Cryphonectria parasitica overwinters in lesions and colonized bark. The spores are produced in wet weather and at any time of the year pending mild temperatures. The spores are dispersed via wind, rain or vectors such as insects and birds. Infection is initiated when spores land on freshly wounded bark. The susceptibility of the host tissues decreases after a few days. The fungus kills the bark and cambium by a combination of mechanical and chemical actions. It is also reported that the pathogen can exist as a saprobe (i.e. it lives and feeds on dead organic matter) for at least two years on moribund bark. Long distance spread is by movement of infected plants, wood or bark. Risk of transmission by fruits or seeds is small.
There is also evidence that the pathogen can weaken in virulence in Europe and some American localities due to infection of the fungus by a virus (dsRNA hypovirus CHV1). The virus limits the ability of the pathogen to produce spores, reduces its growth and its ability to produce enzymes that kill the host tissues. This phenomenon, known as hypovirulence, allows the trees to recover. The discovery of hypovirulent strains of the fungus raised hopes for biological control of the disease. However the success has been limited because the virus does not spread readily between strains and hypovirulent strains do not compete very well with virus-free fungal strains. Scientists are working towards manipulating hypovirulence to develop it as an efficient method of biological control.
Image: © Forestry Commission Picture Library