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The battle against potato black dot: A comprehensive review of two decades of research and management strategies

This article was written and prepared by Jorge Luis Alonso G.

Researchers at Cranfield University and Aberystwyth University in the UK carried out a study examining pre- and post-harvest strategies over the past 20 years related to potato black dot. The goal was to identify knowledge gaps and enhance the management of potato black dot.

This article is a summary of their findings.

The potato, known as Solanum tuberosum L., is important to global agriculture, with production increasing by about 20% over the past 30 years. China is the leading producer, while the United Kingdom consistently produces about 5.5 million tons annually.

Potatoes are classified according to their use (e.g., processing or fresh consumption) and maturity. When stored at temperatures below 4°C with high humidity, they can remain fresh for up to ten months. These conditions promote wound healing and limit the growth of pathogens and sprouts.

There’s a growing preference for pre-packed potatoes in the UK, but their marketability is linked to their appearance. Skin diseases such as black dot and silver scurf detract from their visual appeal. Specifically, black dot, caused by the fungus Colletotrichum coccodes, produces blemishes on potatoes that result in economic losses.

Controlling this disease at harvest plays a key role in minimizing its spread during storage. Several common control methods are used. These include the use of fungicides, the practice of crop rotation, and careful temperature management. However, silver scurf can be difficult to identify. Its symptoms are very similar to black dot, making an accurate diagnosis difficult.

Technologically, diagnostic tools that combine computer vision and machine learning offer hope for early disease detection. Studies are exploring algorithms, including the Visual Geometry Group (VGG), to detect potato diseases.

While the subtle nature of these symptoms and their similarity to other diseases pose challenges, automated detection could revolutionize the prediction of optimal storage periods based on disease presence and severity.

This review highlights the evolving strategies over two decades to manage potato black dot and underscores the importance of integrating technological solutions for effective disease intervention and diagnosis.

Preharvest factors affecting black dot development in potatoes

Potato black dot is a significant problem for potato growers because it causes symptoms on tubers, stems and leaves, resulting in crop losses and reduced yields. The disease has a complex life cycle; sclerotia can infect potatoes and persist in fields for up to eight years, thriving on seed tubers and crop debris. In addition, internal growth of mycelial hyphae allows the fungus to colonize various parts of the potato plant.

Recent evidence suggests that planting clean seeds in contaminated soil or using infected seeds in clean soil results in rapid plant infection. Surprisingly, C. coccodes has appeared in fields where potatoes have not been grown for years. While seeds transmit the disease, soilborne sources are the primary culprits. The presence of fungal DNA in the soil indicates potential infection, so selecting fields with low fungal activity is essential. Soil structure influences fungal spread more than soil type.

External factors such as climate change, with its rising temperatures and unpredictable rainfall, complicate matters for UK potato growers. Increased heat and moisture can exacerbate potato late blight. As global climate change continues, these fungal threats are expected to increase in frequency and severity, threatening potato yields.

Managing black dot in the field

Black dot management in potato crops is challenging due to limited treatment options. The main methods are crop duration management, crop rotation and fungicide application. The fungicide azoxystrobin can reduce the severity of black dot, particularly in areas with low soil inoculum levels, but its efficacy decreases in areas with high inoculum levels. Although azoxystrobin is registered for soil use in the UK, it hasn’t yet been registered for foliar use.

Given the genetic diversity of C. coccodes, research into fungicide resistance is essential.

Direct soil fumigation has been considered as a solution, but results are inconclusive. A sustainable strategy requires an understanding of black dot resistance mechanisms. While the genetic basis of this resistance remains elusive, certain potato lines exhibit resistance. In particular, metabolites such as the glycoalkaloid alpha-chaconine appear to enhance resistance, which may be useful in breeding for sustainable production.

Longer harvest times can increase the risk of black dot, especially in areas with high soil inoculum concentrations. Infections likely begin during early root growth, and crop duration has a significant impact on disease severity. Effective management includes planting in low-inoculum fields, starting azoxystrobin applications early and adjusting harvest timing.

However, how these strategies affect tubers in post-harvest storage needs further investigation. Adherence to these practices can help reduce black dot in storage.

Influences on potato black dot after harvest

Long-term storage of potatoes is essential to ensure a consistent supply for fresh consumption and processing. Some potatoes are stored for up to ten months. The initial storage period often includes a curing period, where potatoes are exposed to temperatures between 10°C and 14°C for about two weeks after harvest to help the tubers heal.

