Primary symptoms

A good wheat crop should still have 2.5 green leaves per spike at anthesis (i.e. when yellow anthers are clearly visible on spikes).

Summary

The crop fills its grains by using carbohydrates from two sources, either produced freshly from photosynthesis in the green leaves or from storage in the stem. Stored material alone can only produce a small grain yield of less than 1 t/ha, so for high yields it is important to keep several leaves green and active on each shoot during the grain filling process.

Growing grains need nitrogen as well as carbohydrates. The plant extracts little nitrogen from the soil after anthesis so the developing grains, which grow entirely after anthesis, have to get virtually all their nitrogen from storage in the plant. An important store is the green leaves.

A main reason that leaves lose their green colour and die is because their nitrogen is removed and redistributed to the grains. The less nitrogen stored, the more quickly the leaves die. How much is stored depends on how much was available in the soil before anthesis, as determined by management.

The figure below indicates how many green leaves each shoot should have at each stage of grain growth to achieve yields ranging from 4 to 10 t/ha. Any diseases such as rusts (yellow rust, stem rust, leaf rust) and leaf blights that reduce the number of green leaves and the green leaf area at this time also reduce yield. Field sheets can have a column for recording green leaf number.

Does the crop have enough green leaves after heading?

• Count how many green leaves there are on average shoots and add the number to the appropriate column in the field sheets. Check your count against the figure to see what your crop can yield. For example, if shoots have three green leaves at mid flowering, the potential yield of the crop is around 6 t/ha. If your crop has one or less green leaves at this stage, yield will be very poor. The crop to this stage has been limited by nitrogen, soil conditions, disease or water (too little or too much)
• Have the upper leaves lost green coloration? Are the symptoms caused by leaf diseases or poor nutrition? Count the green leaf number on the average shoot (see where, how and what to sample in the field) and assess whether the leaves are diseased. From the above figure estimate the impact of disease on yield. The figure shows how quickly disease can reduce potential yield.

Causes of leaf death after heading

• In nutrient-poor soils, nitrogen was not applied at first node (Z3.0) or later.
• The crop is short of water or waterlogged.
• Leaf diseases, root rots or insect attacks have caused leaf death.

Making sure there are enough green leaves

• Nitrogen: Apply nitrogen to the soil. If nitrogen is readily lost through leaching as from a sand loam, split the applications between sowing and first node.
• Irrigation: Ensure that irrigation is done at recommended or calculated intervals, supported by the wilting score.
• Pests and diseases: Control diseases and pests. Next season use a more disease-resistant variety.