White mould or Sclerotinia disease is caused by the fungus Sclerotina sclerotiorum. This disease is known to infect a large variety of ornamental and vegetable plant species in Canada and around the globe. It is one of the most destructive diseases of dry beans and can lead to a considerable yield loss if not properly scouted for and treated timely and efficiently.
S. sclerotiorum overwinters in the soil or in plant residues as hard sclerotia bodies. These sclerotia germinate to form apothecium which can release millions of spores into the canopy. Spores must come into contact with dead or dying plant tissue (typically flower petals) in order to develop. These petals are often found in branch axils or stuck to developing pods and initial damage from the disease begins at these locations. Watery soft lesions develop, followed by white, fluffy mycelium on infected tissues. The formation of black sclerotia inside the plant occurs as the disease matures. The disease is favored by wet conditions and a dense canopy – a reason why the crop density plays a role in the development and outbreak of the disease.
Preventive measures to decrease the likelihood of infection are highly recommended and include the following: the use of fungicide, the adequate spacing of row crop, low density planting and crop rotation.