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Press
Release: Syngenta
Zoospore explosion threatens tuber blight infection
July 20, 2004: Cool conditions encourage a greater production of zoospores from blight lesions hidden deep in the potato crop canopy and stems. Even relatively low levels of foliar blight can still result in high levels of tuber blight; growers must target Shirlan applications to reduce the risk of infection.
Low levels of foliar blight in Great Britain has reduced pressure on potato crops this season, but cool summer temperatures are increasing the risk of tuber blight infection; chilly nights and dull damp conditions induce blight lesions to create a higher proportion of zoospores - the most infective stage for causing tuber blight.
SAC blight specialist, Dr Ruairidh Bain, warns that some crops could still be harbouring blight lesions deep in the canopy and at the base of stems, even if weather conditions have prevented spread on to clean upper leaves that have been well protected by fungicides. “Close inspection may reveal hidden lesions from early in the season that have, hitherto, remained dormant. Growers should remain vigilant for any signs of blight infection,” he says.
In cool conditions each lesion will produce millions of zoospores, which can be washed down onto the soil surface and right through to tubers. “Zoospores are far smaller than sporangia, so can move easily in the soil moisture, particularly in sandy soils. Tubers are still vulnerable to infection, through open lenticels and eyes, right up until the skins have set at harvest,” reports Dr Bain.
Rain running down the stem can gather and wash zoospores to the soil surface, which can then move through soil moisture to attack tubers. Just a few zoospore producing blight lesions could produce hot-spots of blighted tubers going into store.
Dr Bain advises growers to take extra care with foliar blight applications right up to desiccation, aiming for good spray penetration of the crop canopy, including targeting lower leaves and stems – an angled spray pattern can make a significant contribution. “Select a product with strong activity on zoospores and tuber infection. Add a product with translaminar activity, such as cymoxanil, to help achieve good leaf coverage and a curative effect, particularly if protectant fungicide spray intervals have been stretched,” he recommends.
Last season independent SAC trials of blight fungicides, for the BPC, revealed that Shirlan achieved the best results in minimising levels of tuber blight damage in low foliar blight conditions. Levels of tuber blight in the trial were reduced to less than 0.25% by Shirlan used continuously from the fourth application to the end of season; cymoxanil + mancozeb in the same scenario resulted in over 2% of blighted tubers. A third of tubers from the untreated crop were blighted.
Dr Bain’s views are echoed by Jon Ogborn of Syngenta, who urges growers to remain vigilant and maintain blight protection programmes on foliage. “Foliar blight could still increase dramatically in favourable conditions, undoing all the good achieved by growers so far this season.
“The spread of planting dates and canopy sizes does create additional problems for growers in selecting fungicides. With its inherently strong activity against zoospores Shirlan is the best option for completing the programme through to desiccation. Furthermore, tank-mixing Shirlan with Reglone for desiccation will give a single and significantly lower cost solution, compared to more expensive acid and a separate fungicide application.
“Syngenta recommends that Reglone should normally be used as a split dose, using an initial low dose to open up the canopy, prior to second main dose. Including Shirlan in the second Reglone application, making use of its short 5 day spray interval, will maintain protection until all green material is destroyed. With the canopy already opened up, this second application will give better protection on the lower leaves and soil surface, further enhanced by application with the angled Syngenta Potato Nozzle,” he adds.
For further information please contact:
Lesley Nichols
Syngenta Crop Protection
Tel: 01223 494143
Email: lesley.nichols@syngenta.com
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