Potato Review

20 POTATO REVIEW SEPTEMBER/OCTOBER 2021 SEED CARE HOW TO CONDUCT A POTATO SEED TREATMENT RISK ASSESSMENT Certis has teamed up with SAC Consulting to produce the new Seed Potato Storage Treatments Guide, which provides a detailed insight into best practice use of products that control storage diseases. Kyran Maloney said that a risk assessment for a particular crop will start before a tuber is planted, with the farm and field history and the variety’s characteristics underpinning the whole process. Input stocks should also be assessed for infection and if any signs of the key storage diseases are uncovered, then he urges a rethink on how that stock is used. “Following planting, climatic conditions through the season should be considered and any signs of disease during yield digs should feed into risk assessments too,” Kyran said. “How long the tubers have been in the ground and wet conditions at harvest are also key factors for diseases like gangrene, silver scurf and skin spot, whereas incomplete skin set and dry conditions and damage at lifting increase risk of dry rot infection.” While many seed growers are experienced enough to know what to look for in input stocks and the growing crop, plant clinics run by SASA, FERA and SAC can help identify problems in the laboratory if there is any uncertainty. If bacterial rots get in to a stock, it can be difficult to identify underlying disease symptoms, while gangrene and dry rot can sometimes be confused. Similarly, watery wound rot can get pustules around the decaying tissue and leading to confusion with dry rot. Silver scurf and advanced black dot symptoms are similar to the naked eye and it is important to establish which one it is to decide on treatment. Black dot cannot be controlled by storage treatments, but Gavel is effective against silver scurf. “If there is any doubt, seek a professional opinion,” said Kyran. A full version of SAC’s Seed Potato Storage Treatments Guide can be found at www. certiseurope.co.uk/tubercare , including detailed advice on seed tuber treatment and decision support flow charts. only treat seed with a fungicide, allowing oversize or “tops” to be sold as ware. The survey didn’t ask when the “at grading” applications were made and Kyran is concerned they may be later than optimal. He said that storage diseases such as dry rot, gangrene, skin spot and silver scurf cannot be dealt with once tubers are infected as seed treatment fungicides are solely protectant, not eradicant. This means it’s important to treat as soon after harvest as possible to maximise protection during the handling and storage process. “Depending on how each business is set up, grading will take place at different times. Some will be selling into the export market, so grade and treat from October onwards. “However, where seed producers are grading to order and not until after Christmas we feel that is too late. It will help prevent further spread, but won’t achieve the best results.” Too late? Despite the popularity of treating at harvest, Kyran said this method isn’t without its problems, particularly as the majority of these applications are through bespoke equipment. Nozzle placement – which varies depending on harvester and applicator set up – may be sub-optimal and environmental conditions also have a huge influence on coverage, with windy weather potentially blowing spray solution off target and risking operator exposure. “Some growers are compensating for this by using high water volumes, but that has implications later on when tubers are loaded into store. Free moisture is one of the critical risk factors for disease, particularly silver scurf,” said Kyran. Furthermore, if soil conditions are wet at harvest and tubers are muddy, spray solution won’t make it in to nicks, scrapes and wounds where it needs to be to prevent disease ingress. The operator may also struggle to maintain an even flow of tubers through the spray solution, making under or over application an issue. Kyran said that in an ideal world, growers should set up to split grade into store, but acknowledges that the convenience of applying at harvest is a discouragement from doing so. Dual purpose crops Certis Commercial Development Manager Neil Beadle warns that the label on its seed treatment Gavel stipulates that any tubers treated with the fungicide cannot be sold for ware or into the animal feed chain. He adds that there are now similar restrictions on insecticides used to reduce virus transmission that are also hindering the ability of growers to market outgrades. One example is Insyst (acetamiprid), which can only be applied once to a ware crop, but twice to seed, so seed growers applying robust aphicide programmes each season should not be selling tops for ware either. Frontier potato agronomist and seed treatment specialist Darren Wonnacott “Free moisture is one of the critical risk factors for disease, particularly silver scurf.” SAC’s Kyran Maloney, Thin-skinned varieties and tubers harvested in warm and dry conditions are most at risk from dry rot, which caused by several Fusarium species. Photo: Blackthorn Arable