However, prolonging this curing heat can promote fungal diseases, increase weight loss, and reduce quality attributes such as fryability and shelf life. One method to counteract this is to rapidly cool potatoes in storage, which helps control potato black dot in colder climates.

Some research suggests that omitting the curing period may reduce the incidence of black dot, while prolonging the curing period may increase its growth.

Potato black dot remains dormant in cold storage and doesn’t spread between potatoes. However, infections may intensify and spread during storage. Maintaining an appropriate storage environment, particularly in terms of temperature and humidity, is therefore crucial. In the UK, where the focus is on washed and pre-packed potatoes, the ideal temperature for extended storage is between 2.5°C and 3.5°C.

Research has shown that C. coccodes can cause tuber lesions when stored for extended periods between 5°C and 15°C. Therefore, cooler storage temperatures are essential to control disease, prevent lesions and reduce moisture loss.

Innovative techniques for detecting potato black dot disease

In agriculture, proper diagnostics are essential for managing diseases such as potato black dot, both in the field and post-harvest. Thorough inspection in packinghouses is necessary to ensure that potatoes are of consistent quality and disease-free. The traditional inspection method of manually inspecting the potato skin is subjective and prone to error. It’s also labor-intensive and often costly.

Modern techniques use computer vision coupled with machine learning to provide a more efficient quality assessment system, resulting in reduced post-harvest losses.

Machine learning, particularly convolutional neural networks (CNNs), has become prominent in plant disease detection. The design of CNNs mirrors the human visual processing system, allowing them to effectively handle complex tasks. However, they require extensive data for training. Fortunately, they can use images from inexpensive cameras, which simplifies data collection.

The success of CNNs depends on both the quantity and quality of the data.

Deep learning has achieved significant accuracy in plant disease detection. Some studies report between 83% and 96% accuracy in detecting potato blemishes using CNN. These networks are fast at classifying and localizing problems, making them ideal for wider industrial use. They can identify multiple defects in a single plant.

Current systems can detect potato skin problems, but not early signs of potato black dot. New technologies such as Mask R-CNN provide an unbiased way to detect such dots and could predict the shelf life of the potato.

Researching new techniques for managing black dot in potato storage

Postharvest research on potatoes reveals a gap in knowledge about black dot development during cold storage and methods to control it. Among the techniques being investigated, exogenous ethylene and plant volatile organic compounds (VOCs) are of significant interest. Ethylene, commonly used to prevent sprouting in potatoes, may also affect fungal growth.

The role of ethylene in influencing black dot remains unclear, although there is some evidence that it may enhance potato defenses against certain diseases. Essential oils, particularly spearmint, orange, and caraway, have antifungal properties, with the compound carvone showing potential in inhibiting fungal progression and suppressing sprouting.

The potential of black spruce essential oil in these areas is also being investigated, although rigorous studies of its efficacy against black dot are lacking. Plant VOCs are emerging as potential antifungal agents.

Preliminary results suggest that they may address the problem of black dot in storage, but rigorous trials are needed. If confirmed, these methods could lead to fewer fungicide applications and reduced environmental impact.

Conclusion and future prospects

Management of potato tuber blight requires strict pre-harvest measures. A key step is to avoid planting contaminated seed in virgin fields to minimize the risk of future soil contamination. While chemical treatments, particularly fungicides, are widely used, there’s growing pressure to limit their use for environmental reasons. During the post-harvest phase, it’s critical to maintain ideal curing conditions, modulate cold temperatures, and maintain appropriate humidity levels.

However, these factors are affected by variables such as potato variety and seasonal shifts. As a result, the industry is driving the development of rapid, non-invasive diagnostic tools using technologies such as machine vision and machine learning.

These innovations can assess storage time based on disease severity and prevent the shipment of affected potatoes. In essence, a holistic disease management strategy ensures a consistent, high-quality tuber supply and reduces food waste in the potato production process.

Source: Sanzo-Miró, M., Simms, D.M., Rezwan, F.I. et al. An Integrated Approach to Control and Manage Potato Black Dot Disease: A Review. Am. J. Potato Res. (2023).
Author: This article was written and prepared by Jorge Luis Alonso G.
Photo: Credit Bayer Crop Science UK

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