The wheat crop is host to a wide range of fungal pathogens. In the UK those which are most important to the crop are Septoria tritici, yellow rust, mildew, brown rust, Septoria nodorum, eyespot, fusarium ear-blight and take-all. Most of these infect the leaves and reduce the photosynthetic area and may induce other physiological changes that affect both yield and quality. Take-all affects the roots of plants which may lead to the death of the plant and eyespot affects the stem bases. Fusarium (several species) infects both stem bases and the ears of wheat plants. Ear infections result in the production of mycotoxins which are a potential food safety issue.
There are also a range of lesser diseases such as bunt, loose smut, ergot and several virus diseases which are either found infrequently of have a relatively minor effect on the plant. Sooty moulds are caused by a mixture of fungi which grow on the glumes in wet weather. They cause little yield loss but may discolour grain which affects the visual appearance of milling wheat.
Wheat diseases are important to farmers not only because of their direct impact on the yield of the crop but also on their choice of wheat varieties. Millers may be interested in wheat diseases for a number of reasons. Outbreaks of certain diseases in some years may have significant effects on yield and quality thus affecting the supply of home-grown wheat. Sometimes the media seizes on such outbreaks and this may create concerns amongst flour customers or secondary processors.
Disease control is a significant and costly input for wheat production. Applications of fungicides are usually made as part of a programme using specific timings. For most wheat crops these are:
T0: In the spring when the crop is at Growth stage (GS) 25 - 30
T1: GS 31 to 33, up to second node detectable
T2: The most important timing (GS 37-49) when the flag leaf has emerged
T3: Usually mid-June when the ear has fully emerged (GS59)
As with diseases there are a number of pests that attack and damage the wheat crop. Some such as grain aphids and the virus-vector aphids occur in most localities in most years. Others such as wheat bulb fly are geographically restricted whilst some, such as slugs, are more significant on heavier soil types. Others may be more sporadic in their importance such as wheat blossom midge, yellow cereal fly, and gout fly. Some pests may be associated with wheat in grass rotations (wireworms, frit fly and leatherjackets) although most milling wheat is not grown in these situations.
Control thresholds exist for many pests and farmers may use cultural controls together with agrochemicals within integrated control strategies.
Weeds are plants growing within the crop. The damage they cause to the crop depends on weed species, weed density, the competitive ability of the crops and the growth stage when weeds compete. While some weeds are highly competitive, others pose little threat and may be valuable to wildlife.
Weeds are usually divided into two broad categories:
Grass weeds: e.g. Black-grass, Italian rye-grass, wild oat
Broad-leaved weeds: e.g. Chickweed, poppy, scentless mayweed, cleavers
Non-chemical control methods are increasingly important to reduce weed numbers and the need for herbicides. These include the use of rotations, cultivations, crop choice and drilling dates. However, herbicide applications are still essential to give effective and efficient control; the optimisation of herbicide timings is very important.
AHDB produces a broad range of information about pests, diseases and their control which can be found on www.ahdb.com.