Plant Disease: Diagnosis, Organic Remedies & Prevention

Overview

Diagnosing plant disease in the vegetable garden demands a disciplined, evidence-based methodology. Most plant diseases — approximately 85% — are caused by fungal or fungal-like organisms, with bacterial and viral pathogens accounting for roughly 10% and 5% respectively. However, abiotic stressors (nutrient deficiencies, environmental extremes, mechanical injury) frequently mimic biotic disease, and misdiagnosis leads to wasted interventions or, worse, the spread of active infection. This masterclass presents a structured three-tier framework: first, a rigorous visual diagnostic protocol; second, evidence-backed organic intervention strategies; and third, a cultural prevention architecture grounded in peer-reviewed science.

Part I: Visual Diagnostics — A Systematic Observation Checklist

1.1 The Disease Triangle: Understanding the Prerequisite for Infection

Before symptom pattern recognition, it is critical to internalize the disease triangle — the foundational model in plant pathology. A disease occurs only when three conditions converge simultaneously: a susceptible host, a virulent pathogen, and a favorable environment. When a plant shows symptoms, always ask which leg of the triangle is primarily responsible. An isolated plant showing yellowing after a cold snap is more likely environmental stress than a pathogen; a cluster of plants with spreading concentric-ringed lesions after two weeks of rain almost certainly signals a fungal pathogen.

1.2 Step 1 — Survey the Population, Not Just One Plant

The single most important diagnostic rule is population-level observation:

• If all plants of the same species, age, and bed show identical symptoms simultaneously, the cause is almost certainly abiotic (nutrient deficiency, pH imbalance, environmental stress, herbicide drift).
• If symptoms appear erratically — some plants severely affected, adjacent ones unaffected — suspect a biotic pathogen (fungal, bacterial, or viral).
• If symptoms track insect feeding paths or tool-use corridors, consider mechanical injury or vector-transmitted diseases (bacterial wilt via cucumber beetles; mosaic viruses via aphids/whiteflies).

1.3 Step 2 — Map Symptoms to Plant Architecture (Mobile vs. Immobile Nutrients)

Nutrient deficiencies follow a predictable gradient determined by whether the nutrient is phloem-mobile (can be redistributed from old to new tissue) or immobile (fixed where it lands):

Nitrogen deficiency presents as uniform chlorosis (yellowing) of the oldest leaves, followed by necrosis and potential leaf abscission. Potassium deficiency produces marginal scorch (necrosis at leaf margins) on old leaves, often with brief chlorosis at tips first. Calcium deficiency — the cause of blossom end rot in tomatoes, peppers, and squash — manifests as decayed, sunken, leathery tissue on the blossom end of fruit. Because calcium is immobile, the deficiency appears in the youngest, fastest-growing tissues (fruit tips) even when soil calcium is adequate; the root cause is usually inconsistent watering that impairs calcium uptake.

Phosphorus deficiency produces deep green foliage with purple/red pigmentation (especially in cool soils), along with stunting and fewer roots. Iron deficiency causes interveinal chlorosis on young leaves (the veins stay green while the tissue between turns yellow) — a pattern easily confused with manganese deficiency and with certain viral symptoms.

1.4 Step 3 — Distinguish Environmental Stress from Biotic Disease

Environmental stressors produce bilateral symmetry (both sides of a leaf affected equally) and are typically non-progressive once the stressor is removed. Pathogen symptoms often show asymmetrical progression and continue to advance even after environmental correction. Key environmental stressors to rule out before assuming pathogen:

• Drought stress: Leaf scorch, tip burn, wilting that recovers overnight. Occurs across entire bed uniformly.
• Overwatering/poor drainage: Root rot, yellowing, wilting that does not recover after watering. Check roots for brown, mushy tissue.
• Herbicide drift: Characteristic curling, distortion, elongated growth patterns; no spore-like structures present.
• Sunscald: Bleached white patches on sun-facing surfaces of fruit and leaves, sharply delineated.
• Hail damage: Random tearing, bruising, and puncture patterns distributed by wind direction.

1.5 Step 4 — Pathogen-Specific Symptom Fingerprinting

Once abiotic causes are eliminated, use the following diagnostic matrix to distinguish pathogen types. As Michigan State University Extension notes, there is significant overlap between categories — a confirmed diagnosis should involve a plant disease diagnostics clinic when in doubt.

