Primary symptoms

Symptoms associated with leaf diseases are sometimes similar to those resulting from nutritional imbalance.

Leaf diseases reduce green leaf area and yields

Summary

Are leaf diseases a problem?

Rusts: Look for coloured spores on the surface of leaves, stems or spikes that rub off with a finger. These are rusts. White or grey patches are mildew.

• Yellow rust (Puccinia striiformis fsp. tritici) has bright yellow spores that appear as stripes on the upper surface of leaves. It prefers moist conditions and low temperatures (8-15°C), particularly cool nights (<10°C).
• Stem rust (Puccinia graminis fsp. tritici) has dark brown pustules that can tear through either leaf surface and spread to stems and spikes. It appears late in the season particularly in humid, warm areas (15-30°C).
• Leaf rust (Puccinia recondita) produces light brown spores mostly on the upper surface of the leaf. It causes no rupture of the epidermis. As with stem rust, the pustules may spread to stems late in the season. Leaf rust is in all cereal growing areas. It thrives in moist conditions with temperatures of 15-25°C.
• Powdery mildew (Erisyphe graminis fsp. tritici) has white cotton-like spots piled in patches on any green surface. Infected areas turn dull grey and may contain black spherical points that are the fruiting bodies. It occurs in damp conditions.

Look for blotches with yellow margins on your crop leaves. Blotch diseases are more difficult to identify than rusts, but check the descriptions below.

• Septoria tritici (blotch) produces irregular lesions with black specks. Septoria nodorum (glume blotch) produces lesions that have no black spot. Late in the season and under humid, warm conditions, the disease infects the glumes and spike causing grey blotching and shrivelled seeds.
• Tan spot (Pyrenophera repentis) produces symptoms that are similar to those of glume blotch, but the disease occurs in cool climates and is restricted to leaves.
• Spot blotch or leaf blight (Bipolaris sorokiniana) has become significant in the warmer humid areas of SE Asia causing major yield losses in Bangladesh (Alam et al 1994). It can be seen as small blotches with minimal necrosis on the lower leaves of seedlings, but during stem elongation and heading it can spread rapidly up the plant to damage most leaves and eventually infect the seed with black point.

Solutions to disease

Rusts & powdery mildews

• Variety: Sow more resistant varieties, and change varieties as often as practicable.
• Rotation: Avoid monoculture of a variety over large areas.Fungicides: Apply fungicides if an epidemic level is reached, particularly for yellow rust.
• Fertilizer: Avoid excess application of nitrogen fertilizer.

Smuts

• Variety: Use resistant varieties.
• Clean seed: Use only clean seed harvested from a clean field. Ensure the harvester is not contaminated from the previous field
• Seed treatment: Use treated seed or treat at the farm using available recommended fungicide.
• Shallow planting: Avoid deep planting particularly in fields where smut disease was present in previous years.
• Soil moisture: Pre-irrigate or plant into moist soil. Ensuring fast germination minimises early infection.
• Rogue: Rogue out any infected heads of loose smut and burn them outside the field.

Blotch diseases (Septoria spp., tan spot)

• Variety: Use resistant varieties and change varieties as often as realistically possible.
• Rotation: Avoid monoculture of a variety over large areas. Avoid continuous cropping of wheat on the same field. Rotate wheat with a legume crop.
• Residue management: Avoid planting into the residues of the previous crop.
• Clean seed: Use plump and clean seed uncontaminated with plant debris.
• Fertilizer & seed rate: Avoid excess nitrogen fertilizer and high seeding rates. Dense foliage encourages faster development and spread of disease.