Potato Review

www.potatoreview.com POTATO REVIEW NOVEMBER/DECEMBER 2019 53 FOCUS: Rhizoctonia solani Research suggests survival in the soil decreases over an 18-month period, but with such a vast host range, combined with seed transmission and infected volunteers, its proliferation is very di cult to supress during a cropping rotation. As a soil-borne disease, R. solani is a ected by soil conditions. Symptoms tend to be more common and severe in cool, moist soils at temperatures of 16C-23C. When soil temperature is above 25C, stem canker reduces in severity. e disease can also be more severe on dry, light soils or where pH is neutral to acid. It is very common in all UK soils, although it does have a greater preference for soils where there are pores or cracks, as mycelium can grow and infect hosts more readily. e most important part of the R. solani lifecycle is this asexual phase. It does have a sexual phase where basidiospores are produced, but its importance and impact in potato production is not well understood. When and how does initial infection occur? Whether R. solani is present in the soil or introduced on seed tubers with black scurf or less visible mycelium on the surface, initial infections take place shortly after planting. e fungus is stimulated by carbohydrates produced by the plant as it grows. ese same secretions guide hyphae to the surface of the shoots, stolons or stems, where they aggregate to form a structure known as an infection cushion. is secretes toxins and a series of cell wall-degrading enzymes which soften the plant tissue. e combination of pressure from the cushion and the cell wall-degrading enzymes allow the fungus to invade, macerate and kill the surrounding tissue. As R. solani is a saprophyte, is survives on dead material and aims to kill as much of the plant as possible. What are the symptoms? ere are two phases to the disease in potatoes. e rst is the stem and stolon phase, which is typically seen when infection occurs soon after planting. Symptoms include stolon pruning, where lesions develop on tips of stolons within the soil, or lesions on stems known as stem canker. is can cause uneven emergence and growth and as plants have fewer primary stolons, there is an increase in very small and very large tubers, so less marketable yield in the size brackets packers and processors are looking for. Optimal timing of inputs such as herbicides, fungicides and desiccants becomes more di cult. A further consequence of stolon infection at tuber initiation is a hard, necrotic patch at the rose end of the tuber. As the tuber bulks up, this is stretched and creates an unsightly scab, which resembles an elephant’s hide. In the worst cases, deep cracks can develop as soft tissue grows around the scarring, forming jigsaw-like necrotic pieces that can sometimes run down the length of the tuber. e nal evidence of R. solani infection can be seen towards the end of the season in the form of black scurf. It forms as black sclerotia on the surface of the tuber as the crop begins to senesce and can be scratched o easily with a ngernail. Potatoes free of black scurf may still be infected with R. solani, because hyphae barely visible to the naked eye can be growing across the surface. ese thin black lines can be picked up by using a x10 hand lens. How damaging is R. solani? As a rule of thumb, yield loss is often cited as being about 30%. However, where poor size distribution and skin blemishes are rife, rejections or downgrading of marketable yield can make losses much worse. Can you test for it? Seed tuber infection not visible to the naked eye can be tested using real-time PCR assay extraction of the pathogen’s DNA and is considered a very reliable way of assessing the need for seed treatments. More recently, a test for detection and quanti cation of soil inoculum has been developed in the UK and Australia using the same method and can help potato growers develop more robust management strategies. Commercial testing services for presence of diseases like R. solani on seed tubers are available at FERA and NIAB CUF in England and SASA in Scotland. How is R. solani managed? 1. Source clean seed Good seed health is important in minimising risk. Sourcing clean, certi ed seed is ➜ “Potatoes free of black scurf may still be infected with R. solani, because hyphae barely visible to the naked eye can be growing across the surface.” Rhizoctonia causing uneven emergence - copyright Blackthorn Arable Stem canker - copyright Blackthorn Arable

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