The Ooze Test for Bacterial Wilt: Cut a wilted stem near the base and hold the two ends together briefly, then slowly separate. Bacterial wilt caused by Erwinia tracheiphila (cucumber, melon, squash) will produce a sticky, thread-like ooze stretching between the two ends — a definitive field test.

1.6 Disease Profile Cards: Common Vegetable Pathogens

Fungal Diseases

Early Blight (Alternaria solani — tomato, potato, eggplant) Diagnostic hallmark: concentric-ringed "target" lesions (dark brown with yellow halo) beginning on the oldest, lowest leaves, progressing upward. Conidia survive on soil surface and spread via rain splash and wind. Disease development is strongly linked to plant development stage, environmental conditions (warm temperatures, leaf wetness), and cultivar susceptibility.

Late Blight (Phytophthora infestans — tomato, potato) Water-soaked, rapidly expanding lesions on leaves (grayish-green, then brown and leathery). White, cottony sporulation appears on lesion undersides in high humidity. Fruit develops firm, leathery brown rot. This pathogen caused the Irish Potato Famine and remains one of the most devastating vegetable pathogens globally.

Powdery Mildew (various Erysiphe spp. — cucurbits, peas, tomatoes) Unmistakable white, powdery fungal growth on upper leaf surfaces. Unlike most fungi, powdery mildew thrives in warm, dry conditions with low relative humidity, though it still requires surface moisture for spore germination. Underlying tissue gradually turns chlorotic, then necrotic.

Downy Mildew (Peronospora spp. and others — cucurbits, brassicas, spinach) Yellow or pale green angular spots on the upper leaf surface, with grayish-purple sporulation on the underside of leaves. Spreads rapidly under cool, wet conditions — the opposite of powdery mildew.

Gray Mold / Botrytis Blight (Botrytis cinerea — broad host range) Gray, fuzzy sporulation on affected tissue. Commonly infects wounded tissue, senescing flowers, and dense canopy areas with poor airflow. Favors cool, humid conditions.

Fusarium Wilt (Fusarium oxysporum spp. — tomato, pepper, basil) Asymmetric yellowing and wilting — often on one side of the plant first. Stem cross-section reveals brown vascular discoloration. Pathogen persists in soil for years.

Septoria Leaf Spot (Septoria lycopersici — tomato) Small, circular spots with dark brown margins and tan/gray centers, progressing from bottom up like early blight but with smaller, more uniform lesions and visible dark pycnidia (fruiting bodies) within the lesion center.

Bacterial Diseases

Bacterial Wilt (Erwinia tracheiphila — cucurbits): Sudden, dramatic wilting — leaves take on a dark green color initially, then become chlorotic and necrotic. Transmitted exclusively by spotted and striped cucumber beetles.

Bacterial Leaf Spot / Speck (Xanthomonas and Pseudomonas spp. — tomato, pepper): Small, water-soaked spots with yellow halos. Spots remain bounded by leaf veins, giving them an angular appearance. Pseudomonas syringae pv. tomato produces bacterial speck — tiny, raised lesions with yellow halos.

Black Rot (Xanthomonas campestris pv. campestris — crucifers): Characteristic V-shaped yellow lesions forming from leaf margins inward, following vascular bundles. Vessels turn black when stems are cut.

Viral Diseases

Viral pathogens leave no visible sign (the virus cannot be seen). Diagnosis relies entirely on symptom patterns: mosaic/mottled leaf patterns (intermingling light and dark green), leaf distortion and curling, stunting, and misshapen or discolored fruit. Tomato Spotted Wilt Virus (TSWV), Cucumber Mosaic Virus (CMV), and Zucchini Yellow Mosaic Virus (ZYMV) are among the most common in vegetable gardens. Vector management (aphids, thrips, whiteflies) is the primary control lever since no curative treatment exists.

Part II: Organic Remediation — Step-by-Step Intervention Strategies

2.1 The Golden Rule: Protectants Must Be Applied Before Infection

A critical concept governing all organic fungicide and bactericide use is that most organic materials are protectants, not eradicants — they form a chemical barrier on plant surfaces preventing spore germination and infection, but do not cure tissue that has already been colonized. As Cornell University Plant Pathology states, copper ions kill pathogens by "denaturing proteins and enzymes in cells of pathogens they contact that have not yet infected the plant" and "have no post-infection activity". The practical implication: begin applications at first disease risk (after a rain event during susceptible growth stages, or at first sign of disease on lower leaves), not after widespread infection.

