📈 Bunt Disease of Wheat — Smut, Karnal Bunt, Powdery Mildew

Beyond rusts, wheat is affected by several other important diseases including bunt (common bunt / stinking smut), loose smut, flag smut, Karnal bunt, and powdery mildew. These diseases affect grain quality and marketability, and some have major quarantine significance.

Bunt Disease of Wheat

Bunt disease of wheat encompasses a group of fungal diseases caused primarily by Tilletia species. The two main types are common bunt (T. caries / T. foetida) and Karnal bunt (T. indica). Both replace grain contents with dark teliospores and emit a characteristic fishy odour (trimethylamine).

Common Bunt of Wheat (Stinking Smut)

Causal Organism

Tilletia caries (syn. T. tritici) and Tilletia foetida (syn. T. laevis)

Both species are members of the order Ustilaginales (class Teliomycetes).

Symptoms

• All or most grains in an ear are converted into bunt balls — dark olive-green to black sori filled with teliospores
• Infected ears appear greener than healthy ones and remain erect (normal ears nod)
• At harvest, bunt balls break open releasing masses of black powdery spores with a strong fishy/herring-brine odour (due to trimethylamine)
• Infected plants are slightly shorter; ears mature slowly
• A single bunt ball contains ~2–12 million teliospores

Disease Cycle

• Teliospores contaminate seed surface or persist in soil for 5–7 years
• Spores germinate at 5–10°C (cool, moist soil) during seedling emergence
• Basidiospores (primary sporidia) fuse in pairs → infects coleoptile/leaf sheaths of germinating seedling
• Systemic mycelium invades growing point → colonises developing ovary
• Entire grain replaced by teliospores at maturity

Favourable Conditions

Management

• Hot water treatment: Soak seeds 4 hrs cold water → hot water 52°C for 10 min (or 54°C for 5 min)
• Seed treatment with Carboxin 75 WP @ 2 g/kg seed (first systemic fungicide for bunt)
• Vitavax (Carboxin + Thiram) @ 2.5 g/kg — combined systemic + protectant action
• Thiram 75 WP @ 2.5 g/kg — surface protectant
• Resistant varieties: HD 2281, HD 2733, PBW 502
• Avoid very early or very late sowing; use clean certified seed

Economic Importance

• Yield loss: 5–30% depending on inoculum load
• Severely infected grain unfit for human consumption, seed, or export
• Even 1% contamination renders entire lot unmarketable due to odour

Loose Smut of Wheat

Causal Organism

Ustilago tritici (syn. Ustilago nuda f. sp. tritici)

Symptoms

• Infected ears emerge earlier than healthy ones
• Entire ear is replaced by a mass of dark brown to black powdery spores enclosed in a thin membrane
• Membrane ruptures at heading, releasing spores that are wind-dispersed to healthy florets
• After spore dispersal, only the bare rachis remains

Disease Cycle

The fungus is internally seed-borne. Teliospores land on open florets, germinate, and infect the developing ovary. The mycelium remains dormant in the embryo until the seed germinates next season — a classic systemic infection.

Management

• Hot water treatment: Soak seeds in cold water for 4 hours, then immerse in hot water at 52 degrees C for 10 minutes
• Seed treatment with Carboxin 75 WP (2 g/kg seed) — most effective systemic fungicide
• Use certified disease-free seed

Flag Smut of Wheat

Causal Organism

Urocystis agropyri

Symptoms

• Long, dark grey to black stripes on leaves, sheaths, and stems running parallel to veins
• Stripes contain masses of spore balls covered by a silvery membrane
• Affected plants are stunted, leaves become twisted and shredded
• Severe infection causes poor ear emergence

Management

• Seed treatment with Carboxin or Thiram (2.5 g/kg seed)
• Crop rotation with non-host crops for 2-3 years
• Deep ploughing to bury inoculum

Karnal Bunt (Partial Bunt)

Causal Organism

Tilletia indica (syn. Neovossia indica)

First reported: Karnal, Haryana, India — 1931 (by K.C. Mitra). The disease is named after Karnal city.

Symptoms

• Only part of the grain is converted to black powdery spore mass — hence "partial bunt"
• Infected grains emit a fishy odour (trimethylamine) when crushed
• The pericarp is intact but bulges slightly; the remaining portion of the grain appears normal
• Grain weight reduction: 40–50% in severely infected grains
• Incidence in field: 1–20% grains affected in an infected crop

Quarantine Significance

Karnal bunt is a quarantine pest (A2 list, EPPO). It is absent from most wheat-importing nations (USA, EU, Canada, Australia). Even one infected grain in a consignment can lead to rejection of the entire export lot. This makes it one of the most economically significant bunt diseases globally.

Favourable Conditions

Management

• Grow resistant or tolerant varieties (HD 2781, WH 147)
• Avoid late sowing — early varieties escape critical infection window
• Deep summer ploughing to reduce soil-borne teliospores
• Seed treatment with Thiram + Carboxin or Trifloxystrobin
• Foliar spray of Propiconazole at booting/heading stage

Powdery Mildew of Wheat

Causal Organism

Blumeria graminis f. sp. tritici (syn. Erysiphe graminis)

Symptoms

• White to greyish powdery patches on upper leaf surface, sheaths, and ears
• Patches consist of mycelium and conidia of the fungus
• Later, patches turn grey-brown with embedded black cleistothecia (fruiting bodies)
• Reduces photosynthetic area, leading to poor grain filling

Favorable Conditions

• Cool temperatures (15-20 degrees C) with high humidity
• Dense crop canopy and excessive nitrogen
• Prevalent in northwestern India (Punjab, Haryana)

Management

• Resistant varieties with Pm genes
• Avoid excessive nitrogen and dense planting
• Foliar spray of Sulphur WP (0.2%) or Triadimefon 25 WP (0.1%)
• Spray Propiconazole or Tebuconazole at early symptom appearance

Comparison: Bunt Diseases of Wheat at a Glance

Cheat Sheet — Quick Facts for Exams

• Common bunt = stinking smut; pathogen T. caries; entire grain destroyed; fishy odour; Vitavax/Carboxin seed treatment
• Karnal bunt first reported Karnal, Haryana 1931; partial grain; quarantine (EPPO A2); Propiconazole foliar at heading
• Loose smut = internally seed-borne; bare rachis left after spore dispersal; hot water treatment (52°C, 10 min)
• Flag smut = Urocystis agropyri; grey stripes on leaves; Carboxin seed treatment
• Trimethylamine causes fishy odour in all Tilletia bunt diseases
• Carboxin = first systemic fungicide developed for smut/bunt diseases
• Teliospores of common bunt survive in soil for 5–7 years

Summary Cheat Sheet