Primary symptoms

Many diseases which attack seeds and seedlings reduce plant density and plant vigor. Stalk rots can kill plants if they attack early in the cycle, and thus reduce plant density. Later in the cycle, stalk rots cause lodging. Leaf blights cause a reduction in photosynthesizing leaf area.

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There are many diseases that attack wheat some causing seedlings to die even before emergence (bare patch) or shortly thereafter (damping-off). Infections can continue throughout crop growth and impacts be seen right through to harvest.

Fungal diseases are the most common and widespread diseases. They have distinctive symptoms that can appear on spikes as well as on leaves and stems when the infection is severe. Symptoms on leaves range from small, infected patches (necrotic lesions) to discolouration and premature drying of whole leaves. Flat spots and blotches that progressively increase in size and spread from lower to upper leaves are generally due to fungal diseases that are transmitted in stubble or splashed up by water. These include septoria and tan spot. Rust spores cause protruding dusty spots or streaks on the surface of leaves, stems and spikes. The colour of the spots or pustules indicates the type of rust present. Rust spores leave a coloured smear on fingers when touched.

Whitened spikes in the crop commonly indicates the presence of root rot pathogens such as take all, a Fusarium disease or Bipolaris sorokiniana. These white heads are completely dead having had their vascular links to water and nutrients completely severed by the root or crown rot disease. The plants have probably been diseased for some time prior to heading. Symptoms associated with seed and soil-borne diseases also usually become apparent on the crop after heading. Smut diseases for example are seed borne and very common in areas where farmers recycle their own seed.

Considering the diversity of wheat diseases, it is critical to keep an eye open for problems right from the beginning. Loss of plants and loss of green leaf area reduce the potential yield of a crop. Many of the problems are seed borne, while others are already in the soil at planting, carried over from the previous crop or in residues, or even associated with perennial weeds. Conditions that are less than optimal for the crop are often better for disease infections. Poor aeration, poor nutrition and water logging are examples. However, dense crop canopies necessary for highest yields are also perfect for rapid spread of diseases (and pests).

Consequently, good management with regard to disease is a continuous balancing act. It starts before the crop is sown with a clean field and choice of the most disease resistant wheat variety. It demands vigilance right through until harvest as the crop is continually exposed to new disease threats as it develops. Finally, it requires thought and action about the crop rotations best able to prevent build up of diseases across seasons.

Causes of plant diseases

  • Resistant varieties were not used. Some resistance is available to the rusts, to septoria, to cereal cyst (Heterodera sp.) and stem nematodes (Pratylenchus sp.) and to a lesser extent to crown and root rots. Check what variety was planted. Were other varieties planted in the area less affected? If so, were the farming methods also different?
  • Infected seeds were planted. Was black point apparent in the seed sample? Where did the seed come from? Was it from last season's crop on that field or purchased from a seed merchant? Absence of black point is unfortunately no insurance that the seed is clean of Bipolaris sorokiniana E.
  • Diseased standing crop residues were not removed prior to planting in a disease prone area, or an alternate host is present close by (grass species harbor yellow rust while non-grass species harbor leaf rust and stem rust). Look for remains of crop residues and other plants that could host the disease.
  • No seed treatment or an inappropriate treatment was applied. Get details of the seed treatment and check whether it is an acceptable treatment for the disease identified.
  • The soil was waterlogged during the seedling stages slowing emergence and early growth. Check the soil type and look for evidence of poor drainage. Was there heavy rain during seedling growth?
  • Was wheat planted after wheat or after barley? Is the wheat crop next to a field of maize?