IDM combines them.
• Fungicide types — protectant (surface, multi-site, no resistance risk) versus systemic (translocated, single-site, resistance-prone).
• Indian legal frame — Insecticides Act, 1968 with CIB&RC registration; DIP Act, 1914 with PQ Order, 2003 for quarantine; FSSAI sets MRLs.

Formulas and examples

Example 1 — Incidence and severity are different numbers. In a 100-plant tomato plot, 40 plants show early blight. Severity on a 0–5 scale: 10 plants at 1, 20 at 2, 8 at 3, 2 at 4, remaining 60 at 0.

• PDI (incidence) = 40/100 × 100 = 40%.
• Σ(rating × plants) = (10×1) + (20×2) + (8×3) + (2×4) = 10 + 40 + 24 + 8 = 82.
• Maximum possible = 100 × 5 = 500.
• Severity index = 82/500 × 100 = 16.4%.
• Conclusion: 40% of plants are affected but only 16.4% of the assessable tissue — a wide, mild infection, so a protectant spray is enough; incidence alone would overstate the threat.

Example 2 — Apparent infection rate of a polycyclic disease. Late blight severity rises from 1% to 25% in 12 days.

• x₀ = 0.01 → ln[0.01/0.99] = ln(0.010101) = −4.595.
• x = 0.25 → ln[0.25/0.75] = ln(0.3333) = −1.0986.
• r = (−1.0986 − (−4.595)) / 12 = 3.4964/12 = 0.291 per unit per day.
• Interpretation: at r ≈ 0.29/day the disease roughly needs only about 8 days to go from 25% to 75% (ln(3) − ln(1/3) = 2.197/0.291 ≈ 7.5 days) — a true compound-interest epidemic that demands preventive, not curative, spraying.

Example 3 — AUDPC comparison of two wheat varieties. Severity (%) recorded on days 0, 10, 20, 30.

• Variety A: [(2+10)/2 × 10] + [(10+30)/2 × 10] + [(30+60)/2 × 10] = 60 + 200 + 450 = 710 %·days.
• Variety B: [(1+4)/2 × 10] + [(4+9)/2 × 10] + [(9+18)/2 × 10] = 25 + 65 + 135 = 225 %·days.
• Relative AUDPC of B = 225/710 = 0.317 → about 68% reduction; B is partially (horizontally) resistant, not immune.

Example 4 — Fungicide dose and cost. Mancozeb 75% WP is to be applied at 1.5 kg a.i./ha in 500 litres of spray fluid per hectare, over 2.5 ha; the product costs ₹420 per kg.

• Formulation needed per ha = (1.5 × 100)/75 = 2.0 kg/ha.
• For 2.5 ha = 5.0 kg; spray fluid = 500 × 2.5 = 1,250 litres.
• Spray concentration = 2.0 kg / 500 L = 0.4% (4 g per litre).
• Cost = 5.0 × 420 = ₹2,100 for the material (about ₹840/ha), before labour.

Example 5 — Avoidable loss and the value of protection. A groundnut trial for tikka disease: protected plot 1,850 kg/ha, unprotected 1,250 kg/ha. Pod price ₹60/kg; protection cost ₹3,200/ha (three sprays plus labour).

• Avoidable loss = (1,850 − 1,250)/1,850 × 100 = 600/1,850 × 100 = 32.4%.
• Yield saved = 600 kg/ha → gross benefit = 600 × 60 = ₹36,000/ha.
• Net benefit = 36,000 − 3,200 = ₹32,800/ha; benefit : cost = 36,000/3,200 = 11.25 : 1.

Where it is used

• Seed and planting-material industry: seed health testing and indexing (ELISA, PCR, grow-out tests) at seed companies and at ICAR-IARI/NBPGR; certified-seed disease tolerance limits under the Seeds Act, 1966.
• Agrochemical and bio-input companies: UPL, Coromandel, Bayer, Syngenta, Dhanuka — field trial officers, bio-efficacy and residue trials for CIB&RC registration; biocontrol production units making *Trichoderma* and *Pseudomonas* formulations (talc-based, often supplied through State Agriculture Departments and KVKs).
• Plant quarantine and trade: DPPQS stations at ports and airports, NBPGR for germplasm import — issuing phytosanitary certificates, intercepting pests under the PQ Order, 2003; critical for India's export of mango, grape (residue-free grape protocol for the EU), banana and rice.
• Research and teaching: ICAR institutes (IARI, IIRR Hyderabad, CPRI Shimla, IIHR Bengaluru, ICAR-NCIPM), SAUs and KVKs — ARS/NET-recruited scientists working on resistance breeding, surveillance and disease forecasting.
• Extension and advisory: crop surveillance schemes (CROPSAP in Maharashtra), Kisan Call Centres, agri-advisory apps and FPO-based agronomy services; plant clinics diagnosing farmer samples.
• Plantations and horticulture: tea (blister blight, *Exobasidium vexans*, in Assam and the Nilgiris), coffee rust, coconut root wilt in Kerala, black pepper quick wilt, citrus greening management in Punjab and Vidarbha.