2.2 Strategy 1 — Physical Removal and Cultural Pruning

Before any chemical or biological intervention, physical removal of infected tissue is mandatory. Skipping this step and spraying over heavily diseased plants merely wastes product.

Step-by-step cultural pruning protocol:

1. Assess before cutting: Identify the extent of infection. If more than 50% of the plant's canopy is affected by a non-systemic pathogen (e.g., early blight), consider whole-plant removal.
2. Sanitize tools before starting: Dip pruners/scissors in 70% isopropyl alcohol or a 10% bleach solution (1 part bleach to 9 parts water) for 3–5 minutes. Bleach achieves surface pathogen kill within 30 seconds and is the most effective disinfectant, though corrosive to metal tools.
3. Remove infected material: Cut affected leaves and stems, working from the outside of the lesion into healthy tissue to avoid contaminating the cut surface. Remove entire leaves showing lesions — do not leave partial infected leaves on the plant.
4. Dispose, do not compost: Place infected material into sealed trash bags. Fungal spores and bacterial pathogens routinely survive home composting temperatures.
5. Re-sanitize between plants: Carry alcohol wipes or a small dip container when moving from plant to plant. This is especially critical for viral diseases where mechanical transmission via sap is possible.
6. After pruning, apply a protectant spray to the remaining canopy to prevent new infections at pruning wounds.

2.3 Strategy 2 — Copper-Based Fungicides and Bactericides

Copper-based products are the most broadly effective organic-approved tools available for both fungal and bacterial vegetable diseases. They are approved under the USDA National Organic Program when used according to label directions and are listed with OMRI (Organic Materials Review Institute).

Target diseases: Bacterial leaf spots, blights, angular leaf spot, black rot, downy mildew, early blight, late blight, Septoria leaf spot, and Alternaria leaf blight.

Available formulations (OMRI-listed):

• Copper hydroxide: Kocide 3000-O, Nu-Cop HB (50% MCE)
• Copper octanoate: Cueva (1.8% MCE — lowest accumulation risk, lowest phytotoxicity)
• Copper oxychloride + copper hydroxide: Badge X2 (28% MCE — dual-action particle size coverage)
• Cuprous oxide: Nordox 75 (75% MCE — highest metallic copper equivalent)

A decade-long field study published in Plant Health Progress (University of Idaho) confirmed that copper fungicide treatments "consistently slowed disease spread and improved yields" when applied on a 7-day schedule starting at row closure, with treated plots yielding at least 3 tonnes per hectare more than untreated controls. The study underscored that consistency of application schedule, not just product choice, determines efficacy.

Application protocol:

1. Timing: Apply before anticipated infection periods — before rain events that will create prolonged leaf wetness, or 5 days before symptoms appear (the minimum incubation period for most foliar pathogens).
2. Application window: Early morning on low-wind, overcast days. Allow adequate drying time before rain or dew.
3. Coverage: Spray both upper and lower leaf surfaces thoroughly; most pathogens initiate infection on the underside of leaves.
4. Interval: Reapply every 7–10 days during high-disease-pressure periods, or after heavy rain events that wash off residues.
5. Harvest: All labeled copper fungicides carry a 0-day pre-harvest interval (PHI) for most vegetable crops, though the 48-hour re-entry interval (REI) applies.
6. Phytotoxicity caution: Do not apply when temperatures exceed 90°F, or when plants are heat or drought-stressed. Copper can accumulate in soil with intensive use over many years, so rotate with non-copper alternatives when possible.

Resistance management: Bacterial pathogens can and do develop resistance to copper. Cornell Extension explicitly notes that "bacterial pathogens have proven adept at developing resistance to copper, which can render copper fungicide ineffective". Rotating copper applications with biofungicides and potassium bicarbonate reduces resistance selection pressure.

2.4 Strategy 3 — Biofungicides: Bacillus subtilis and Trichoderma spp.

Biofungicides represent the most exciting frontier in organic disease management. They act through multiple mechanisms simultaneously — direct antagonism (mycoparasitism, antibiosis), competition for nutrients and colonization sites, and critically, Induced Systemic Resistance (ISR) — priming the plant's own immune system to respond more rapidly to pathogen attack.

