Potato Review

www.potatoreview.com POTATO REVIEW MARCH/APRIL 2023 35 IRRIGATION He nds that the application of gypsum helps open up soil structure, and therefore can be useful in reducing common scab. Although hydrogen ions do not cause scab soil pHmay also be a factor, he says, noting that the e ects of lowering soil pHby an application of ammoniumsulphate in the ridge are inconsistent. is is down to the slow rate of the nitrogen breakdownwhich slightly increases acidity Sulphur pellets have been used to increase ridge acidity but owing to the speed at which the prills break down, the crop is already past the susceptible tuber initiation stage, says Andrew, “Another point about bringing down pH is that you reduce the plant’s ability to uptake nutrients, so you may be creating more problems than you solve,” he said. Manure can work as a useful soil conditioner, and may even do more than applying a biostimulant at planting. He gives the example that in Herefordshire many of the soils are low in calcium and high in magnesium, so applications of poultry manure which help correct not only calcium de ciency but also lower pH because of the nitrogen levels it contains. “Scab control If the crops are under stress at this time, they can also be more susceptible, so anything you do to reduce this may be useful. Keeping powdery scab at bay Powdery scab (Spongospora subterranea) is a fungal blemish disease of potatoes which often occurs when there are cool, wet growing conditions, or potato crops have received too excessive irrigation, says Andrew. e powdery scab pathogen can sometimes carry and transmit Spraing (potato mop top virus or PMTV). He notes that crops grown in heavy soils can be more susceptible to the pathogen, particularly when there is poor drainage or standing water. Infection can occur from diseased seed or it can be soil-borne, he adds noting that elds can carry inoculum if manure from stock fed with diseased tubers is applied, or if there is ine cient volunteer control after powdery- scab infested crops. “If you are renting land for growing potatoes, you should look always carefully at the eld history before taking it on,” said Andrew. Potato crops are most susceptible to the pathogen at tuber initiation. However it can take between four and eight weeks for symptoms to show. “Part of the problem is that the ‘powder’ is made up of spore balls (cystosori) that are released into the soil can survive in the soil for up to ten years. ese spores release motile (swimming) zoospores that penetrate root hairs and stolon epidermal cells, lenticels and tuber eyes. ey can also enter via wounds on the developing tubers.” Given the lack of e ective treatment, particularly in ware crops, avoiding diseased seed is key, and he emphasises that careful checking of seed is crucial, particularly as tolerances are quite high. “It can be confused with common scab so I prefer to use an eyeglass when doing his inspections as it can be di cult to distinguish between powdery and common scab. e shape of the lesion is slightly di erent. e powdery scab pathogen produces circular pustules on the tubers which can act as ‘volcanos’ after they rupture. “Another sign to look for is the size of the lesion – those made by powdery scab are often larger because they have merged together. Also, if you scratch the lesion with a ngernail, it leaves a greater depression on the surface than you get with common scab. “If you suspect powdery scab, it is always a good idea to send a sample to a diagnostic laboratory for con rmation.” Once in the soil, the zoospores can move through water across short distances to infect the new crop, and, if the mother tuber is diseased, they will move to the daughters. “ e zoospores infect the root hairs and stolons before the tubers develop, and can cause galls on the roots. e developing tuber produces corky tissue to halt the growth of the disease. “We have found that some varieties are more susceptible to others, so it is always a good idea to check susceptibility of the variety you intend to grow alongside the eld conditions and soil type to judge risks of growing in that particular eld.” Higher risk elds where seed potatoes are to be grown can be treated with an in-furrow treatment of uazinam at 3l/ha, but this then means that the fungicide cannot be used as part of a blight control programme. “If you are growing a ware crops, it is more di cult. A zinc-based treatment (such as zinc oxide or zinc sulphate) applied to the soil could be useful, but at present there are no such treatments approved for powdery scab.” Top tips for reducing the risk of powdery scab • Grow on well-drained land or improve drainage (if it is poor on own land) • Check varietal susceptibility • Use disease free seed • Maintain rotation of at least six years • Take soil samples to determine presence of the disease in the eld Drip irrigation is not suitable for rectifying a problem as not enough water can be applied quickly. Drip irrigation is not suitable for rectifying a problem as not enough water can be applied quickly.