24 May 1996

Take care with cereal spraying times – ADAS

By Robert Harris

TRADITIONAL fungicide timings will be unreliable on many cereal crops this season. Both wheat and barley carry high levels of disease even in dry areas, and growers must monitor crops closely.

So says Bill Clark, ADAS national cereal disease specialist at Boxworth, Cambs. "This is turning out to be one of the most difficult seasons in terms of crop management I can remember."

The wheat problem stems from the long, cold spell which appeared to hold crops back. In fact, although nodal development was delayed, leaves continued to grow, he explains.

Leaf three appeared early, often at first node detectable stage (GS31) rather than the usual second node detectable (GS32) when fungicides to control Septoria tritici are usually applied.

"In some cases, leaf three had been out for 10-14 days by the time it was sprayed, and insufficient chemical was applied to eradicate disease. That means leaf two and the flag leaf will be emerging through an infected canopy."

Although septoria may not yet be apparent, leaf two is likely to be heavily infected. Even rain-starved crops are at risk. "Plants have been sopping wet with dew for several hours a day for weeks, allowing disease to travel up from one leaf to the next."

Mr Clark urges growers in this situation to spray earlier than normal to eradicate disease. "Trials work shows delaying a spray by two weeks in these conditions can reduce yields by 15-20%," he notes.

Growers should check which crops are at risk by walking through them. "If leaves three and four are showing disease, then you will need to spray at about GS37 (flag leaf just visible) rather than GS39 (flag leaf emerged)."

Half rates or more of even modern chemistry like Opus (epoxiconazole) should be used to provide enough kick-back to protect leaf two, he adds.

Mildew is also causing trouble in wet areas and on drought- or manganese-stressed plants. "On forward crops, growers can probably wait until flag leaf before spraying. But if mildew is on the top three leaves in backward crops, it will need treating now."

Net blotch is worrying many barley growers, adds Mr Clark. Despite splitting early doses to try to eradicate it, active disease can be found on leaf two. It should be controlled now, and a repeat treatment made in about three weeks time to protect awns to avoid a potential yield loss of up to 10%, he advises.

Again, he warns against cutting rates too hard. "Net blotch is very difficult to control. Its going to be expensive – some people will end up applying half to one unit more fungicide than planned."


DISEASE CONTROL


&#8226 Beware traditional timings.

&#8226 Heavy disease in some wheats and barleys.

&#8226 Spray early to eradicate infection.

&#8226 Maintain rates for best control.