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.
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.
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.
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.