Interview questions

• Q: State the disease triangle and its practical use. A: Disease needs a susceptible host, a virulent pathogen and a favourable environment together; management works by weakening any one side — resistant varieties, sanitation to cut inoculum, or changing sowing date and spacing to alter the microclimate.
• Q: Why can Koch's postulates not be completed for a rust fungus? A: *Puccinia* is an obligate biotroph and will not grow in pure culture on ordinary media, so step 2 fails; pathogenicity is proven by controlled inoculation with urediniospores plus microscopic and molecular identification.
• Q: Differentiate incubation period from latent period. A: The incubation period ends when symptoms become visible; the latent period ends when the pathogen starts producing inoculum. In rusts the latent period can be shorter than full symptom development, which is why epidemics run ahead of visible damage.
• Q: Explain the gene-for-gene hypothesis with its outcomes. A: For each host *R* gene there is a matching pathogen *Avr* gene; only the combination dominant *R* + dominant *Avr* gives incompatibility (resistance), while *R*+*avr*, *r*+*Avr* and *r*+*avr* all give susceptibility.
• Q: Why do single-gene resistances break down? A: Selection pressure favours rare pathogen genotypes that have lost or altered the matching *Avr*; with high pathogen populations and sexual or mutational variability a new virulent race takes over — the boom-and-bust cycle, which is why gene pyramiding and varietal diversification are used.
• Q: How would you distinguish a bacterial wilt from a fungal wilt in the field? A: Cut the stem and suspend it in clear water — *Ralstonia* gives a milky bacterial ooze within minutes, while *Fusarium* shows vascular browning with no ooze; confirm with isolation on nutrient agar versus PDA.
• Q: Why is the Indo-Gangetic plain largely free of wheat rust inoculum in summer? A: Rusts are obligate and cannot survive the hot, wheat-free summer there; inoculum oversummers in the Nilgiri–Palni hills (black and brown rust) and the northern Himalayan hills (yellow rust) and blows back in — the basis of the Indian rust-surveillance and barrier-variety strategy.
• Q: What is the difference between a protectant and a systemic fungicide, and why does it matter for resistance? A: Protectants stay on the surface and act at multiple biochemical sites, so resistance is rare; systemics move inside the plant and usually hit a single site, so point mutations confer resistance — hence FRAC-group rotation and mixtures.
• Q: How does *Trichoderma* control soil-borne disease? A: By mycoparasitism (coiling and lysing hyphae with chitinases and glucanases), antibiosis, competition for space and nutrients in the rhizosphere, and by inducing host defences; applied as seed treatment, nursery mix or FYM enrichment.
• Q: What is SAR, and how does it differ from ISR? A: SAR is a broad, long-lasting resistance induced by a localised infection, salicylic-acid/NPR1-dependent and marked by PR proteins; ISR is induced by beneficial rhizobacteria and runs through jasmonate–ethylene signalling without necrosis.
• Q: Which Act and which body regulate fungicide registration in India? A: The Insecticides Act, 1968 with the Insecticides Rules, 1971; the Central Insecticides Board and Registration Committee registers products and approves label claims, while FSSAI notifies maximum residue limits under the Food Safety and Standards Act, 2006.
• Q: A farmer's field shows uniform yellowing in a sharp rectangle. Biotic or abiotic? A: Almost certainly abiotic — sharp geometric boundaries and uniform expression suggest herbicide drift, fertiliser or irrigation effects; biotic disease spreads from foci in a random or gradient pattern.
• Q: Why is seed treatment effective for loose smut of wheat but not for sheath blight of rice? A: Loose smut is internally seed-borne and monocyclic, so one systemic seed treatment (e.g. carboxin) removes the inoculum; sheath blight comes from soil sclerotia and spreads by plant-to-plant contact through the season, so seed treatment alone cannot stop it.

Common mistakes

• Confusing disease incidence with severity — reporting one number and calling it "PDI" without saying which; they answer different questions and are computed differently.
• Writing outdated pathogen names (*Pyricularia oryzae* for *Magnaporthe oryzae*, *Pseudomonas solanacearum* for *Ralstonia solanacearum*, *Xanthomonas campestris* pv. *oryzae* for *X. oryzae* pv. *oryzae*) and mixing genus with the disease name.
• Calling *Phytophthora* a "true fungus" — it is an oomycete with cellulose walls and diploid mycelium, which is why metalaxyl works on it and carbendazim does not.
• Mixing up the hypersensitive response (host defence) with hypersensitivity in the sense of susceptibility; and treating tolerance or escape as resistance.
• Reversing gene-for-gene logic — thinking resistance follows from *Avr* alone or susceptibility from *R* alone; only *R* + *Avr* is incompatible.
• Writing a spray dose in a.i. per hectare on the label line instead of converting to formulation quantity, or ignoring the percentage of active ingredient; also quoting doses without checking the current CIB&RC-approved label claim and waiting period.
• Prescribing a curative spray after symptoms are widespread in a polycyclic disease, when the model shows protective sprays before the epidemic's exponential phase are what work; and repeating one systemic fungicide until resistance develops.

Related topics

Mycology: fungal groups and life cycles · Plant–microbe interactions and plant immunity · Plant breeding for disease resistance · Economic botany · Microbiology and microbial techniques · Agricultural entomology and insect vectors