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Bacterial Blight of Soybean

Bacteria

Pseudomonas savastanoi pv. glycinea


In a Nutshell

  • Small yellow to brown spots on leaves.
  • Later turn dark brown, irregular or angular.
  • Yellowish-green "halo".
  • Lesions appear also on pods.

Symptoms

Infections early in the season are characterized by brown spots on the margins of the cotyledons. Young plants may be stunted and die if the growing point is affected. Plants infected later in the season develop small yellow to brown spots on leaves. Young leaves are usually more susceptible than older ones and symptoms are mainly found in the mid- to upper-canopy. Over time, as the spots coalesce, they turn into dark brown, irregular or angular lesions of different sizes. A yellowish-green "halo" will appear around the edge of water soaked tissue that surrounds the lesions. The center of the lesion gradually dries out and eventually falls out, giving the foliage a ragged appearance. If the infection takes place during pod formation stage, the lesions may also occur on the pods, giving them a shriveled and discolored appearance. However, seeds usually do not show symptoms.

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Hosts

Trigger

Bacterial blight is caused by the bacterium Pseudomonas savastanoi. It is a seedborne disease that also overwinters in the field on plant residues. Early infections in the seedling stage are usually a sign of contaminated seeds. On older plants, the initial infection usually occurs when dormant bacteria are spread via wind or splashing water droplets from plant residues to the lower leaves. Wet leaf surface will favor the development of the pathogen, which at some point will enter the tissues via wounds or leaf pores. Rain and wind will also favor the secondary spreading within the plant or between plants. The disease if favored by cool (20-25 °C), wet and windy weather (rainstorms) and limited by hot and dry weather.

Organic Control

Copper fungicides are recommended for the control of bacterial blight on soybeans. To be fully effective treatments should be carried out early in the disease cycle to be effective, that is, when symptoms are first detected.

Chemical Control

Always consider an integrated approach with preventive measures together with biological treatments if available. Copper fungicides can be used for control of bacterial blight on soybeans but need to be applied early in the disease cycle to be effective. However, it is recommended to follow core integrated pest management practices as fungicides are often not effective against this pathogen.

Preventive Measures

  • Choose varieties that are resistant to the disease.Do not work in the fields when plants are wet to limit the spreading of the disease.
  • Incorporate crop residue by tillage after harvest to reduce the amount of inoculum the next season.
  • Plan a crop rotation with non-susceptible hosts such as maize, wheat and other non-legumes.

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