Potato Scab: An Overview

Potato scab is a common disease that affects potatoes worldwide. It is caused by a bacterium-like organism known as Streptomyces scabies. This disease primarily impacts the appearance and marketability of tubers intended for table stock.

Impact on Marketability

When potatoes meant for table stock become infected with scab, the economic losses can be significant. The overall appearance of the tubers is crucial in this market, making scab lesions a major concern. While superficial scab lesions may not greatly affect processing potatoes’ marketability, deep-pitted lesions increase peeling losses and diminish the quality of the processed product.

Preferred Growing Conditions

Streptomyces scabies thrives in soils with a pH above 5.5 and temperatures ranging from 10 to 31 degrees Celsius. Interestingly, these conditions closely resemble the optimal growing conditions for potatoes, which contributes to scab’s prevalence in potato-growing regions.

Variability and Seasonal Impact

The occurrence and severity of scab can vary from season to season and from field to field. Factors such as host resistance, the aggressiveness of the pathogen strain, timing of infection, and environmental conditions influence the type and extent of scab lesions.

Life Cycle and Survival

Streptomyces scabies is a soil bacterium that can persist in the soil for extended periods, particularly in the presence of decaying plant matter. The pathogen can also survive in lesions on stored tubers; however, it does not spread or increase in severity during storage. The disease is transmitted through infected seed tubers, wind, water, and even fresh manure, as the bacterium can survive the digestive system of animals.

Infection and Symptoms

The bacterium enters potato plants through pores/lenticels in stems, wounds, and the skin of young tubers. Initial infections result in reddish-brown spots or lesions on the tuber surface. As the tubers grow, these lesions expand, becoming corky and necrotic. The lesions can affect small portions or completely cover the tuber surface, sometimes appearing as broken concentric rings.

Individual scab lesions are circular but can merge to form larger scabby areas. The type of lesion depends on factors such as host resistance, pathogen aggressiveness, timing of infection, and environmental conditions.

Severe cases of scab can cause deep pitting and cracking, creating entry points for secondary pests and diseases to invade the tuber’s flesh. While potatoes affected by scab can still be consumed after removing the scabby areas, they are not suitable for the farmer’s market due to their compromised appearance.

Management and Control

Chemical Methods

To control common scab disease, copper-based fungicides are commonly used. These fungicides are most effective when applied directly to the soil. Examples of such fungicides include Trinity Gold 425WP, Greencop 500WP, and Colonizer 440WP. Additionally, spraying Optimizer can boost the plant’s immunity, while Pyramid 700WP helps suppress the bacterium’s activity within the plant. It is advisable to mix any fungicide with Integra (3ml/20l) to enhance its effectiveness.

Non-Chemical Control Methods

Non-chemical approaches to managing potato scab include:

1. Planting resistant varieties in fields where scab is a recurring problem.
2. Using certified seed that is free from scab.
3. Preventing the introduction of the pathogen into fields.
4. Treating seeds before planting.
5. Avoiding overwatering the garden.
6. Practicing crop rotation with non-host plants.
7. Maintaining soil pH levels between 5.0 and 5.2 using acidic fertilizers.
8. Preventing moisture stress during the 2-6 weeks following tuberization.

Conclusion

Potato scab, caused by Streptomyces scabies, poses a significant threat to the marketability and quality of potatoes. Understanding its life cycle, preferred conditions, symptoms, and management strategies is crucial for farmers and gardeners aiming to control and mitigate its impact. By employing appropriate chemical and non-chemical control methods, it is possible to minimize the occurrence and severity of scab and ensure healthier potato crops.

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