The term “scab” refers to a variety of fungal and bacterial infections primarily caused by bacteria in the Streptomyces family and fungi in the Spongospora family. Though it can be caused by different species, the symptoms (brown lesions or powdery brown spots) are similar, and abnormally wet or dry weather is a frequent cause.
The various fungal and bacterial infections deemed “scab” do more than just cause spots: they can severely distort the tubers or roots of affected plants and cause anything from mild discoloration to total crop spoilage.
Scab is spread through bacteria in the soil (in the case of Streptomyces) or spores in the soil (in the case of Spongospora).
While scab infections are present in the majority of soil to a very mild degree, weather is most often the catalyst for the worsening of an infection:
Interestingly, overly alkaline soil can also contribute to the problem: the more alkaline the soil, the more likely a scab infection is to take root.
Preventing scab is far easier than treating it, although no method is a surefire “firewall” in this case. Scab is present in a large percentage of soil to some degree, so you’re likely to see mild effects on a few tubers no matter what you do. It is, however, important to keep scab from getting out of control.
Generally speaking, it is better to employ as many preventative techniques as possible.
Scab can be highly infectious and has the potential to ruin an entire crop if the tubers or roots of a plant split—this will cause rot to set in. However, mild infections may cause only cosmetic imperfections. A little patience with scraping off the affected areas of your tubers can leave your harvests perfectly suitable for cooking.
Fungicides and other chemical control methods are not effective methods of scab control, so organic solutions are the only solutions in this case.
Scab can overwinter on plant matter, so remove as much of it as you can at the end of the season to lessen the risk of re-infection.
This is both a preventative and curative technique, although it’s most effective before an infection occurs. Rotate your plant beds, making sure that scab-susceptible plant varieties are planted in the same bed no more often than every three years.
If you have persistent problems with scab, test your soil every three months or so and amend it with sulfur products as necessary.