Ascochyta blight (Ascochyta fabae) occurs in all faba bean growing areas of Western Australia. It is yield limiting in the medium and high rainfall areas of the central and southern agricultural regions. The minor, usually non-yield limiting diseases Alternaria and Cercospora can both be confused with ascochyta blight.
Symptoms
Ascochyta blight usually appears within eight weeks of sowing.
- The first symptoms are grey spots which show through on both sides of the leaves.
- As the spots on leaves, stems and pods enlarge, they develop grey centers which contain many black specks. These are the fruiting bodies (pycnidia) of the fungus.The center of the spots may fall out to produce a 'shot hole appearance'.
- Alternaria spots have a red-brown margin and contain obvious concentric rings but do not develop black specks (pycnidia).
- Cercospora spots are darker and have irregular edges. They may develop a ring pattern inside the spot. The spots do not develop black specks on a grey centre.
- Herbicide damage (particularly simazine) can be mistaken for ascochyta blight. It does not have the grey center with black specks and is usually confined to the edges of leaves.
- Infected seed have dark lesions that result from pod infections.
Disease risk
A faba bean crop may be infected by ascochyta blight from two major sources:
- Sowing infected seed. This is the primary way ascochyta blight is introduced into new areas. In areas where faba beans have been grown for several years it is not a significant cause of infection.
- Spores produced on stubble from the previous year's faba bean crops are the main cause of infections.
The weather is the principal factor in translating disease risk into disease severity. The disease is favoured by rainfall and temperature conditions in the high rainfall (above 450mm) areas of the southern agricultural region. Infection can occur at any stage of plant growth following either rain or heavy dew. Ascochyta blight requires cool conditions for infections to grow and promote the development of ascospores. Frequent rain moves spores about and provides moist conditions for their germination as leaves remain wet for longer periods.
Infection cycle
The ascochyta fungus survives on faba bean stubble for 2-3 years. After autumn rains and exposure to cold temperatures (less than 10°C) wind dispersed spores (ascospores) are produced in fruiting bodies (pseudothecia) on the stubble. They can be blown further than a kilometre, but most land within a few hundred metres of the source. Ascospores are the main way new crops become infected.
After the initial infection, the disease spreads within the crop. Spores (conidia) are produced in the black specks (pycnidia) in leaf and stem infections. These spores are splashed by rain to adjoining leaves. Some spores may be blown several metres by the wind that accompanies rain fronts. After landing on a susceptible leaf, the spore will germinate and initiate a new infection if the leaf surface remains wet for 12-24 hours.
Control
Use varieties that have some resistance
In the central and southern agricultural areas varieties that are at least moderately resistant to ascochyta blight (such as Fiesta) should be used. More resistant varieties (such as Farah or Ascot) will be required in southern high rainfall areas that favour the disease.
Rotate crops
Avoid paddocks in which faba bean has been grown in the past 2-3 years or which are within 500 metres of stubble from the previous year's crop.
Use clean seed
In areas where faba bean has not been grown, or not grown for several years, take care not to introduce the disease by sowing infected seed. No seed treatment is recommended.
Foliar fungicide
Resistant varieties (such as Ascot) should not require fungicide sprays for ascochyta blight. On more susceptible varieties, in areas where ascochyta blight occurs regularly, fungicidal sprays may be required early in the season.
Note: recommendations were current at the time this page was prepared.