Growers are being strongly advised to test their seed tubers for evidence of Rhizoctonia solani prior to planting this spring to avoid the financial penalties caused by black scurf and stem canker.
Eye plug testing and visual examination are essential to establish the presence of the disease and to provide the basis on which an informed decision can be made on whether to treat seed tubers. This view is widespread among potato specialists, including David Kenyon at NIAB, who has already seen samples in which there are worrying levels of the disease.
"Due to the generally cool, wet conditions last summer, this is unlikely to be a particularly high Rhizoctonia year, but there have been samples with big problems," he reports. " In the absence of any scientifically-based economic threshold for treatment, growers should be going in with as clean a tuber as possible to avoid introducing the disease just at the point where you don't want it."
Testing of samples in Scotland by SAC this winter has revealed that more than 40 per cent had evidence of black scurf. Stuart Wale there regards treating the seed tubers as an insurance policy, especially if the crop is destined for packing, for example. "It's slightly arbitrary," he explains, "but a general guide is that if one in 100 tubers are showing black scurf, then it will probably pay to treat them."
This approach is endorsed by Denis Walsh, senior regional agronomist with Greenvale, who says varietal susceptibility and end markets should be taken into account when making the decision. "Conditions at planting time will affect the risk, too," he points out. "Early growers, or those with a strict growing programme, may be tempted to begin planting before the ground is right and this could increase the disease risk.
"If scurf is seen on 1-2 per cent of seed, then treatment is likely to be justified.
Bayer CropScience technical manager Nigel Adam says the company fully supports the principle of growers having their seed tubers tested before planting. "Frequently, Rhizoctonia symptoms are sub-clinical and cannot be detected by eye," he explains. "Planting such seed can lead to financial penalties if harvested tubers are rejected for the pre-pack market due to poor skin quality.
"Conversely, testing will reveal whether the cost of chemical treatment is unwarranted."
However, Mr Adam maintains that even with very low levels of infection, the Monceren product range produces cost effective returns in several ways. "Both are highly effective in controlling the development of black scurf symptoms on the daughter tubers – confirmed again in 2004 trials," he continues, "and their use results in more even tuber shape and size by reducing the stolon pruning and stem canker effects of Rhizoctonia.
"In addition, the use of Monceren seed treatments prevents the contamination of healthy soil caused by using untreated infected seed stocks. Preventing disease symptoms caused by infected soil is a good deal more difficult and expensive than preventing disease originating from infected seed.
"But most importantly," Mr Adam concludes, "growers should be careful not to underestimate the risk this disease poses and the financial implications of failing to take preventative action."
