Colletotrichum coccodes on potatoes: what treatments?

Potato black rot ( Colletotrichum coccodes) is a major disease of potatoes, tomatoes and other Solanaceae plants as well as some Brassicaceae, Cucurbitaceae and Asteraceae. Although it does not systematically affect overall crop yield, it significantly affects the appearance of tubers, leading to commercial devaluation. It therefore represents a major challenge.

At AGROBIOTOP, we are developing natural solutions that provide an effective response to this problem.

Description of the disease Colletotrichum coccodes

Black dot disease is a fungal infection caused by the fungus Colletotrichum coccodes , an ascomycete belonging to the Phyllachoraceae family . This fungus is widespread throughout the world and mainly affects potato crops. 

Its presence has been reported in major producing regions, including Europe, North America and Asia, making dartrosis a cosmopolitan problem for agriculture.

Which crops are affected by Colletotrichum coccodes?

Although potatoes are particularly vulnerable, Colletotrichum coccodes also affects other crops, such as tomatoes , eggplants, peppers and other members of the Solanaceae family .

Other plants susceptible to this disease include Brassicaceae such as watercress and white mustard, Cucurbitaceae such as zucchini, and Asteraceae such as lettuce and chrysanthemum. This wide range of host plants increases the potential for dissemination of blackhead and makes its management more complex.

Biology of the fungus Colletotrichum coccodes

The fungus Colletotrichum coccodes persists in the soil in the form of microsclerotia, survival structures capable of persisting for several months or even years in infected plant debris . These microsclerotia allow the fungus to remain viable even in the absence of host plants.

In the spring , microsclerotia germinate and produce mycelium. This then forms acervuli, structures containing spores that are disseminated into the environment. Spores are transported primarily by wind, rain, or irrigation . They enter plant tissues through the epidermis, or through wounds caused by insects or other diseases. The fungus can also be spread by splashing water, workers, farm equipment, or nutrient solutions in soilless crops.

Infection usually begins in underground organs , such as roots or stolons, and then progresses to the aerial parts of the plant. When an injured stem or leaf is infected, the fungus grows toward the roots, causing progressive deterioration and, in some cases, premature death of the plant.

What are the favorable development conditions?

• Heat : The optimum temperature for the development of Colletotrichum coccodes is between 25 and 30°C.
• High atmospheric humidity : encourages spore germination
• Rainy periods : or sprinkler irrigation facilitates the spread of the fungus.
• Monoculture : when practiced on the same soil year after year, increases the inoculum rate and exacerbates root symptoms.
• Mismanagement of irrigation : whether it is too much or too little water 
• Inadequate fertilization

What are the symptoms of Colletotrichum coccodes?

The damage observed is as follows:

On potato tubers

Dartosis is manifested on tubers by the appearance of light or darker gray spots or discolorations . These spots have irregular borders and may contain microsclerotia, especially visible under humid conditions. These fruiting bodies are generally larger than those observed during silver scurf infection . After a period of cold storage, tubers may develop depressions, sometimes accompanied by dry rot.

On fruits

On fruits, Colletotrichum coccodes is responsible for anthracnose, and mainly affects mature fruits . This infection results in brown lesions that develop into circular spots. Microsclerotia as well as masses of salmon-colored spores may appear on the surface of the lesions in the presence of humidity. The rot caused by the fungus can gradually invade large portions of the affected fruits.

On the roots

The roots of plants affected by dartroth show brown lesions , accompanied by destruction of the rootlets and progressive rot. The cortex of large roots may also be affected, taking on a grayish to brownish tint . Black pits, formed by microsclerotia, are often visible at the base of stems and roots.

On leaves and stems

On leaves and stems, dartosis is identified by round, brownish lesions surrounded by a yellow halo . Infected leaves show general yellowing, followed by curling

What is the impact on potato crops?

Although black spot does not cause a significant reduction in yields, it can still have a significant impact on the commercial quality of tubers . Cosmetic damage caused by spots and black spots makes potatoes less attractive for the market, which can cause economic losses for producers.

Additionally, under stressful growing conditions or in the absence of adequate crop rotations, black rot can weaken potato plants, leading to premature plant death and compromising the harvest.

How to fight against Colletotrichum coccodes?

The fight against Colletotrichum coccodes is based initially on prevention and management of conditions favorable to the development of the fungus.

• Rotate crops for at least four years between potato plantings to reduce inoculum in the soil, avoiding host plants (Solanaceae, Brassicaceae, Cucurbitaceae, Asteraceae).
• Use healthy, certified seeds , and favor resistant varieties when possible.
• Stimulate soil microbial life by adding organic amendments to improve soil quality and limit fungal development.
• Ensure rational management of irrigation and fertilization to avoid water stress which weakens plants.
• Destroy diseased tops and remove crop debris from previous seasons to reduce sources of inoculum.
• Maintain a delay of 3 to 4 weeks between top dressing and harvesting to minimize the risk of tuber contamination.
• Dry tubers before storage to avoid condensation and store tubers at a temperature below 5°C to slow the development of the fungus.

Agrobiotop solutions against Colletotrichum coccodes

We have developed natural solutions that allow the improvement of soils (Contact us for any information on their application protocol on your crops).

We also recommend protecting plants and tubers using these solutions: