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CHAPTER 10 – DISEASES THE WHEAT BOOK OCCURRENCE, RECOGNITION AND CONTROL (continued) because considerable BYDV spread can occur with low aphid numbers, and virus symptoms are difficult to see in winter. Five years of insecticide experiments with a range of cereals in WA, showed that foliar sprays of synthetic pyrethroids at 3 and 7 weeks after full crop emergence decreased BYDV spread by up to 87% and increased grain yields by up to 41%. These pyrethroids (e.g. alphacypermethrin (Dominex or Fastac) or beta-cyfluthrin (Bulldock)) control BYDV better than pirimicarb or dimethoate because they not only kill aphids but also have an anti-feeding effect that deters new aphids from feeding for a further 3-4 weeks. The cost of spraying can often be reduced, as pyrethroids may be tank-mixed with many of the autumn-applied herbicides. Controlling aphids with synthetic pyrethroids early to stop BYDV spread can also prevent them building up sufficiently to cause feeding damage in spring. However, in bad aphid years applications of insecticides from other chemical groups may be required in spring if feeding damage becomes a problem in cereal crops. 210
THE WHEAT BOOK CHAPTER 10 – DISEASES OCCURRENCE, RECOGNITION AND CONTROL (continued) Wheat rusts In Western Australia, rust has been a sporadic problem for wheat growers. Since the first recording of stem rust in Western Australia in 1906, there have been less than 10 epidemics. Small outbreaks of stem rust are more common and can cause severe losses over a limited area. Since the first leaf rust epidemic in Western Australia in 1992 significant outbreaks of this disease have recurred. Rusts require living plants on which to grow and reproduce, so they must continually infect new hosts. To infect crops during the season they must survive over summer by infecting volunteer cereals or grass hosts. This occurs during seasons where summer rainfall is plentiful for volunteer cereals or weed hosts to survive. Dry summer conditions usually preclude significant carryover of rust (Figure 10.13). Fig. 10.13 Rust risk. Table 10.12. Distinguishing features of the wheat rusts 211 The three types of rusts that can occur on wheat are stem, leaf and stripe rust. In the field, the shape and colour of the pustules (Table 10.12) can distinguish the type of rust. It is important to submit samples of rust to AGWEST. These are submitted to Sydney for typing. This then informs us if the rusts are changing in terms of resistance and helps the breeding program in developing varieties resistant to rust. So far, stripe rust (a serious disease in the Eastern States) has not crossed the Nullarbor. Its arrival in Western Australia may increase the importance of rust control here. Pustule character Stem rust Leaf rust Stripe rust Colour† Reddish brown Orange brown Yellow Shape Oval to elongated with conspicuously tattered stripes edges Circular to oval Small, circular, in yellow Location on plant On both sides of leaf, on Chiefly on upper surface of Chiefly on upper surface of leaf sheaths, stems and leaf leaf, on leaf sheaths, awns outside of head and inside of glumes † Colour depends on the freshness of the pustule. Most spores are produced overnight so that pustules are best observed in the morning before spores are spread by wind.
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