New blight strains need more control

Potato growers relying on protectant fungicide activity alone for mid-canopy blight sprays could find that more aggressive strains of potato blight catch them out this season, according to Masstock technical support manager Malcolm Smith.

Smith warned that, as the more virulent A2 'Blue 13' strain now dominates potato blights' population, a strategy that controls, rather than just protects against, blight will better tackle the disease.

He said: "Employing protectants alone is now a very risky strategy.

"The blight we are now dealing with is more aggressive. It can develop and complete its life cycle faster, and at both higher and lower temperatures. There is also greater opportunity for blight to infect crops between sprays - the older chemistry is less effective and it's more important to optimise the fit of blight sprays into the programme."

He recommended that growers should have a seven-day spray interval and adjust the products they use to the blight pressure - as opposed to adjusting the interval - and incorporate the kind of chemistry at mid-canopy that will control and protect against blight.

He suggested alternating Valbon (benthiavalicarb + mancozeb) with protectants like Roxam (mancozeb + zoxamide) or Ranman (cyazofamid) mixed with cymoxanil. This strategy saves products with a greater degree of tuber activity like Infinito (fluopicolide + propamocarb) for later mid-season.

"Valbon combines long-term protection against blight with 48-hour kick-back, delivering good control of recent infection. And as a product that contains both mancozeb and benthiavalicarb, the mix is good for resistance management."

He added that there are a good number of blight treatments for the mid-canopy slot, but stressed that it is still important to alternate fungicides to avert resistance.

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