Cereal virus serious threat—M.A.F.

Press, 5 July 1985, Page 10

Cereal virus serious threat —M.A.F.

Barley yellow dwarf virus disease is potentially a serious problem in autumnsown wheat and barley crops in Canterbury because of the mild winter, according to Drs Nizar Mohamed, pathologist, and Barney Stephenson, entomologist at the Plant Protection Centre of the M.A.F. at Lincoln.

BYDV, an aphid trasmitted disease of cereals and grasses, is widespread throughout the South Island. In most years, this disease is not a major problem in autumn sown cereals as aphid flights (which introduce BYDV into crops) are over before crops emerge. However, when the autumn

and early winter are mild, aphid flights continue until late June or early July when most wheat and barley crops have emerged and are therefore susceptible to infection by BYDV. This year, two factors have combined to make

barley yellow dwarf disease a potentially serious problem: • Mild weather during May and June has meant that in many areas, flights of cereal aphids have continued into July. • There has been an increasing tendency to plant cereal crops, and especially wheat, in late April or early May so that emerging crops have been particularly susceptible to virus infection during June and early July. BYDV has therefore been introduced into many emerging cereal crops and there is a danger that further spread of the disease could occur now, if the winter continues to be mild, or spread could occur in early spring as temperatures begin to rise. In barley crops BYDV disease appears as a bright yellow discolouration at the leaf tip and margins and moves rapidly down the

whole leaf. This symptom can be confused with nitrogen deficiency. But BYDV infected plants may be found scattered throughout the crop, while nitrogen deficiency generally appears more uniform. In wheat the leaves turn yellow and sometimes show a little reddening; and occasionally the edges of wheat leaves may be serrated. The strains of BYDV present in New Zealand are transmitted mainly by the cereal aphid, Rhopalosiphum padi. This aphid usually introduces the virus into a crop from either infected volunteer plants or infected pastures. Autumn flights of winged aphids introduce the disease into an emerging cereal crop by feeding on scattered plants. Secondary spread of virus then occurs as wingless aphids multiply and move to adjacent plants. This secondary spread can be

particularly damaging when it occurs in late autumn and early spring. Autumn-sown plants that become infected later in spring do not suffer appreciable losses although they can serve as reservoirs of infection for late spring sown cereals, especially barley.

In most years, losses due to BYDV infection are of the order of one to three per cent but in years when there is an epidemic of the disease, yield losses can go as high as 20 or 30 per cent according to the scientists.

The traditional method of controlling BYDV has been to delay sowing autumn cereals so that they emerge after aphid flights have stopped. This year aphid flights persisted longer as a consequence of the mild weather and later sowing may not be a very effective method of avoiding the disease.

The Plant Protection Centre has recommended that growers check crops now for winged aphids and if any are found, apply a registered insecticide. Aphids have been found in significant numbers on newly emerged crops.

Growers should also try and prevent further secondary spread of virus by checking crops regularly for aphids, particularly if the weather remains mild, and apply insecticide if any wingless aphids are found. This application may coincide with herbicide application in early spring, in which case tank mixing will reduce costs. Note that infection with BYDV is not significant if it occurs after tillering is completed. Cereal aphids should be controlled during early to mid-tillering, al Feekes growth stages twc and three.