- Soybean

Soybean Soybean

Ascochyta Blight

Fungus

Didymella rabiei


In a Nutshell

  • Water-soaked spots on leaves, stems or pods.
  • Turning brown.
  • Dark concentric rings on leaves.
  • In fields, patches of blighted plants can be seen from the distance.

Symptoms

On older plants, the disease first appears as pale water-soaked spots on leaves. Over time, these lesions become brown and small black specks start to develop toward the center, forming concentric rings with darker margins. Elongated to oval brown lesions with black specks may also form on the stem. In severe cases, they are girdled and eventually break off during adverse weather. Pod lesions are similar in appearance to leaf lesions. Entire plants may become blighted, something that is visible as brown patches in the field. Seeds may be infected and carry the disease to seedlings, which develop dark brown lesions at the base of the stem.

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Hosts

Trigger

Symptoms are caused by the fungus Didymella rabiei, formely known as Ascochyta rabiei, thereby the name of the disease. It can overwinter on plant residues for several years. Under favorable conditions, it produces spores that are later spread by wind and rain splashes, sometimes over distance of several kilometers. Cool and wet weather, high humidity, morning dew and prolonged leaf wetness (2 hours or more) favor the spread of the disease. The fungus can develop over a wide range of temperatures (5-30°C) but optimal growth is reached between 15-25°C . Multiple cycles of infection can occur during the growing season if conditions are favorable.

Organic Control

Sorry, we don't know of any alternative treatment against Ascochyta rabiei. Please get in touch with us in case you know of something that might help to fight this disease. We look forward hearing from you.

Chemical Control

Always consider an integrated approach with preventive measures and biological treatments if available. Seeds may be treated with a seed dressing based on thiram or thiram + thiabendazole before sowing. Preventative fungicides (for example chlorothalonil) may be applied before the flowering stage to avoid the development of the disease. Once the disease detected, a rotation of foliar fungicides with a systemic mode of action are recommended (boscalid, mancozeb, pyraclostrobin + fluxapyroxad or triazolinthione class of products). Treatments may need to be applied throughout the growing season to avoid serious yield losses.

Preventive Measures

  • Choose more resilient varieties if available.
  • Use certified disease-free seeding material.
  • Alternatively, use seeds from an healthy field.
  • Follow recommendations in regard to seed rates.
  • Plant later to avoid worst effect of the disease.
  • Monitor fields for signs of the disease.
  • Rogue volunteer plants and weeds in and around the field.
  • Harvest as early as possible to avoid the worst effects on yields.
  • Practice good farm sanitation practices, for example washing booths and clothes after field inspections.
  • Plant chickpeas only once in three years in the same field (crop rotation).

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