Destroying mite's 'green bridge' keeps virus at bay
By Steven Simpfendorfer, NSW Agriculture, Tamworth
While surveys have confirmed that the Wheat Streak Mosaic Virus (WSMV) discovered last year occurs in most states of Australia, there is as yet little evidence of observable yield loss. Furthermore, there are steps growers can take to minimise any effect of the disease.
The virus is transmitted by the wheat curl mite and is a common disease in a number of wheat-growing countries. The best way to manage the virus is to break the ‘green bridge’ that hosts the mite and the virus between seasons.
This means destroying all cereal volunteers (especially wheat) and grass weeds at least two weeks before sowing in autumn. This should include adjacent paddocks, borders of paddocks and around grain storages. This will remove the host plants on which the mites survive until the next crop.
Wheat curl mites colonise protected areas of the plant (under leaf sheaths, within curls on new leaves, and later in the glumes of heads) so spraying with miticides is generally not effective.
Plants infected with WSMV initially have light green streaks in the leaves which later develop into discontinuous yellow stripes, streaks or dashes running parallel to the leaf veins. Affected plants can also be stunted, depending on how early they were infected.
Tillers on infected plants also tend to be less erect than those on uninfected plants. Infected plants usually first occur along paddock edges or in patches near volunteer wheat plants, which reflects the mites’ feeding pattern. Symptoms develop at temperatures above 10°C, so may be masked during winter.
A sign that the wheat curl mite is feeding, especially in large numbers, is when leaves tend to remain erect, with their lateral margins rolled toward the upper midrib. This tight curling can trap new leaves and result in rolled, twisted plants. Heads may also be trapped in the boot.
The wheat curl mite (Aceria tosichella) is the only known carrier for WSMV. It is tiny (0.3mm) and white with a cigar-shaped body, and needs a microscope or strong magnifying glass to be seen. The mites can survive for up to 14 days away from hosts and have the potential to catch a ride on other insects and animals or other materials including clothing. Most infestations will occur close to the source.
For further information:
Dr Steven Simpfendorfer, NSW Agriculture, 02 6763 1261, firstname.lastname@example.org
To obtain a copy of the WSMV advice sheet, contact Ground Cover Direct, 1800 11 00 44, email@example.com
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