Potato Review

12 POTATO REVIEW MAY/JUNE 2022 PEST OUTLOOK he prefers not to stretch intervals as it can be a ‘nightmare’ if the weather changes and spray operators to not have time to get round all the vulnerable crops. “If the Hutton Criteria indicates that pressure is very high, you need to opt for the more robust blight sprays, as otherwise you could be putting your crop at risk of serious yield losses which could wipe out profits.” Application technique is worth considering he adds, drawing attention to Syngenta’s new designs of 90 percent drift reduction nozzles which are suitable for of blight foliar treatments and desiccation. “These can improve spray penetration throughout the crop canopy by utilising larger, higher energy spray droplets. The angled 3D 90 percent drift reduction nozzles improve consistency of coverage on lower leaves and stems, which is important for effective blight prevention.” The spray pattern produced can also significantly reduce the risk of drift that ensure more of the product applied stays in the field target, he advises, noting that with finer droplets wind gusts can shift the spray pattern and result in patches of the crop receiving less application and therefore lower protection from infection. Andrew likes to ‘mix and match’ actives according to conditions at the time the Hutton Criteria is triggered, using different fungicide groups to avoid putting a single active under pressure. For example, he advises reaching for Zorvec (oxathiapiprolin) in high blight pressure situations because of its ten-day persistence. This offers much-needed flexibility when the spray operator needs to stretch the time limits for getting round all the potato fields. When he uses it in irrigated crops, Andrew prefers to irrigate the day before application so the crop can benefit frommaximumprotection time. “Although Zorvec is one of the more expensive options, it is very useful at mid- season when the pressure is really high. We tend to use it in sequence with Ranman Top, Infinito or Shirlan either side.” Another good-value fungicide is Revus (mandipropamid), and Andrew finds it works particularly well as a second spray, particularly when it is mixed with cymoxanil. “We may use it in up to four or five rounds across the season, depending on the weather conditions at the time.” The future of one of the most cost-effective actives, mancozeb, is now uncertain, and as manufacturers do not want to be caught out with left-over stock, the availability of Curzate (mancozeb + cymoxanil) is limited. “I find this a useful mix because it also has activity against Alternaria, so we are mixing out own and including it in our blight programmes.” Combining blight spraying with micronutrient or herbicide applications is a useful strategy, saving fuel and labour costs. Andrew has also found. For example, Ranman Top works well in a mix with herbicides such as Titus (rimsulfuron ), because of its good wetting agents, he said. “Growers should always follow manufacturer’s and Fungicide Resistance Action Group (FRAG) guidelines to protect the lifespan of all active ingredients.” Late season blight control Reflecting on the limited actives available for late blight, Andrew notes that growers still have some effective options for foliar blight, although there are fewer which offer protection against tuber blight. Desiccation for harvesting has become more challenging since the withdrawal of the desiccant diquat because the currently available actives are slower working so the haulm takes longer to die off. As a result, the crop remains green for longer, and therefore susceptible to blight, said Andrew. “It is crucial for blight spraying to continue until the crop is completely dead, or growers risk taking tuber blight into the store, which can cause devastate quality and causes significant losses.” He finds it useful to hold back for a couple of days after the first desiccation to give time for the outer leaves to dry and be taken away by the wind, to make it easier for the stem to be coated with desiccant and blight spray. “Infinito ( fluopicolide + propamocarb) has good activity for tuber blight and it fits in well with other actives. However, growers should avoid using it when the weather is particularly wet and drizzly.” “Ranman Top also controls zoospores which cause tuber blight so I often find it useful to apply it for the last two sprays before potatoes are loaded into store. One of its strengths of is rainfastness, which can be particularly useful later in the season and coming up to harvest.” Top tips for late blight control • Maintain recommended spray intervals • Attention to detail on the correct water volume; initial doses should be at 200l/ha but as the canopy grows, this should be increased • Ensure maximum speed is 12km/h; any increase on this will affect spray deposition and therefore efficacy • Pay careful attention to going round obstacles in the field such as trees and poles, and also boom speed when turning on the headland as parts of the crop may be left unsprayed • Control groundkeepers • Be aware of proximity to allotments and vegetable gardens as they can be a source of inoculum Keeping ahead of Alternaria Crops under stress – particularly when they are short of key nutrients - become more susceptible to Alternaria, says Andrew. Potato crops infected with Alternaria (also known as early blight) can create AND be subject to rapid loss of green area, with the subsequent effect on yields and tuber size. There are two strains of the Alternaria pathogen that affect British potato crops; A. alternata which infects crops early in