Global: Common Scab Control Depends on 80–85% Soil Moisture in the First Weeks After Tuber Initiation
For growers now planting in the Southern hemisphere, the scab decision comes a few weeks after emergence. Irrigation research from the UK and North Dakota shows a short window when moist soil protects young tubers.
Common scab is a skin disease of potato caused by Streptomyces bacteria in the soil. It leaves corky, rough lesions on the tuber surface. The damage lowers the value of ware and seed potatoes, especially for washed markets.
Young tubers are most vulnerable in the first three to four weeks after tuber initiation, according to North Dakota State University (NDSU) Extension. By six to eight weeks after initiation, they are much less susceptible. This short window is when irrigation can make the biggest difference.
How much moisture
NDSU advises keeping soil moisture at 80–85% during tuber initiation. This level should hold until tubers are 1 to 1.5 inches, or about 2.5–3.8 cm, in size. Infection is favoured by low soil moisture of less than 65–70%. Soil pH between 5.2 and 8.0 and temperatures above 70°F, or 21°C, also favour the disease.
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A trial funded by the levy board AHDB in the United Kingdom gives similar timing. The project ran from 2009 to 2010 and tested five irrigation regimes. The critical period for scab control by irrigation was between one and three weeks after tuber initiation. In dry soils, populations of pathogenic Streptomyces increased between three and four weeks after tuber initiation. Levels were lower in irrigated soils.
Control was most effective when plots stayed at field capacity after tuber initiation, the project found. However, over-irrigation was detrimental to canopy duration and yield.
Other tools
No single method gives complete control, NDSU said. In its research, a seed treatment with fludioxonil reduced common scab by 58%. A biopesticide based on Bacillus subtilis reduced it by 56%. Resistant varieties and certain rotation crops can also help. These include buckwheat, canola, oat, rye and millet. NDSU also advises laboratory diagnosis to separate common scab from powdery scab, which needs different management.
Why it matters
In the Southern hemisphere, spring planting is under way in countries such as Argentina, Chile, South Africa and Australia. Tuber initiation in these crops will come in the next weeks. Irrigation plans should mark that date in advance.
The challenge is balance. Moist soil protects young tubers from scab, but waterlogged soil cuts yield and raises other disease risks. Growers need soil moisture sensors or regular checks during this short window.
Photo: Common scab (Streptomyces scabies) on potato tubers (illustrative). Rasbak / Wikimedia Commons, CC BY-SA 3.0
