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Crop physiology

Important phases of yield formation Understanding how yield forms and how disease affects crop growth can help target fungicide treatments. Foundation phase After crop establishment in autumn and the start of tillering, little canopy expansion occurs during winter due to low temperatures and lack of sunshine for photosynthesis. Construction phase Canopy growth: The rate of canopy expansion increases in April/May when temperatures rise and the large upper leaves emerge. The green area of a canopy (measured as green area index ­ GAI) is at its maximum when the ear emerges, just before grain filling starts. Stem reserve accumulation: During stem extension, stored soluble carbohydrates accumulate in the stem to act as a buffer, allowing some growth to continue when photosynthesis is inadequate. Production phase ­ the grain filling period Soon after grain filling begins, stem reserves move to the grain and stem dry weight reduces by about 3t/ha between flowering and harvest. The contribution of stem reserves to grain filling is particularly important if disease causes premature canopy senescence. Reserves can contribute 20% to 50% of grain fill. After flowering, most dry matter produced by photosynthesis goes directly to the grain. Grain filling usually lasts 6­7 weeks and, in bright days, yield typically increases by 0.2t/ha/day. Maintaining green canopy area is crucial to preventing a premature end to grain filling. If GAI reduces below the optimum (GAI = 6 approximately), light interception and crop yield will fall. If the canopy dies ten days early due to disease, yield will be reduced by about 2t/ha and specific weight will fall. See The wheat growth guide, HGCA (2008).

Illustration of GAI = 2 (two areas of green leaf and stem to one area of ground)

Crop dry weight increase over the season

The wheat disease management guide ­ Spring 2008


Crop physiology and disease control

Yield formation depends on light interception by the crop canopy. Disease can reduce canopy size at establishment, during tillering, and during canopy expansion and senescence. Most foliar diseases exert their main effect on yield by hastening senescence of the top three leaves during grain filling. Seed-borne, as well as root and stem-base diseases, can affect establishment and canopy growth. which usually occurs before foliar diseases reduce canopy size and hence light interception. Production phase The main aim of sprays is to protect the top three leaves, as up to 80% of the yield in wheat is derived from these leaves ­ hence the importance of the T1 and T2 sprays, which are applied to protect upper leaves as they emerge. Effect of disease on canopy expansion and survival in relation to growth stage

Foundation phase Seed-borne diseases such as Microdochium nivale, Fusarium graminearum and Septoria nodorum can cause seedling losses and poor In very thick crops, with large crop establishment. canopies, even leaf 3 may contribute little to yield. In thin crops leaves 3 Construction phase Stem-base and root diseases such and 4 are more important. It is as eyespot and take-all can restrict more important to control disease canopy growth. Foliar diseases rarely on the upper three leaves (those emerging after GS32, particularly impair stem reserve accumulation the flag leaf) than on lower leaves. Up to 80% of wheat yield comes from the top three leaves, so it is vital to protect them

The wheat disease management guide ­ Spring 2008




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