Potato Review

COVER CROPS Andrew emphasises that biofumigants need to be treated as cash crops with a difference and he recommends using either N or chicken manure so the crop grows as big and lush as possible, with a large canopy which will come into flower in the spring. Destruction should take place at the onset of flowering and not be done later than 5 per cent flowering. “This is the time when it has the highest level of glucosinolates, and so it will provide the most effective biofumigant properties. But you need to treat it like you would a chemical application.” When it comes to destruction and incorporation, timing is key to making sure the compounds toxic to potato cyst nematodes are incorporated into the soil before they volatilise, which happens in as little as two hours. “Flailing and incorporation should be done as with one pass, using a flail on the front of the tractor with a plough/rotavator/spader behind. Of course, maceration technique and timing are key to ensuring you get the maximum damage possible to the crop through both bruising and chopping which stimulate the release of gas.” “Biofumigants are useful for getting on top of a PCN problem, but it is only one part of the picture, and is not a one-pronged solution.” How does a biofumigant work? Maceration and bruising a brassica crop, such as Indian mustard, releases products from the plant’s metabolism, which are known as glucosinolates. These are then broken down by enzymes in the presence of water, to produce isothiocyanates (ITC) and volatile sulphur compounds, which have a biocidal effect. “Cover crops are not a quick fix and in heavy soils it can take more than one cover crop in the rotation before the benefits can really be assessed.” WAKE-UP CALL ON PCN TESTING Prior to renting, soil samples should be done well ahead – about 10 to 11 months - to give you time to put a cover crop in before the potatoes are planted, says Andrew, adding that this should not be a last-minute task. Andrew also warns growers to avoid skimping on soil sampling when testing for PCN, particularly if they are planning on renting a field for growing potatoes or you suspect a field may be infested, warns Andrew. This is because the pest occurs in hotspots, so it can easily be missed, and although in the West, PCN levels tend to be lower than in the Eastern counties of England, the pest is present in some fields. If it is found to be present, integrated control, which will include the application of nematicides plus an Indian mustard biofumigant high in glucosinolates, will be necessary, and this implies costs of getting the field into conditions for growing a potato crop. “There are only two nematicides, Nemathorin ( fosthiazate) and Velum Prime ( fluopyram), which have approval for use, so we may end up using all of these options,” said Andrew. ADVANTAGES OF COVER CROPS INCLUDE: • Helping to suppress weeds with their biomass • Improving drainage and reducing runoff • Mopping up excess N and slowly releasing nutrients ready for the next crop • Helping build soil organicmatter and creating better conditions for beneficial soil micro-organisms www.potatoreview.com POTATO REVIEW NOVEMBER/DECEMBER 2022 25