Malcolm Smith
Potato growers relying on protectant fungicide activity alone for their mid-canopy blight sprays, could find that the more aggressive strains of potato blight catch them out this season, warns Masstock's technical support manager Malcolm Smith.
"Of all the treatments used in a season, two-thirds will be during the mid canopy stages of the crop, so it's worth investing in," he says, noting that protectant fungicides have historically dominated this phase of the crop's development.
"But employing protectants alone is now a very risky strategy," says Malcolm. "There has been a big change in the resistance status of blight. We have seen a significant increase in the A2 blight strain, and in particular during 2007 the shift has been to the A2 'Blue 13'
genotype that now dominates.
"What this means is that 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 demonstrably less effective; and it's even more important to optimise the fit of blight sprays into the programme."
Malcolm Smith recommends a seven day spray interval as standard, adjusting product to blight pressure rather than adjusting the interval, and advises incorporating chemistry at mid-canopy that will control - and not just protect against - blight.
Recommending alternating Valbon (benthiavalicarb + mancozeb) with protectants like Roxam (mancozeb + zoxamide) or Ranman (cyazofamid) mixed with cymoxanil for curative control, Malcolm notes that this strategy saves products with a greater degree of tuber activity like Infinito (fluopicolide + propamocarb) for later mid-season.
"Valbon combines both long-term protection against blight with 48 hours 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 notes.
Malcolm adds that unlike the rapid growth stage of the crop, there are a good number of blight treatments for the mid canopy slot, although he notes the importance of alternating fungicide treatments to guard against resistance developing.