Common symptoms of nutrient deficiency include chlorosis, necrosis, reduced growth and reduced tillering/yield. Main symptoms are described in (Bowden et al. 2008). Nutrient deficiency is often linked to enzyme dysfunction or degradation. Rubisco (among other chloroplast proteins) is degraded during or after abiotic stresses, to allocate nitrogen to other compartments and organs (Feller et al. 2008). Nitrogen, phosphorus, potassium and sulphur deficiencies have been linked to a decrease in nitrate reductase activity (Harper & Paulsen 1969). Boron toxicity has also been observed in wheat (Bolland et al. 2000). Aluminium toxicity has also been noted in acidic soils in Western Australia (GRDC 2015c).
6.1.2 Temperature and water stress
Most wheat in Australia is grown as a dryland crop, with irrigated wheat contributing only a very small proportion to total production (Turner 2004). The wheat growing areas in Australia generally have a climate that is considered Mediterranean, with a concentration of rainfall during the winter months while summer months are drier. Wheat is subjected to drought and heat stress (as well as frost, to a lesser extent). Heat stress generally results in suppressed growth and lower yield but is rarely lethal. Heat stress can affect both vegetative and reproductive tissues, impacting on photosynthesis efficiency or male and female fertility (Gusta & Chen 1987). Under heat stress hormone homeostasis is altered, the rate of carbon assimilation decreases as Rubisco activity is impaired and oxidative damage is frequently observed (see (Barnabas et al. 2008) for review).
Freezing can impact grain yield and quality. Cold injuries increase with the length of exposure to low temperatures. Depending on the stage of development, yield can be reduced to zero. Frost during flower and seed development causes more damage than during vegetative growth (Gusta & Chen 1987).
Water deficit affects every aspect of wheat development, from germination to yield. Protein synthesis, photosynthesis, respiration and transpiration are most impacted by water stress (Gusta & Chen 1987; see also Barnabas et al. 2008 for review). Drought and heat stresses are often combined, and have a greater detrimental effect on growth compared to individual stress. Stomatal conductance is especially impaired (Barnabas et al. 2008).
6.1.3 Salt stress
Salinity is a significant issue in south-western Australia and in some parts of the Murray Darling Basin. In 2000, 5.7 million hectares of Australia were assessed as having a high potential to develop salinity. More than two million hectares of broadacre farmland were estimated to be affected by dryland salinity, with more than half in WA (ABS 2002). Predictions indicate that unless effective solutions are implemented, the total area affected by soil salinity could increase to 17 million hectares by 2050, more than 11 million hectares of which is agricultural land (ABS 2013c).
Salt stress impairs wheat growth and development, reduces photosynthesis efficiency, decreases respiration and protein production. Salt can also trigger physiological responses seen in drought stress (see Sairam et al. 2002 for review).
6.2 Abiotic tolerances
Some wheat varieties display tolerance to aluminium toxicity (Trainor et al. 2015). Likewise some varieties show tolerance to boron toxicity (Bolland et al. 2000; GRDC 2014; GRDC 2015b).
Section 7 Biotic Interactions
A number of weeds occur in Australian wheat crops, however not all warrant control in wheat production or in all seasons. In the western growing region the most common weeds in wheat crops include wild radish (Raphanus raphanistrum), annual ryegrass (Lolium rigidum Gaudin), wild oats (Avena fatua L.) and brome grasses (Bromus spp.) (GRDC 2015c). In the northern wheat growing region, the main weeds are wild oats (A. fatua), paradoxa grass (Phalaris paradoxa L.), awnless barnyard grass (Echinochloa colona (L.) Link.), annual ryegrass (L. rigidum) and fleabane (Conyza spp.) (GRDC 2014). Herbicide tolerance has become an important problem in the management of weeds in wheat crops. Several of the most common weeds, namely wild radish, annual ryegrass (in both western and northern regions) and awnless barnyard grass, exhibit resistance to herbicides including glyphosate (GRDC 2015c; GRDC 2014). See Section 2.3.3 for information on weed management practices in Australia.
