14 June 2002

Downy mildew alert for spring bean growers

By Andrew Swallow

SPRING bean growers should check crops for symptoms of downy mildew and be ready to apply fungicides if thresholds are breached. Bruchid beetle may also warrant attention.

Recent warm and humid conditions have been ideal for the disease, highlighting the value of varietal resistance, warned PGRO technical director, Anthony Biddle, at the organisations Thornhaugh Open Day earlier this week.

"Some varieties now have good field tolerance, for example Compass. That is standing up to the disease pressure extremely well. It may not earn a premium for export, but it will give a good yield and could save the need to spray," he says.

Visitors to the Open Day could see that for themselves, with susceptible variety Victor approaching treatment thresholds while Compass was completely clean next to it.

"The fungicides are quite expensive so have to be deployed quite carefully. That is why we have a relatively high threshold – 20% of plants showing symptoms on the upper leaves."

Folio (chlorothalonil + metalaxyl) is the only approved product and for control 2 litres/ha is needed, he stresses.

"We find you need the full rate if you are going to stop the disease, it can be quite devastating. Yield losses from severe infections are typically 25-30%," he notes.

Sprays for downy mildew may be combined with bruchid beetle controls, a spray of Decis (deltamethrin) or Hallmark (lambda-cyhalothrin) at six flowering trusses and again ten days later being recommended if crops are targeted at seed or export markets.

DOWNY MILDEW ALERT

&#8226 Ideal disease conditions

&#8226 Spray at 20% infection on top leaves

&#8226 Up to 25-30% yield loss

&#8226 New warning period soon?