Wheat streak mosaic virus
Wheat streak mosaic virus (WSMV) was found in commercial wheat crops in Queensland during the 2003 season.
However, its geographic distribution throughout the region is still unknown as are the potential impacts from the virus on yield.
Wheat plants infected with WSMV have discontinuous yellow-streaked and mottled leaves.
Plants infected before tillering are often stunted, discoloured and rosetted.
The host range of WSMV is wheat, barley, oats, rye, maize and some grass species including brome grass, barley grass, ryegrass, phalaris and liver seed grass.
In Australia, the virus has been found on wheat, Setaria and Urochloa.
WSMV is unable to spread without the aid of a vector to transmit it from a diseased plant to a healthy plant. The wheat curl mite (Aceria tosichella) is the vector for WSMV. Wheat curl mites are microscopic, about 0.2 mm long and can only be seen with magnification.
The mite consumes plant sap from a diseased plant and the virus remains alive in the mite´s mouthparts and is transmitted to other plants as the mite feeds and moves between plants. Wheat curl mites cannot survive for long periods off living plant material.
WSMV cannot survive outside of a host plant or the vector, the wheat curl mite. Therefore, during the period between summer harvest and planting of the next wheat crop, WSMV persists in the ‘green bridge’ of volunteer wheat plants and other grasses.
Disease management should involve eliminating the ‘green bridge’ by controlling:
- wheat volunteers between crops
- grass hosts growing on the borders of areas to be sown to wheat
- grasses in fallows.
This means green plant material should be dead, as a minimum, two weeks before sowing the next wheat crop.
Nematodes: root-lesion nematode (RLN)
Root-lesion nematodes (RLN, Pratylenchus thornei and P. neglectus) are migratory root endoparasites that are widely distributed in the wheat-growing regions of Australia and can reduce grain yield by up to 50% in many current wheat varieties.
P. thornei is the most damaging species and occurs commonly in the northern grain region. P. neglectus occurs less frequently than P. thornei but is still quite common. A third nematode, Merlinius brevidens (stunt nematode), is the most commonly identified plant-parasitic species but is not thought to be damaging to wheat.
Two species of root-lesion nematodes (Pratylenchus thornei and P. neglectus) occur in the soils of southern Queensland and northern New South Wales. In a survey of soil samples from 596 paddocks in this region, 42% had P. thornei alone, 27% had both P. thornei and P. neglectus, 5% had P. neglectus alone, while 26% had neither species.
These nematodes can severely affect wheat yields and the term ´wheat sickness´ is commonly used to describe deteriorating yields in a succession of wheat crops. Some other crop species are also hosts of root-lesion nematodes and care should be taken in sowing wheat after crops such as black gram and chickpeas in nematode-infested paddocks.
Nematodes penetrate the plant root, digesting the cells’ contents and laying eggs within the roots. High populations develop quickly following planting, so that the root systems become inefficient in absorbing water and nutrients.
Crops are patchy, show lower leaf yellowing and appear drought affected and nutrient deficient. As peak nematode numbers often occur at depths in the soil of 30-60 cm, the condition of the plant deteriorates as the roots go deeper.
Grain yield can be severely reduced in susceptible varieties.
Nematodes can spread through a district in surface water (e.g. floodwater) and can be moved from one area to another in soil adhering to vehicles and machinery. They have the ability to quickly build up populations in the roots of susceptible crops and remain in the soil during fallow. As a result the yield of following wheat crops can be significantly reduced.
There are no wheat varieties fully resistant to root-lesion nematodes. Nematodes can be managed through crop rotation and varietal selection. The wheat varieties Sunvale, Baxter, Hartog, Sunstate and Kennedy are tolerant to P. thornei and will yield well in nematode-infested soil.
Most barley varieties and the durum wheat, Yallaroi, are also tolerant to P. thornei. The wheat varieties with tolerance to P. thornei are not necessarily tolerant to P. neglectus and care must be taken to diagnose which species are present in a paddock.
Root-lesion nematodes can take half your yield. The Department of Employment, Economic Development and Innovation’s (DEEDI) Leslie Research Centre can test for and diagnose their presence for $169.40/paddock (inc. GST).
Test your farm for nematodes.
Crop rotation with resistant crops such as grain sorghum, millet, sunflower and canary will reduce the numbers of nematodes in the soil to a level where susceptible wheat varieties can be grown but will not eliminate them completely.
It is important to know whether nematodes are on your farm and if so, which species are present. This is important because varietal tolerance information for P. thornei does not hold true for P. neglectus, as they are distinct species.
Proper species identification can help minimise losses that arise from planting intolerant varieties in nematode-infested land. The nematode status of any paddock can be determined by laboratory analysis of appropriately sampled soil at a cost of $169.40/paddock (inc. GST).
For further information contact:
Soil Microbiology Section
Leslie Research Centre
PO Box 2282
Toowoomba Qld 4350
13 Holberton Street
Toowoomba Qld 4350
Phone: 07 4639 8888
Fax: 07 4639 8800