7.2 Pests and pathogens
7.2.1 Vertebrate pests
Damage to wheat crops by birds has been noted in Australia and around the world (Temby & Marshall 2003; Jones 1987; Massam 2001; Jarman & McKenzie 1983; Davies 1978; Coleman & Spurr 2001; Massam 2000). In Australia, birds such as the sulphur-crested cockatoos (Temby & Marshall 2003), cockatiels (Nymphicus hollandicus) (Jones 1987), long-billed corellas (C. tenuirostris), galahs (Temby & Marshall 2003), tree sparrows (Passer montanus) and house sparrows (P. domesticus) (Massam 2000) and emus (Davies 1978) are known to cause damage to cereal crops. Birds such as cockatoos damage the cereal crop most during germination in autumn (Temby & Marshall 2003). When feeding on seed, cockatiels appear to prefer softer, younger seed to harder, mature seed (Jones 1987). Emus feed on a great variety of plant material, but prefer succulent foods, such as fleshy fruits, rather than drier items (Davies 1978). Birds and mice may also damage grain stored on farm (GRDC 2014; GRDC 2015c).
Kangaroos are reported to damage grain crops by feeding on seedlings or trampling mature plants. Eastern grey kangaroos (Macropus giganteus), for example, may feed on young green cereal crops when native grasses are dry and producing no new growth (Hill et al. 1988). Kangaroos are more commonly a problem in dry years (Hill et al. 1988). Like kangaroos, rabbits prefer soft, green, lush grass (Myers & Poole 1963) and select the most succulent and nutritious plants first (Croft et al. 2002).
The main rodent pest in Australian wheat crops is the house mouse causing average annual losses to Australian agricultural crops of US$10 million (ACIAR 2003). Rodents are opportunistic feeders and their diet can include seeds, the pith of stems and other plant materials (Caughley et al. 1998). Rodents may eat seeds, thus destroying them, at the seed source or they may hoard seed (AGRI-FACTS 2002). The average territory size of mice varies between breeding and non-breeding seasons, from 0.015 to 0.2 hectares respectively (Krebs et al. 1995). Mice have been noted as moving up to 300m in a day and numbers can build up and decline quickly (GRDC 2015c).
7.2.2 Invertebrate pests
The five most important invertebrate pests associated with wheat crops in Australia are redlegged earth mites (Halotydeus destructor), blue oat mites (Penthaleus spp.), locusts (various spp.), lucerne fleas (Sminthurus viridis) and slugs (various spp.) (Murray et al. 2013). These five pests cost the wheat industry over $100 million annually in losses and controls costs (Murray et al. 2013). Other invertebrates, such as snails, brown wheat mites (Petrobia latens), wheat curl mites (Aceria tosichella, a vector of wheat streak mosaic virus), various armyworms, cutworms and budworms (Lepidoptera: Noctuidae), beetles, earwigs and slaters infest wheat, but cause less overall losses to crops, however this analysis does not include the impact of invertebrate pests on grain quality (Murray et al. 2013). Rutherglen bug (Nysius vinitor Bergr) is also noted as an insect pest of wheat in Australia (Miller & Pike 2002; ALA 2010). Invertebrate pests are also found in the post-harvest grain. Lesser grain borers (Rhyzopertha dominica), rust-red flour beetle (Tribolium castaneum), rice weevils (Sitophilus oryzae), saw-tooth grain beetles (Oryzaephilus surinamensis), flat grain beetles (Cryptolestes ferrugineus) and book lice (order Psocoptera) all cause substantial economic losses when they attack stored wheat (GRDC 2013a; GRDC 2013b).
Wheat is economically the most important crop in Australia and wheat diseases can reduce the quantity and quality of grain yield (Table 6).
Table 6. Diseases of wheat reported in Australia (Murray & Brennan 2009).
Incidence and severity of disease varies across and within wheat growing regions in Australia. One example is Septorium nodorum blotch which casue major loss in the Western region, but has negligible effect in other regions, while cereal cyst nematode causes significant losses in the South Australian and Western Victorian zones, but effectively no loss in other zones of the Southern region (Murray & Brennan 2009). It should be noted that in comparing three studies published over a 21 year period, the diseases ranked highest in terms of their potential or present loss5 varied between studies (Table 7). This highlights the fact that disease and pathogen incidence, severity and/or costs can vary over time as new challenges arise or previous challenges are managed.
Table 7: Ranking of losses from major wheat pests and diseases in Australia.