engleski

Naturally occurring hypovirulence is necessary for the survival of the chestnut

Cryphonectria parasitica Murill Barr is an introduced ascomycete pathogen that caused serious destruction of chestnut trees (Castanea sativa Mill.) in Europe since its introduction in the first half of the XX century. It has spread throughout the entire chestnut areal in Europe and around the Mediterranean. It was reported in Croatia in 1955 and quickly spread though the country causing canker wounds and dieback of the infected trees. Fortunately, a virus which reduces fungal virulence was also introduced in Europe – Cryphonectria hypovirus 1, which induces a phenomenon called hypovirulence – reduction of the fungal aggressiveness and reproduction capacity: the mycelia no longer expands as quickly, and healing cankers which close the wound are induced. However, C. parasitica populations in Croatia and surrounding countries have a high genetic diversity, which may hinder natural spread of this biocontrol agent. We have selected two chestnut populations infected with C. parasitica and observed the disease progress on the infected trees over the year: Buje in Istria and Hrvatska Kostajnica in continental Croatia. Previous studies have indicated high proportion of naturally occurring hypovirulent fungal isolates in H. Kostajnica – 50.8%, and low in Buje – 12.7%. Our recent observations in 2014 indicate this number remains high in H. Kostajnica: 34.4% and low in Buje: 12.8%. We have also assessed the condition of the trees and cankers in the sampled chestnut populations and found more active cankers in Buje (60.3%) than in H. Kostajnica (42.9%). Over the year the number of active cankers in Buje decreased only slightly to 49.2%, while in H. Kostajnica about 14.5% of cankers could still be considered as active, while many stopped growing, becoming superficial. In H. Kostajnica active cankers increased in size on average by about 5.3% while in Buje by 33.6%. Furthermore, in Buje 11 out of 63 trees died since 2014, while none in H. Kostajnica. In conclusion, the chestnut forest in Istria is in much worse state than in H. Kostajnica, probably because the naturally occurring hypovirulence is not well established and the most aggressive fungal strains are unhindered, destroying the trees.

biocontrol ; chestnut blight ; hypovirulence

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