As its name suggests, potato scab disease causes lesions on potatoes (Solanum tuberosum). Depending on the kinds of potatoes you are growing, the tubers can be marked with scabs that are cork-like in texture, or are simply pit-like depressions. No matter what the lesion looks like, scab disease can ruin some or all of each potato, likely running through the entire crop -- and often persisting in the soil for years. Some organic growers recommend using molasses to halt the spread of pathogens.
The Streptomyces pathogen causes potato scab, and it can lurk in the soil, in manure used to fertilize the potato patch or in the tubers themselves. According to the book "Recent Developments in the Management of Plant Diseases," the Streptomyces pathogen that causes potato scab can be controlled with substances high in volatile fatty acids. These fatty acids can kill the pathogen within 48 hours. Molasses is one of the substances high in volatile fatty acids, along with liquid swine manure, fish emulsion and some composts.
The potato nursery Ronninger's suggests applying a molasses solution to the potato patch as many as four times over the course of each growing season. Blend 1 cup molasses with 5 gallons of water to create a solution. It's best to let the mixture sit for about 24 hours, stirring frequently to release beneficial microbes. Pour the solution along the rows near where the potato root systems are, rather than over the sprawling foliage of the vines. Along with directly killing pathogens, blackstrap molasses promotes the proliferation of beneficial microorganisms and fungi in the soil, according to the University of Wyoming's Cooperative Extension program.
Using methods such as molasses applications -- or applications of other materials high in volatile fatty acids -- can have unpredictable results. Applying a liquid solution of a material containing volatile fatty acids works well only in acidic, or low-pH soils. If your potato patch has a pH balance of 4.8 or higher, using molasses may not help prevent potato scab. The alkalinity of higher pH soils can neutralize the acidic nature of molasses, and the acidic forms of its molecules are the only ones that kill the potato scab pathogens.
If scab is a continual problem with your potato crops, consider adding control measures along with, or instead of, the molasses treatment. The University of California's Integrated Pest Management program recommends buying potato sets that are certified to be free of scab. The program also suggests practicing crop rotation, so that potatoes aren't planted in the same area of your garden year after year. This can keep soil-lurking scab pathogens away from future crops -- but avoid planting other root vegetables in the bed immediately after potatoes. In addition, avoid animal manures in favor of compost, blood meal or other nutrient-rich fertilizers.
- University of California IPM: Potato -- Common Scab
- Recent Developments in the Management of Plant Diseases: U. Gisi, Ilan Chet, et al.
- University of Wyoming Cooperative Extension: Gardening Organically
- Ronninger Potato Farm Growing Guide
- Photo Credit Martin Poole/Digital Vision/Getty Images