Get Ready to Scout Dry Beans for Anthracnose

Anthracnose continues to be a concern for dry bean producers. In 2003, a number of white bean and black bean fields in Ontario were identified with the disease. The occurrence of anthracnose means it is imperative that growers scout their fields for the disease.

The seed borne, fungal disease affects all bean types and can severely reduce yields and down-grade seed quality. The first lines of defense are to use disease free seed, select varieties with resistance to known races of anthracnose, and treat seed with DCT. Now, growers also have a second line of defense with the registration of 2 new fungicides - Quadris from Syngenta Crop Protection, and Headline from BASF.

Appearance

Early signs of infection appear on the lower leaf surface along the veins, which show a brick-red to purplish-red discolouration. Later, such discolouration also appears on the upper leaf surface. Since the disease is initially restricted to the lower leaf surface, plants may appear quite normal until the disease is well advanced. Therefore, it is important to thoroughly examine plants that have any unusual appearance. On stems, an early sign of infection is dark brown eye-spots that develop longitudinally. In young seedlings, this may cause the stem to rot off. At later stages, the most striking disease symptoms are seen on the pods. The pod symptoms begin with rusty-brown spots that later become brown sunken cankers. Seed from infected pods will show brown to light chocolate coloured spots on the seed coat.

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Disease Spread

The disease favours moderate temperatures of 13 – 21º C (optimum 16ºC). Temperatures over 28ºC will halt its spread, but symptoms can develop if the night temperatures drop below this. A minimum of 10 mm of rain is required to establish infection. The disease is spread in a field by movement of water. A driving rain will spread the disease 3 – 5 meters by splashing raindrops. The movement of animals, machinery or people through the beans when the plants are wet will also spread the disease. Humidity alone is not enough to spread the disease, and the canopy must be wet from heavy dew or rain.

Management

Early infection can be detected at the 3^rd to 4^th trifoliate stage. Early identification of the disease is made difficult by the fact that the disease starts on individual plants and spreads out from these foci. Detection will be easier at late-vegetative stages to early-flowering. If anthracnose is identified, two foliar fungicides, Headline and Quadris, are effective in decreasing the spread of the disease. Timing of application is critical to reducing disease severity and seed infection. Application is recommended at the early-bloom stage to minimize yield losses and seed infection. Fungicide application at an earlier vegetative stage, while effective in reducing the disease spread, may not provide protection during pod development. Application at the end of the flowering to pod-fill stage will not always prevent yield loss or a high incidence of seed infection. A second application may be warranted 10 –14 days after the initial application if weather conditions continue to favour the spread of the disease and infection is detected.

Both fungicides are locally systemic within contacted leaves, while Quadris has some upward systemic activity in the plant. However, thorough plant coverage is recommended for complete protection. Achieving penetration into the lower crop canopy is also important because anthracnose spreads upwards in the crop canopy.

If infection is found, I would be interested in collecting pod samples in order to identify the race of anthracnose in a co-operative project with Dr Bob Conner, AAFC.

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