AN EPIDEMIC OF GNOMONIA PASCOE ON NUTS OF CASTANEA SATIVA IN THE CUNEO AREA

Abstract

Since 2005, chestnut growers have observed increasing losses in yield in some sweet chestnut orchards lying to the South of Cuneo. In 2007-2008 a health survey was carried out on branches, flowers and developing nuts. A fungus was constantly isolated from the bark, but not from the xylem, of 1 to 3-year old branches, fruit stem, burrs, and with a variable frequency from the stigmas and styles. The fungus could not be isolated from the inner layer of the hylum. It was isolated sporadically from the nuts just after the fruit setting and with frequency increasing 25-80% in 2007 and 4-84% in 2008, in completely ripened nuts. In the diseased nuts, small chalky areas originate at the ripening time from the endosperm outer layers, and develop towards the centre of the kernel; progressively their color turns brown and the endosperm mummifies; the pathogen colonizes the pellicle and the shell on which it produces 122x85 - 251 x 171 μm, globose, black, pycnidium-like acervuli, that split open along the top and release a slimy, globular mass of conidia. Conidia are pale brown, one-celled, ovoid-oblong, biguttulate, 6.1-7.32 x 2.44-2.68 μm. This fungus, being able to colonize asymptomatically the branches and the immature nuts, has an endophytic life style with a true pathological behavior only on the ripened nuts. It is likely that it is unable to colonize the endosperm through the stem. It reproduced the disease in artificially inoculated healthy nuts, and endophytically colonized artificially inoculated chestnut branches. On the basis of such characteristics, the pathogen has been identified as Discula pascoe sp. nov., the anamorphic stage of Gnomonia pascoe sp. nov., a pathogen recently reported in the rainy areas of Australia and New Zealand. The territorial distribution, the epidemiology and the systematic characterization of the pathogen through phylogenetic analyses by unilocus and multilocus markers are under investigation.