Primary symptoms

Root diseases reduce moisture and nutrient uptake

Root diseases like take-all reduce water and nutrient uptake.

Summary

Are root diseases a problem?

Seedlings (Before Z1.3)

Are there sections of crop where plants are missing or seedling leaves look discolored or weak? Carefully dig up seedlings and wash out their roots. Are the roots stunted, are they discolored, either brown or grey?

Pythium species cause short stubby main roots with poor development of laterals and brown soft tissue near or at root tips. It occurs in very wet or waterlogged soils and results in poor emergence and seedling death.

Jointing (round Z3.3)

Check for poor plant vigour in the crop looking particularly for defined patches of low vigour. Are any leaves yellow? Dig up plants and wash out their roots. Are roots stunted with discolored, sharp tips?

Rhizoctonia sp. can produce bare plant-free patches in the crop (hence its name 'bare patch'). There may be some surviving plants in these patches but they will be very stunted. Their roots are likely to be brown and spear-shaped and even largely rotted away. Rhizoctonia sp. occurs mainly where weeds had not decomposed before the crop was sown. It can be associated with waterlogging and is common in winter rainfall areas and in poor calcareous soils.

Cereal cyst nematodes (Heterodera sp.) also cause patches of weak crop growth, often with some yellow leaves, that may be several metres across. Dig up and examine the roots and crown regions of plants. Are the secondary roots white or discoloured; are they short, twisted and knotted in a mass? Cereal cyst nematodes feed on roots and cause these responses. Later, small white cysts develop at the knots or on thickened roots. Cysts finally turn brown.

Check the color of the crown area. Is it black? Take all (Gaeumannomyces graminis var. tritici) can be confirmed in its early stages by breaking a root. Its core will be jet black. In later stages the lower stem and many roots will be black. Plants can be uprooted easily and the roots are stunted. Take all affects all the tillers of the plant and might infect patches of crop several meters across. It spreads faster under wetter conditions, particularly in neutral to alkaline soils. It is also called white heads because dead ears become increasingly apparent in expanding infected patches of the crop as the season draws to a close.

Is the lower stem brown? This can be due to the crown rot (Helminthosporium sativum). There may also be pink coloring of the sub crown internodes, and the lowest nodes of the stem under the leaf sheaths. It usually affects the primary tillers; one or two shoots on each plant. Pink color is associated with Fusarium species of the root pathogen complex. It is more evident when plants are water stressed.

Around heading to anthesis (Z5.5 to Z6.5)

The symptoms of seed and soil-borne diseases usually appear on the crop after heading. Smuts are seed-borne.

Are some spikes completely white and dead? Take all or crown rot causes this problem. The disease will by this stage have completely destroyed the conducting system of the stems, thus killing and whitening the spikes. See above for earlier symptoms. Severe frost can produce similar symptoms, but then large continuous sections of the crop will be affected.

Other root rots are prevalent in South Asia, particularly when conditions are warm. Bipolaris sorokiniana causes brown necrotic roots and brown subcrown internodes. The root rot Sclerotium rolfsii is also prevalent.

Is one or more spikelets or the entire head bleached or partially dried out? Do these spikelets contain shrivelled and often discoloured seeds? This is likely to be caused by fusarium head scab (Fusarium graminearum) that is common on wheat grown in humid and warm areas. Spikes can be partially pink.

Are plants short with dark green leaves and an unpleasant fishy smell? These plants are infected with bunt (Tilletia tritici, Tilletia laevis). Bunt balls infect grains. At harvest the infected grains burst to release their mass of black spores.

Are plants relatively tall with smutted spikes? This is loose smut (Ustilago tritici). All that eventually remains of infected spikes is the bare central stalk.

Solutions to disease

Root rot disease-complex

• Variety: Use resistant varieties
• Rotation: Avoid cereal monoculture (wheat after wheat) and rotations that include consecutive host crops for the diseases.
• Include a legume or biofumigant crop in the rotation. These are not hosts for the disease so avoid build up of disease inoculum. Diseases like take all may require two seasons without wheat to kill all inoculum (pulses, oilseeds, potatoes, dry beans, lentils and sunflower are not hosts).
• Avoid growing wheat after maize or even near a maize field if Fusarium head scab is present in the area.
• Residues: Remove standing crop residues and any alternate hosts for the disease.
• Vigor: Ensure fast crop emergence by avoiding deep planting and by pre-irrigation.
• Soil moisture: Do not sow into a dry soil and then irrigate shortly after as this encourages Pythium species particularly if the soil remains wet for a long period. Irrigate before sowing and do not irrigate again until the crop is tillering (Z2.1).
• Nutrition: Improve crop nutrition by the balanced use of fertilizers. Phosphate and potassium fertilizers will reduce infection by dry land root rot.
• Aeration & drainage: Improve soil aeration and drainage by cultivation and increased organic matter.
• Seed treatment: e.g., use general purpose Vitavax 200 in Bangladesh has increased yields in farmer's fields by a minimum of 10% (Meisner et al 1994).