Potato Review

www.potatoreview.com POTATO REVIEW MAY/JUNE 2022 13 PEST OUTLOOK the season, and themore aggressive A. solani, whichmakes a later appearance. This sequence has been verified by a project undertaken by Syngenta, with independent potato agronomists sampling suspected cases throughout the season and sending them for laboratory analysis by NIAB specialists to determine the species of Alternaria present. In the latest published results received from Syngenta, up until a sampling date of mid-July, 85 percent of the positive Alternaria infections identified were A. alternata, with just 15 percent as A. solani at low levels. However, after mid-July, 81 percent contained A. solani and less than 10 percent solely A. alternata. Interestingly, around 20 percent of samples tested were identified with mixed Alternaria populations present. No A. alternata was identified in the sampling after mid-August. Andrew said the pathogen can overwinter in soil and infected groundkeepers. Spores are then released in the spring, providing a primary inoculumsource which can spread by wind andwater. Alternating wet and hot, dry weather promotes spore production and therefore Alternaria pressure. Although initial infections can create similar symptoms, there are some important differences: with A alternata lesions aremore angular while A. solani causes interveinal yellowing and concentric ring symptoms, and lead to tubers with sunken lesions. AAlternata symptoms can bemistaken for magnesium (Mg) deficiency, because Mg deficiency causes chlorosis starting at the intercostal areas of the base leaflets. Crops under pressure frompotato cyst nematode (PCN) damagemay also be more susceptible to Alternata, he says. “There are a number of popular varieties which appear particularly susceptible to the pathogen, includingMelody, King Edward, Markies, Russet Burbank, Vivaldi, Agria andMaris Piper, so attention to detail about micronutrient availability – particularlymagnesium - is critical.” Alternaria has a long latent period of six to eight days between infection and symptoms appearing, so Andrewprefers a prophylactic approach to control. There are a number of treatment options available, which can help limit the damage to yields frompremature defoliation. “At themoment we still havemancozeb, which is used inmost late blight programmes, which is gives useful control. However if it is withdrawn from the toolbox, growersmay have to amend their strategies accordingly. “If environmental conditions promote incidence of the pathogen in susceptible varieties, there are a number of products such as Carial Star (difenoconazole +mandipropamid), Signum (boscalid and pyraclostrobin) and Caligula (carboxamide + triazolinthione) which offer growers different options.” “If you your crops are under stress at the rapid canopy growth phase, and the weather is conducive to Alternaria, protection is amust.” Location, location, location and aphids “Location, environment and season, alongside varietal susceptibility are key to aphid infestation and subsequent virus, so as you can only control two of these, make your choices carefully,” said Andrew. Aphid control is key to the potato sector because they are virus vectors and can devastate quality and yields. These viruses can be persistent (potato leaf roll virus) or non- persistent (including leaf mosaic virus, and potyviruses such as PVA, PVV and PVY, he explains. This is particularly important for growers planning on producing home-saved seed. “Aphids are attracted by soft, sappy leaves, and also the difference in colour between the brown of the soil and the green crop. Once they land, numbers can build very quickly.” With fewer options for chemical control for virus-vectoring aphids, getting the most from those that remain is crucial, says Andrew. This means that timing of applications has to be targeted, and he recommends subscribing to the Rothamsted aphid monitoring project to know when aphids which affect potatoes are likely to fly. He reports that colleagues have been exploring the benefits of planting flower strips on headlands and between crops to encourage predators to thrive. Trials are also underway in Scotland to discover the efficacy of foliar applications of oils and purging strips planted alongside crops. These strips provide a place where the stylet is cleaned before the aphid lands on the potato crop; the idea of using oils is their potential to clog the stylus. “The challenge with biological controls is that there have to be a certain number of aphids for the predators to go into the crop, so there is always going to be a risk to the crop. “Of course, seed crops should be grown in areas where virus is less common, but a change in regulation would be beneficial, so that in addition to inspection, seed crops would have to receive a virus test before sale.” Alternatria solani under the microscope. Photo: Syngenta Leaf infection spotting in the early stages. Photo: Syngenta