Common scab is a disease that affects the appearance of potatoes, causing economic losses for producers due to the downgrading of infected table potatoes. While it does not significantly impact yield or short-term storage, the lesions can serve as entry points for other pathogens such as Fusarium.
Symptoms:
- Superficial, russet/netted, raised or deep, brown to dark brown, corky lesions on tubers.
- Lesions can range from small, isolated corky areas to large, compound lesions (5-6mm in diameter) and even large coalesced lesions covering the tuber’s surface.
- The disease affects all underground parts of the potato plant, with no visible symptoms above ground.
Favourable Conditions for Development:
- Warm, dry environments and repeated cultivation of potatoes or other hosts (groundnut and carrot) promote the development of common scab.
- Cultivating spinach and red clover as green manure crops can also enhance disease development.
- Common scab pathogens are present in most soils that have been used for potato cultivation and can be introduced via infected seed tubers. The disease can also occur in virgin soils if symptomless seed tubers from infected soil are planted, spreading infection to previously disease-free soils.
- Streptomyces species can survive in soil on live or decomposing roots of infected plants. Severe infections are often associated with soils that have received large amounts of compost of animal origin, such as composted cattle manure.
- The disease can also spread through infected soil on implements, windblown soil particles, and water used for post-harvest washing.
Control Measures:
- Grow potatoes in soils with a pH ranging from 5.0 to 5.2.
- Purchase certified seed from reputable suppliers.
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