Bacillus subtilis (OMRI-certified products: Serenade ASO, Cease, Rhapsody)

Research published in the Current Journal of Applied Science and Technology demonstrated that Bacillus amyloliquefaciens strain FZB24 (Taegro®, a close relative of B. subtilis) "exhibited significant potential in reducing leaf spot in tomato and improving growth and yield attributes" compared to untreated controls. Critically, the study found that combining biofungicides with a reduced-rate copper fungicide was not inferior to full-rate copper alone — suggesting a synergistic integration model that reduces chemical load.

A comprehensive Ukrainian field study evaluating Bacillus subtilis-based preparations (Serenade ASO) on tomatoes, cucumbers, and white cabbage demonstrated protective effects against alternariosis (46%), late blight (62%), black bacterial spot (47%), cucumber downy mildew (33%), and vascular bacteriosis of white cabbage (76–77%). Biofungicide treatment increased tomato fruit yield by an average of 18%, cucumber yield by 20–21%, and white cabbage by 51–85%. Critically, "the highest protective effect of the drugs is provided by their prophylactic use" — confirming that biofungicides work best as preventatives.

Application protocol for Bacillus subtilis:

1. Begin applications 1–2 weeks before historical disease onset, or when weather conditions become favorable (warm temperatures, high humidity).
2. Apply as a foliar spray covering all leaf surfaces, including undersides.
3. Repeat every 7–14 days throughout the growing season.
4. Can be tank-mixed with copper fungicides (at reduced copper rate) for enhanced efficacy and resistance management.
5. Do not apply immediately before or after broad-spectrum sprays that may suppress Bacillus populations.

Trichoderma harzianum (OMRI-certified products: RootShield, RootShield Plus)

Trichoderma species are primarily soil-applied biocontrol agents targeting soil-borne pathogens: Fusarium oxysporum, Rhizoctonia solani, Pythium spp., Sclerotium rolfsii, and Thielaviopsis spp.. Trichoderma harzianum directly parasitizes these pathogen mycelia through mycoparasitism — physically attacking and lysing fungal cell walls.

Research in Scientific Reports showed that T. harzianum conidia agents at 10⁶ CFU/g achieved a 83.98% control effect of cucumber Fusarium wilt at the seedling stage, and 70.08% at the adult plant stage, along with significant improvements in fruit yield, soluble sugar, and Vc content. A classic study in Phytopathology confirmed that wheat-bran preparations of T. harzianum incorporated into pathogen-infested soil "significantly reduced bean diseases caused by S. rolfsii, R. solani, or both" across multiple field experiments.

Application protocol for Trichoderma:

1. Soil incorporation at planting: Mix granular RootShield into transplant holes or potting mix according to label rates.
2. Root drench: Apply RootShield WP as a soil drench at transplant and repeat at 4–6 week intervals throughout the season.
3. Do not apply within 24 hours of synthetic fungicide applications, which can harm Trichoderma populations.
4. Most effective when applied preventatively before pathogen establishment in the root zone.

2.5 Strategy 4 — Potassium Bicarbonate and Sulfur for Powdery Mildew

Potassium bicarbonate (Milstop, Kaligreen — OMRI-certified) is both curative and preventative for powdery mildew — a distinction from most organic fungicides. It disrupts the potassium ion balance in the fungal cell membrane, causing cell wall collapse. Unlike baking soda (sodium bicarbonate), potassium bicarbonate does not create soil sodium accumulation issues.

DIY potassium bicarbonate spray: Dissolve 1 teaspoon potassium bicarbonate in 1 liter of water with ½ teaspoon horticultural oil and 2 drops of insecticidal soap as a surfactant. Apply in the evening to avoid phytotoxicity in heat; cover all leaf surfaces thoroughly.

Sulfur (Microthiol Disperss — OMRI-certified): Effective preventative for powdery mildew by preventing spore germination. Critical cautions: Do not use sulfur within 2 weeks of an oil-based application (severe phytotoxicity risk); do not apply when temperatures exceed 90°F; do not use on cucurbits (squash, pumpkin, melon, cucumber), which are highly sensitive.

2.6 Strategy 5 — Neem Oil (Azadirachtin-based products)

Neem oil products (e.g., Neem Bliss — 100% cold-pressed, OMRI-listed) have multimodal activity: fungicidal (suppression of powdery mildew, early blight, downy mildew), insecticidal (aphids, whiteflies — key viral disease vectors), and nematicidal properties. Apply as a foliar spray at 7–14 day intervals, always in the early morning or evening to reduce phytotoxicity. Requires thorough coverage and emulsification with warm water and a few drops of castile soap.

