Chestnut blight caused by Cryphonectria parasitica (Murr.) Barr is wide-spread in northeast Spain. Many fruit orchards and forests have been heavily destroyed and in some orchards the fruit trees are being naturally replaced with pine, as silvicultural measures are lacking since owners have lost their interest in further fruit production due to the violence of the disease attack. At some sites chestnut orchards are converting to forest stands as the result of natural seed-born regeneration. However, at some forest sites infected trees with symptoms similar to those caused by hypovirulent strains, are growing without signs of damage, possibly due to the resistance of these trees. To improve the current situation, field work was performed to study population structure and to search for hypovirulent strains to be used as the biological method in chestnut blight control. So far 13 v/c groups have been found and these may be grouped into two inter-incompatible sets: one in Catalonia (9 v/c types) and the other in Navarra (4 v/c types). Numbers of v/c types varied from 2-5 per site. No hypovirulent strains were observed yet. Virulence tests in field trials were performed to evaluate the effect of site condition and/or possible chestnut tree resistance to the disease. Chestnut trees of different ages were inoculated with various local C. parasitica strains. Inoculation studies showed that the most common v/c type from Catalonia No. 1 (EU-2) is very virulent, causing bark lesion 0.81-2.53 cm(2) per week during the summer and 0.26-0.58 cm(2) during the winter. Some differences in disease development are apparently related to tree resistance.
