POPULATION-STRUCTURE AND DISEASE DEVELOPMENT OF CRYPHONECTRIA-PARASITICA IN EUROPEAN CHESTNUT FORESTS IN THE PRESENCE OF NATURAL HYPOVIRULENCE

The Cryphonectria parasitica populations in two 6-year-old European ch estnut (Castanea sativa) coppices were investigated in southern Switze rland over a period of 4 years. Occurrence of white isolates indicatin g an infection with Cryphonectria hypovirus, vegetative compatibility groups (VCGs), hypovirulence conversion capacity, and mating types wer e used to characterize the populations. Sampling of randomly chosen ca nkers in the first year yielded 59% white isolates in one and 40% in t he other population, The distribution of the VCGs and mating types was similar among white and orange isolates, indicating a homogeneous inf ection of the two populations by the hypovirus. Fourteen VCGs were fou nd in the first population, 16 VCGs in the second. Altogether, 21 VCGs were determined. The same three VCGs dominated in both populations, c omprising more than 60% of all isolates. Several VCGs were represented only by white isolates. Five of the six most common VCGs were cluster ed in two hypovirulence conversion groups, with almost 100% hypovirus transmission within each cluster. Repeated sampling of the same canker s in 1990, 1992, and 1994 did not reveal an increase of white isolates . The portion of blighted stems rose from 37% to about 60% in both plo ts within 4 years. In this time, chestnut blight killed 15% and compet ition an additional 21% of the sprouts. Predominantly, sprouts with lo w diameters at breast height were killed. The growth rate of new canke rs was high in their first year and decreased gradually in the followi ng years. A role of hypovirulence in the decline of disease severity w as evident since (i) cankers yielding white isolates grew slower and k illed considerably fewer sprouts than cankers with orange isolates; an d (ii) the majority of the cankers yielded white isolates at least onc e during the 4-year observation period.