Swiss Fed. Inst. Forest, Snow, Landscape Res., CH-8903 Birmensdorf, Switzerland

Background and objectives

Chestnut blight is successfully controlled in many regions in Europe by virus-induced hypovirulence, either naturally or after biocontrol treatments [1]. How the virus (CHV1) spreads has not been fully explained. Chestnut forests where hypovirulence is widespread are characterised by high disease incidence but low disease severity leaving most chestnut trees with one or more healed, superficial cankers [2]. Because sporulation of the fungus is rare or absent on such cankers, they probably play no major role for the spread of CHV1. In contrast, heavily sporulation has been noticed on the bark of cut and stacked chestnut stems, as well as on dead trees or branches which apparently were saprophytically colonised by the fungus. The objective of this study was to assess the role of this saprophytic activity for the dissemination of virulent and hypovirulent strains of C. parasitica.

Material and methods
Ten chestnut (Castanea sativa) trees harbouring active, healed, or no cankers were selected in each of 5 coppice forests in southern Switzerland. Half of the trees were cut and the stems were stacked in the forest. C. parasitica was sampled and sporulation (stromata production) assessed in the beginning of the experiment (March 1995) and again in November 1995 and April 1996. Bark samples were removed in November and April and the proportion of stromata with perithecia and mature pycnidia was determined under a dissecting microscope. Single conidia and ascospore cultures were isolated to estimate the transmission frequency of CHV1 into each spore type.

Results and conclusions

Sporulation was rare in the beginning of the experiment regardless of the type of canker on the stems. In November, sporulation was abundant on the stacked stems which harboured active or healed cankers at the time of cutting. Stromata was produced on the old canker surface, as well as on newly colonised areas of the stems. Stromata production was also observed on the stacks of formerly blight free stems. In contrast, no or only little sporulation was detected on comparable control cankers on living stems. At an average, only 20% and 5% of the stromata yielded mature perithecia and pycnidia, respectively. Similar proportions were recorded in April. Perithecia were produced in all plots, preferentially on cut stems harbouring cankers without CHV1-infected white strains. All C. parasitica isolates derived from ascospores showed the normal (CHV1-free) culture morphology. The proportion of CHV1-infected conidia in the plots was between 0% and 73%, with an average of 44% in November and 36% in April. Repeated sampling indicated a good survival of the pathogen on dead chestnut bark. In the end of the experiment the proportion of CHV1-infected isolates was higher on the cankers of the cut stems compared to the living stems. The result of this study demonstrated a considerable saprophytic activity of C. parasitica. Dead chestnut stems can serve as a source for the dissemination of both virulent and hypovirulent strains of the pathogen and should be taken into consideration for biocontrol measures.

1. Heiniger U, Rigling D, 1994. Annu. Rev. Phytopathol. 32, 581-599
2. Bissegger M, Rigling D, Heiniger U, 1997. Phytopathology 87, 50-59.