Wheat Streak Mosaic Virus

Early infection of Wheat Streak Mosaic Virus (WSMV) is responsible for the greatest loss of wheat yield. Plants infected between tillering and first node often do not set any seed, while those infected between first node and booting typically have reduced seed size. Infected seedlings often die prematurely, while later infection cause progressively less damage with only slight losses expected when infections occur in spring growing periods.

Visual symptoms of WSMV can be confused with those of other cereal pathogens, nutritional deficiencies and environmental and chemical related damage. Also, plants with low levels of infection may show minor or no visual symptoms, with symptoms becoming more pronounced as the virus advances. Consequently, attempts to identify WSMV using visual symptoms alone may be difficult and require validation using alternative methods of assessment. Modern laboratory techniques provide rapid and reliable assessments of infection status, and are sensitive to both low and high levels of infection. Genetic techniques can also easily distinguish between WSMV and other cereal pathogens. Diagnostic services are available in most states and territories of Australia. Consult your local agronomist or contact your nearest crop testing facility for further information.

Visual Symptoms

Plants infected with wheat streak mosaic virus are typically characterised by leaf mottling and streaking. Light green streaks running parallel to the leaf veins are the first signs of WSMV infection. As the disease becomes more established the streaks turn yellow and develop into blotches, giving the leaf a green and yellow 'mosaic' like pattern. Tillers on affected plants tend to be less erect than those on uninfected plants. Affected plants can die prematurely or fail to grow, becoming stunted relative to healthy plants. Heads on infected plants can be sterile and contain no seed, or can contain small or shriveled grain. Affected plants often occur in patches or at crop boundaries closest to grasses that were growing when the crop emerged. In severe cases, the whole crop can be affected. Symptoms of wheat streak mosaic typically develop at temperatures above 10°C, so they are masked during winter.

Visible Symptoms of WSMV (Image: Gordon Murray)

Mite Infestation

The Wheat Curl Mite (WCM) is generally not considered a wheat pest, other than in its capacity to vector viruses. WCM generally cause little direct crop damage in the field, although research has shown that WCM can affect wheat yields when found in extremely high abundance. WCM are approximately 0.2mm in length and are extremely difficult to detect without magnification. In the field plants infested with WCM typically display leaf curling (rolling of leaf edges) and the occasional trapping of the flag leaf. Infested wheat spikes generally do not display symptoms.

Signs of WCM infestation (Image: Gordon Murray)