b'TATER TALES Why Control of Common Scab Has Been Unachievable So FarCOMMON SCAB IS one of the oldest bacterial diseases of potatoes. It has been known for more than 100 years and still limits potato production worldwide because of the lack of consistent control. Scab doesnt affect yields, but the corky lesions on the skin, both superficial and pitted, will reduce the quality and marketability of the crop.Common scab is caused by Streptomyces bacteria in the soil. Most species of Streptomyces are non-pathogenic but there are a few that have pathogens. Streptomyces scabies EUGENIA BANKS,(syn. Streptomyces scabiei) is a pathogenic species that has potato specialist with the Ontariobeen investigated more extensively than other common Potato Board scab species. Its a gram-positive, filamentous bacterium that produces both mycelium and spores. It can survive indefinitely in the soil as a saprophyte or on other hosts such as beets, turnips, carrots, parsnips, radishes and A new promising variety but it is susceptible to common rutabagas. The optimum soil pH range for Streptomycesscab.PHOTO: EUGENIA BANKSis 5.5 to 7.5.S. scabies penetrates young tubers through lenticels,structure, pH, organic matter, microbial flora, moisture, stomates or bruises. The small tubers that form at tuberaffect the incidence and severity of common scab. For this initiation are the most susceptible stage. Lesions expand asreason, the management practices that work to reduce the potatoes mature, and mature potatoes have a thickercommon scab in one field does not work in other fields. skin than small tubers and are resistant to infection.III.Any soil treatment must be mixed with two million Keeping the soil moisture at or near field capacity for fourkilograms of soil per hectare. Its difficult to control to five weeks beginning when tubers are forming reducesdiseases by amending the soil. The plow layer, usually the incidence of common scab. The only reliable controlconsidered to be to a depth of 15 cm, means treating two method of common scab is the use of resistant varieties.million kgs of soil per hectare. Its not a simple task to mix Unfortunately, the number of resistant varieties is limited. any product uniformly into that much soil. Why control of common scab has been unachievableIV. Using healthy seed is no guarantee that the daughter for so many years? The factors that are responsible for thistubers will be healthy.Even seed tubers that appear to problem are: be healthy may carry spores of S. scabies in lenticels. This I. The rapid development of new pathogenicexplains how scab can be introduced into a field that was Streptomyces spp creates genetic diversity. Pathogenic S.free of scab. scabies produce a toxin, Thaxtomin A, that inhibits theVariety trials conducted in fields with heavy common production of cellulose in the infected tissues of growingscab infestation provide invaluable information for potatoes. This lack of cellulose results in lesions. The genegrowers. Susceptible new varieties with traits requested responsible for the synthesis of Thaxtomin A is found on aby buyers, should be grown only in healthy fields. Robert pathogenic island, a bit of DNA that also carries the genesCoffin mentioned to me that there have been reports necessary for infection and virulence. Pathogenic islandsof heavy scab infestation of potatoes produced in fields can be transferred by conjugation from S. scabies to non- that never had potatoes before. Therefore, if rented land pathogenic Streptomyces spp. This explains the continuoushas an unknown level of common scab infestation, the appearance of new pathogenic species. infestation can be determined by laboratory analysis of Genetic diversity makes it very difficult to breedsoil. A&L Biologicals in Ontario does this for growers.varieties that are resistant to common scab.Many breeding programs in North America have made II. The relationship between incidence and severity ofresistance to common scab a priority in their breeding common scab and soil chemical composition is complexwork. With time, this may help achieve acceptable control and specific for each soil. Differences in soil texture, soilof this disease.54SPUDSMART.COMFall 2023'