2.7 Integration: The Organic Disease Management Pyramid

Optimal disease management integrates these strategies in a layered program rather than relying on a single tool:

1. Base layer (always): Cultural practices + sanitation (see Part III)
2. Preventative layer (routine): Biofungicides (Bacillus subtilis / Trichoderma) on a 14-day schedule before disease pressure
3. Active pressure layer: Copper fungicide ± potassium bicarbonate on 7-10 day schedule when weather favors disease
4. Reactive layer: Physical removal of infected tissue + spot applications of neem oil or potassium bicarbonate

Part III: Long-Term Cultural Prevention Frameworks

3.1 The Four Pillars of Disease-Suppressive Garden Design

Long-term disease management in the home vegetable garden is fundamentally an ecological engineering problem. The goal is to create a system where the disease triangle is chronically disrupted — reducing host susceptibility, eliminating pathogen carryover, and engineering unfavorable environments for infection.

3.2 Pillar 1 — Crop Rotation

Crop rotation is the most powerful and most frequently underutilized disease prevention strategy. Most fungal and bacterial soil-borne pathogens are host-specific or family-specific — rotating out of susceptible host crops for 3–4 years can dramatically reduce inoculum loads in the soil.

A landmark field study published in the Canadian Journal of Plant Pathology demonstrated that crop rotation with corn, lady's finger, cowpea, or a resistant tomato line significantly reduced bacterial wilt severity caused by Ralstonia solanacearum by 20–26% compared to continuous tomato cropping. Rotation also delayed disease onset by 1–3 weeks and resulted in significantly higher yields. The partially resistant tomato line CL1131 showed both lower disease progress curve values and higher yields than the susceptible check variety.

Rotation guidelines by plant family (minimum 3-year rotation away from each family):

3.3 Pillar 2 — Airflow Engineering

Fungal and oomycete pathogens require a critical leaf wetness period (typically 4–12+ hours, depending on temperature) for spore germination and infection. Any practice that reduces the duration leaves remain wet dramatically suppresses disease. Adequate plant spacing and vertical training are the primary airflow tools.

Spacing and trellising guidelines:

• Tomatoes: Minimum 24–36 inches between plants in-row; 36–48 inches between rows. Train indeterminate varieties to single or double leaders on sturdy stakes/cages; consistently remove suckers to maintain open architecture.
• Cucumbers/squash: Vertical trellising (5–6 foot trellis minimum) improves airflow, keeps fruit off the ground, and substantially reduces powdery and downy mildew pressure compared to ground sprawling.
• Basil: Highly susceptible to downy mildew (Peronospora belbahrii); space at 12–18 inches, pinch centers regularly.
• Brassicas: Wide spacing (18–24 inches) reduces Alternaria and black rot severity.

Irrigation management: Overhead irrigation is the single most preventable driver of foliar disease. Switch to drip irrigation or soaker hoses at the soil level — this eliminates the leaf wetness duration required for pathogen infection entirely. If overhead irrigation is unavoidable, water in the early morning so foliage has maximum drying time before nighttime humidity.

Mulching: A 2–3 inch layer of organic mulch (straw, shredded leaf, wood chip) physically prevents soil-splash inoculation — the mechanism by which Alternaria solani (early blight), Septoria lycopersici, and Phytophthora infestans spores splashing upward from contaminated soil land on lower leaves to initiate primary infection. Mulch also moderates soil temperature fluctuations and reduces weed competition.

3.4 Pillar 3 — Sanitation and Seasonal Hygiene

Garden sanitation is the year-round backbone of disease management. Research from Texas A&M Extension and University of Kentucky confirms that pathogens, viruses, and weed seeds persist on tools, containers, and plant debris between seasons.

End-of-season sanitation protocol:

1. Remove all crop debris: Do not till diseased plant material into soil — this temporarily incorporates pathogen inoculum where it can persist and reinfect. Remove and bag all diseased material.
2. Clean and disinfect tools: Scrub tools with soap and water first to remove organic matter (which inhibits disinfectants), then disinfect with 70% rubbing alcohol (3–5 minute soak) or 10% bleach solution. Thoroughly rinse and dry before storage.
3. Sanitize pots and containers: Soak in 10% bleach solution for 30+ minutes, rinse, and air dry. Reusing contaminated pots without disinfection is a primary cause of seedling damping-off in the following season.
4. Between-plant tool hygiene during the season: Carry alcohol wipes when pruning tomatoes, peppers, and basil. Re-dip or wipe between each plant, especially when working around plants with potential viral infection.
5. Seed sourcing: Purchase from reputable suppliers offering certified pathogen-free, disease-indexed seed. Some viruses (TSWV, CMV) can be seed-transmitted at low rates.

