7 March 1997

Net blotch control slipping until new fungicides arrive

By Robert Harris

BARLEY net blotch is becoming less sensitive to the one group of fungicides that can control it. Continuous cropping and consecutive chemical applications are the main causes.

New fungicides with different modes of action to control the disease are expected soon and should be incorporated as quickly as possible to keep it in check. Otherwise a gradual loss of control is likely, says Tom Locke, ADAS plant pathologist at Rosemaund, near Hereford.

The findings follow an HGCA-funded ADAS survey of 30 infected winter barley crops sampled at random in May and June last year. It was carried out to test the sensitivity of the disease to so-called DMI fungicides, which include several triazoles and prochloraz (eg Sportak 45).

"Net blotch became a problem in the late 1970s, due to the increase in the winter barley area and introduction of some particularly sensitive varieties," says Dr Locke. "Bayleton was also introduced to the market as a cure-all. But it had no effect on net blotch, so by removing other competitive fungal diseases it may have allowed net blotch to colonise leaves more easily."

Yield drops of 30-40% were quite common, costing growers £33m in 1981 alone, he adds. The advent of DMI fungicides appeared to solve the problem. "But in France, the efficacy of some of those fungicides for the control of barley net blotch has declined in the last few years. Fungicide resistance is suspected," says Dr Locke.

A shift in sensitivity was also noted in the UK between 1988 and 1991, along with a few recent reports of control problems. "Unfortunately, control of net blotch is totally reliant at present upon DMI fungicides. Any widespread decrease in sensitivity would have very serious implications for barley growers," says Dr Locke.

Spraying to control net blotch requires careful chemical choice to avoid exacerbating resistance, says Tom Locke. .


NET BLOTCH PROBLEMS


&#8226 Disease becoming less sensitive to DMI (triazole) fungicides.

&#8226 Continuous cropping and repeated DMI use exacerbates problem.

&#8226 Full-blown resistance a potential problem.

&#8226 Yield losses of 30-40%.

&#8226 Use new non-DMI products in spray programmes when marketed.