b'TATER TALES Common Scab, a Problemwithout a Solution?Common scab continues to cause concern for potato growers.COMMON SCAB, caused mainly by the seed and soil-borne bacterium Streptomyces scabies, continues to be a global concern for potato producers, especially for fresh market growers. Common scab does not reduce yield, but the corky skin lesions render tubers unmarketable.Buyers demand a fresh market crop with smooth skin and no blemishesin other words, a perfect potato. Processing potatoes with surface scab are marketable EUGENIA BANKS,because tubers are peeled before frying. However, potato specialistpotatoes with pitted scab are usually rejected because with the Ontario Potato Board pitting leaves cavities in the tubers.The incidence and severity of scab varies from year-to-year and from field-to-field. Management practices to reduce scab have given inconsistent results. There are A cocktail of common scab on potatoes.several factors which explain why common scab hasPHOTO: EUGENIA BANKSbecome such a problem:Spread of common scab on healthy seedPotatobeyonda practice which reduces scab in some fields growers avoid planting seed with visible symptoms ofbut not in others. common scab to avoid introducing the bacterium to Chloropicrin fumigation has reduced common their farm. However, seed which looks healthy can carryscab very effectively in some fields, but not in others. the pathogen in the lenticels or on the tuber skin. Research is needed to determine why this happens. Indefinite survival of S. scabies in soil as aAre there any chances to ever get an effective control saprophyte in the absence of potatoes.method for common scab?According to Vikram Bisht,Soil conditionsLow organic matter and soilpotato plant pathologist with the Manitoba government, compaction are factors which reduce soil health anda knowledge of local Streptomyces species/strains may create an environment that is favourable for scab.prove to be essential. I agree with him.Warmer, drier seasons more common now due toA 2019 soil survey showed Streptomyces stelliscabiei climate change favour scab development. was the predominant species found in Ontario, not S.Genetic variabilitySome Streptomyces strains arescabies. extremely aggressive and can attack resistant varieties. The conditions under which the disease occurs isAny potential treatment would need to be evenlyanother factor which Vikram considers important. The distributed in two million lbs. of soil, which is the plowtype of scab (raised, superficial or pitted) depends layer or top six inches of soil. (Robert Coffins commenton the potato variety, the environment, and the soil at the 2007 International Conference on Common Scab) microbial community. It has proven to be difficult to Its not surprising common scab has been calledidentify the conditions which reduce common scab a problem without a solution. Resistant varieties areconsistently. He suspects a biological seed treatment effective, but there are not always resistant varietieswould be most efficient and cost effective, but a one among those requested by the markets. Growerssize fit all approach is unlikely to work. continue to search for management practices to reduceI tell growers to do their own researchtreat a scab.scabby section of a field with any method they think Charles Emre, a farmer near Simcoe, Ont. has beenmight control scab. Its a practical way of testing evaluating mustards and other rotational crops forscab management practices in their own fields on several years. He says his rotational crop mixtures dothe varieties they grow. This increases the chance of not control scab, but they are keeping it at bay. Emrefinding an effective control method or a combination of also keeps adequate soil moisture at tuber initiation andmethods to control scab. 36SPUDSMART.COM Summer 2021'