3.5 Pillar 4 — Resistant Cultivar Selection

Growing disease-resistant cultivars is the most cost-effective, lowest-effort disease prevention strategy available. Modern plant breeding has developed cultivars with both quantitative (partial, multigenic) and qualitative (complete, single-gene) resistance to many major vegetable pathogens.

A QTL (quantitative trait loci) mapping study published in Theoretical and Applied Genetics identified six QTL regions on tomato chromosomes 1, 2, 5–7, and 9 conditioning resistance to Alternaria solani (early blight), with LOD scores ranging from 3.4 to 17.5 — demonstrating that resistance is genetically complex but real and breeders have successfully incorporated it. The study noted that three of the QTL also conferred resistance to stem lesions, providing multi-tissue protection.

Resistant cultivar selection guide for key vegetable diseases:

Seed packet disease resistance codes are standardized by the International Seed Federation (ISF): "HR" = High Resistance (plant will not express disease symptoms under normal pathogen pressure); "IR" = Intermediate Resistance (may express mild symptoms but significantly less than susceptible lines). Always cross-reference with your regional extension service, as resistance to specific races of a pathogen may vary by geography.

3.6 Pillar 5 — Soil Health as Prophylaxis

Healthy, biologically active soil is the least glamorous but arguably most fundamental component of a disease-suppressive garden. High organic matter supports diverse microbial communities including naturally occurring Bacillus, Trichoderma, Streptomyces, and Pseudomonas species that competitively suppress pathogens. Practices that support soil biological health include:

• Compost incorporation: 2–4 inches of finished compost worked into beds each season provides slow-release nutrition (reducing deficiency-related susceptibility) and inoculates the soil with beneficial organisms.
• Cover cropping: Legume cover crops (crimson clover, hairy vetch) between seasons fix nitrogen and break disease cycles; brassica cover crops (mustard, radish) release glucosinolates during decomposition that have documented biofumigant activity against soil-borne pathogens including Verticillium dahliae.
• Avoid soil compaction: Compacted soils restrict root growth, reduce oxygen penetration, and create anaerobic pockets that favor Pythium, Phytophthora, and bacterial crown rot pathogens.
• pH management: Most vegetable pathogens have narrow pH optima. Maintaining soil pH at 6.0–7.0 creates broadly unfavorable conditions for many soil-borne fungal pathogens while optimizing nutrient availability.

Diagnostic Decision Tree: Quick-Reference Field Protocol

START: Plant showing symptoms
│
├─ Are ALL plants in the bed affected uniformly?
│ ├─ YES → Suspect ABIOTIC cause
│ │ Check: nutrient deficiency (mobile vs. immobile pattern),
│ │ watering irregularity, pH, herbicide drift
│ └─ NO → Continue to Step 2
│
├─ Is there visible growth ON the plant surface?
│ ├─ White powdery coating → POWDERY MILDEW (fungal)
│ ├─ Gray/purple fuzzy coating on leaf undersides → DOWNY MILDEW (fungal)
│ ├─ Orange/rust pustules → RUST (fungal)
│ ├─ Gray fuzzy sporulation on injured/crowded tissue → BOTRYTIS (fungal)
│ └─ No visible growth → Continue
│
├─ What do the lesions look like?
│ ├─ Circular, defined margins, concentric rings → FUNGAL (e.g., early blight)
│ ├─ Angular, water-soaked, bounded by veins → BACTERIAL
│ ├─ Mosaic, mottled, yellowing + distortion → VIRAL
│ └─ Water-soaked + rapid collapse + white mycelium → LATE BLIGHT / PHYTOPHTHORA
│
└─ Is wilting the primary symptom?
 ├─ Sectional wilt + ooze test positive → BACTERIAL WILT (Erwinia)
 ├─ Yellowing + stem cross-section brown → FUSARIUM / VERTICILLIUM WILT
 └─ Wilt with root/crown rot → PYTHIUM / PHYTOPHTHORA (oomycete)

References

Ordered by scientific authority and relevance — peer-reviewed reviews and studies first, university-extension diagnostic and management resources after.

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