Chocolate spot is one of the most severe disease of faba bean in Victoria, SA and NSW, and occurs in all areas where the crop is grown. In light of recent persistent rains across the southern region, frequent rainfall events in combination with wet soils will contribute to humidity beneath the canopy creating favourable conditions for disease development. Under such conditions, chocolate spot can develop quickly, resulting in pod abortion , infection of the flower and damage to the plant through leaf infection and subsequent loss of photosynthetic tissue. In a bad year, the yield loss potential caused by chocolate spot in unprotected crops may be as high as 30-50%. In addition to this, seeds from badly affected plants may have a reddish-brown discolouration, significantly lowering market value.
Typically the disease develops later in the growing season as crops commence flowering and after canopy closure. Whilst most crops will have received canopy closure fungicide applications already, crops will now require follow up fungicide applications with frequency depending on the rain fronts and persistence of wet weather. Even in the absence of rain, wet soils will continue to drive the disease in humid crop canopies.
Chocolate spot symptoms vary from plant to plant and range from small spots on leaves to complete blackening and death of the entire plant. Leaves and flowers are affected, yet under favorable conditions the disease may spread to the stems, flowers and pods. Two stages of the disease are usually recognised with a non-aggressive phase resulting in discrete reddish-brown spots peppered across the leaves and stems. An aggressive phase follows in which spots darken in colour and coalesce to form larger grey-brown target spots that may cover the entire plant.
Outside of the growing season, the disease may survive as sclerotia in the soil or on crop debris, infected seed or on self-sown volunteer plants. Infection is spread when wind carries the air-borne fungal spores from one area to another within 4-5 days of initial infection. Spores can then be formed on infected tissue and initiate secondary spread of the disease within and between crops
For the successful management of chocolate spot of faba bean an integrated approach is key. When selecting a paddock, a break of at least 4 years should be observed between faba bean plantings and a minimum 500m distance kept between the new crop and any of last season’s faba stubble. When planning next season’s crop, a variety with the highest level of resistance should be selected and the ‘cleanest’ possible seed used with nil to 10% levels of chocolate spot present. In addition to this, a successful fungicide program including crop monitoring, correct disease identification and timeliness of foliar fungicide application is crucial for successful disease management. Growers must keep in mind that carbendazin and procymidone can be applied twice each in a crop with the number of sprays restricted to minimise the risk of fungicide resistance.