Dealing with anthracnose

This fungal disease can wipe out a dry bean crop. Here’s how to prevent, and treat, this problem.

Dealing with anthracnose
Also known as black spot disease, anthracnose manifests as small, reddish-brown to purple spots on the pods, which become larger and darker.
Photo: University of Georgia
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Anthracnose is one of the most invasive and destructive dry bean diseases. It can destroy up to 95% of your crop while also threatening the growth rate and seed and pod quality.

The disease is caused by the fungus Colletotrichum lindemuthianum, which is mainly seed-borne. In South Africa, it is most common in KwaZulu-Natal, Mpumalanga and parts of Limpopo.

Initial symptoms are brick-red lesions that appear on the veins on the lower leaf surfaces. These then become darker and spread to the upper side of the leaves and stems.

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Lesions also appear on the pods, starting as small reddish-brown to purple spots that grow to between 5mm and 8mm, becoming darker and sunken.

In severe infections, lesions occur on the seed itself. When the seed germinates, the hypocotyl (growth point) becomes infected and the seedling stem shows signs of infection.

These can weaken the stem so badly that it snaps off.

Farmers who try to save money by planting their own seed held over from the previous season can be in for a 100% yield loss. Even planting seed harvested from a crop with a low level of infection can lead to serious yield loss the following season.

Cool, humid conditions favour the disease. Symptoms appear at 13°C to 26°C, with 17°C the optimal temperature for disease development.

The fungus can survive in crop residue on the land for up to two years.

Rain splash and wind can spread fungal spores throughout the crop, and heavy rain can spread spores up to 4,5m away from an infected plant.

Control measures

  • Use disease-free seed;
  • Once anthracnose becomes a problem in a specific land, don’t plant beans on or near the land for two to three years;
  • Restrict movement within an infected land and don’t cultivate or harvest a crop if it is wet;
  • Rotate the bean crop with non-susceptible crops such as maize, sunflower and wheat;
  • Good sanitation, including removing infected plant residue, will reduce inoculum pressure. A suitable fungicide can help manage the disease if applied in time.

Look under the crop’s canopy during the pod development stage (January to February) to scout for signs of the disease. This will enable you to respond in time.

The disease can become a major problem even if clean, certified seed has been planted; for example, if infested implements are used on a clean land.

Source: Craven, M and Muedi, H. ‘Anthracnose of Dry Bean’. Arc-Grain Crops, Potchefstroom.