Urocystis tritici Körn.

Description

Sori in leaves, sheaths and stems, forming long striae, at first lead-coloured, covered by the epidermis which ruptures longitudinally to expose the granular-powdery blackish brown mass of spore balls; heavily infected plants do not head.

Spore balls subglobose, ellipsoidal to irregular, 20–40 µm long, composed of 1–3 (–5) spores, completely surrounded by sterile cells.

Spores subglobose to irregularly ovoid, rarely elongate, 12–18 (–22) × 11–16 µm, dark reddish brown, smooth.

Sterile cells subglobose to ovoid, 6–12 µm long, yellowish brown, smooth.

Spore germination: (1–) 2–4 (–6) cylindrical basidiospores produced on the tip of an aseptate basidium. Basidiospores 12–30 × 3–5 µm, giving rise to slender infection hyphae and/or secondary sporidia measuring c. 25 × 4.5 µm.

Hosts
   
Host family: Poaceae
   
Host species: Triticum aestivum L.

Distribution

States & Territories: NSW, QLD, SA, TAS, VIC, WA

Comments

Urocystis tritici causes flag smut of wheat. The name of the disease is derived from the main symptom of infection, viz. the presence of sori in the flag leaf. Infected plants are also often severely stunted and tiller excessively. Flag smut was first found in Australian wheat fields in 1868 (McAlpine, 1910). Until the early 1900s, the disease was responsible for widespread and severe losses when popular cultivars were very susceptible and chemical control was not available (Ballantyne, 1993). Flag smut is known throughout the world where autumn-sown wheat is grown in regions with dry summers and mild winter temperatures.

Cross-inoculation experiments (Fischer & Holton, 1943; Rees & Platz, 1973) have shown that spores of U. tritici from wheat can infect some other grasses (Elymus and Hordeum). However, grasses are not considered an important source of inoculum or a factor in the epidemiology of flag smut of wheat.

Spores of the fungus, present in either the soil or as contaminants of seed, germinate and infect wheat seedlings prior to emergence. Soil type, moisture, temperature and sowing depth affect the incidence of disease by influencing seedling germination, as any factor that slows emergence may increase the opportunity for infection. In general, flag smut occurs more frequently in light, relatively dry soils at temperatures between 18° and 24°C. The fungus overwinters as a mycelium within the seedling. In spring, the mycelium systematically invades the host and produces spores within the flag leaf and upper parts of the wheat plant, but not in the developing seed. Spores can survive for up to 7 years in the soil and are spread by wind. Flag smut is readily controlled through the use of resistant cultivars, fungicidal seed treatments and crop rotation.