27 July 2001

Strob failure prompts a mildew strategy rethink

By Andrew Swallow

FIELD trials showing strobilurin mildew control is a thing of the past strengthen the argument for a one-hit combined mildew and eyespot treatment, says a leading Scottish agronomist.

In SAC Scottish Agronomy work near Dundee, Claire sown on Oct 26 last autumn has been treated with 10 different fungicide programmes.

Wherever a strobilurin without a specific mildewicide was used at T1, mildew has developed as if untreated, says business manager Huw Phillips. "Our conclusion is that the strob activity on mildew is nil. It has gone completely."

Including a mildewicide in the T1 mix, or at T0 with the first pgr, is now essential on susceptible varieties, he says. Quinoxyfen or a morpholine may do the job, but with nearly all wheat in Scotland also at risk from eyespot, cyprodinil (as in Unix) is a much better bet. Only wheat, after a break from cereals of two years or more can really be considered safe from eyespot infection, he says.

A wait and see approach to eyespot control is also ruled out as most is now rye strain. That rarely shows up in time to treat at T1, yet causes terrible damage.

"What is so frightening is the lodging eyespot causes. Crops go genuinely flat and dont get back up."

CSLs disease survey data supports his argument, he maintains. In 1999/2000 eyespot was second only to septoria in terms of losses (Arable, Mar 17). Yet few growers use a specific product against it, most relying on the partial control from Landmark (kresoxim-methyl + epoxiconazole), notes Mr Phillips.

Where crops are at high risk of eyespot – second wheats or very early drillings – Mr Phillips recommends a split T0 and T1 approach of 0.4kg/ha and 0.6kg/ha respectively of Unix. For moderate risk crops, anything other than wheats after a two year break in his opinion, a single 0.6-0.8kg/ha hit should be adequate.

That approach should also take care of mildew. But the most mildew prone varieties may justify a quinoxyfen follow-up at flag leaf.

Clearly, septoria also needs cleaning out at T1 and new growth protecting from infection. Using a triazole such as Opus (epoxiconazole) does that and fits in with resistance management strategies.

Strobilurin applications can then be targeted at flag-leaf and the ear-spray where their longer protection will be of most benefit. T3 applications particularly help maintain specific weight.

"On a 10t/ha crop the difference between 72kg/hl and 70kg/hl is worth £20/ha. At 68kg/hl it could be £60/ha, if the crop is not rejected."

That T3 strobilurins delay ripening is a misconception, adds Mr Phillips. "The ear will ripen, it is the straw that hangs on and that has more to do with the T1 and T2. Some people have lost the plot a little in blaming the T3 for the green straw." &#42

FUNGICIDE CHANGES

&#8226 Strob mildew control gone.

&#8226 Eyespot control needed too.

&#8226 Save strob to T2 and T3.

&#8226 Use Unix-based T1.

Compare and contrast… Claires leaf three is covered in mildew where a strob/triazole only T1 and T2 programme was used, but clean where Unix was included at T0 and T1, notes SAC Scottish Agronomys Huw Phillips.