Potato Review

BP2019 14 POTATO REVIEW JANUARY/FEBRUARY 2020 Because of the statistics used, if we find no virus in the test then we have a 95% confidence that sure that virus levels are less than four%, representing a high health crop.” The requested sample size for testing is normally 100 tubers, and this, he said, is sufficient to indicate the virus health of the crop as long as the sample is representative. “Of course, the more you test, the higher the confidence level, but we have to remember that the test is an additional cost burden for the grower, so it needs to be cost effective. If the result is zero, it means there is a low level of virus in the crop, and therefore it is suitable to plant.” He went on to note that in his experience, the two tests are comparable, and any discrepancies are normally biological. “The grow-on test is cheaper but takes five to six weeks, whereas the molecular testing takes just five days, so choice depends on the time frame growers are working to at that moment.” Seeking alternatives after plant protection losses The recent withdrawals of crop protection products across the range from seed to store could lead to lower marketable yields, a fall in quality, higher waste and higher costs, according to potato agronomist Graham Tomalin of VCS Potatoes. Herbicides, following the loss of linuron and diquat, have increased input costs by £40/ha, and seed treatments could rise by £20/ha – going up to £60/ha for higher seed rate salad/seed crops. Desiccation may well double, increasing by between £30-£90/ha where flailing or repeated applications of PPO desiccants are required. Graham has been working with AHDB in the Strategic Potato Farm initiative, exploring the strengths and weaknesses of the available herbicides and their potential as a replacement for linuron. “There are potential substitutes, especially on lighter soils, but costs will increase. Knowledge of the weed spectrum will aid cost- effective control; but if you opt for metribuzin within combinations– which is the most cost- effective – beware of varietal susceptibility on lighter soil types.” Newly-approved Emerger (aclonifen) has the potential to be a useful partner when applied in combination, and Praxim (metobromuron) has useful control of a similar weed spectrum to Linuron. Applications of three residuals at lower rates could be helpful. Alternatives for desiccation are looking at thermal desiccation, electrocution, haulm pulling, and the use of Saltex, with results yet to be published. “We need more research into the use of Saltex with approved desiccants, but also we need to investigate how to improve coverage and performance of remaining actives, so we need to look at nozzle choice, forward speed and adjuvants,” said Graham. “Gaining approval of Saltex as a commodity substance which we can apply to potatoes is essential.” CIPC (chlorpropham) non-renewal Changes within the potato value chain are imminent. This is because the last day that CIPC can be used is October 8th, 2020. After the last use up date, some months later, there will be a drop in the maximum residue level (MRL), said Certis European Development Manager Potato Portfolio, Tjaart Hofman. Graham Tomalin