Chestnut culture is affected by two widespread fungal diseases. The main causes of chestnut decline are: Chryphonectria parasitica, responsible of canker blight and Phytophthora cambivora and P. cinnamomi, the ink disease agents. In addition, nuts are often damaged by insect attacks which cause the loss of an important part of the crop.
CHESTNUT BLIGHT
The agent responsible for chestnut blight is an ascomycetes fungus known for a long-time as Endothia parasitica and today classified as Chryphonectria parasitica.
The fungus was probably brought into Europe during the first world war through the chestnut wood used to pack ammunition transported to England and Belgium (Biraghi, 1950), while it was first reported in Italy in 1938 in the Ligurian hinterland from where it spread to the surrounding chestnut woodland area and then to the rest of the country (Biraghi, 1950).
A classic injury parasite of wounds, it penetrates through damaged bark lesions (damage caused by hailstones, natural bark cracks, cuts made through pruning and grafting). The blight attacks all the aerial parts of the tree: trunk, branches and young suckers. At the infection point, the bark is reddened and cracks longitudinally.
The fungus fructification (pycnidia and perithecia) appears under the bark in the form of orange warts and subsequently layers of felty mycelia. New shoots appear on the trunk at the base of the infection points to replace the blighted underlying vegetation.
The extent of tree damage varies according to the fungus pathology and genetic features of the chestnut tree.
Despite the advances in chestnut blight research, it still remains one of the major causes of the degradation of chestnut woodland in many areas.
The disease is controlled by the selection and use of resistant genetic material and in the implementation of preventive measures such as regular removal of infected wood residues by means of the tools used in tree pruning and grafting.
The most significant operations undertaken in chestnut areas for limiting the disease and promoting new cultivation is the restoration of diseased chestnut trees by cutting away the infected branches.
In order to ensure the success of this reclamation work, these operations should be undertaken on entire woods and not limited to sporadic "leopard spot" interventions.
Another potential means of fighting the disease is by biological control and the spread of hypo-virulent fungi strains in the infected chestnut woods; these strains are easy to identify due to their weak pathogenetic effect and the white colour of the micelium (yellow in the virulent strains, white in the hypovirulent).
A particular feature of these types is their capacity for transferring their properties to the normal (virulent) types by means of union of the fungi hypha (anastomosis), thereby making them harmless.
A true disease control programme has been developed using this method which consists of the artificial introduction of hypo-virulent strains in the chestnut woods. Hypo-virulence also tends to spread naturally in many chestnut woods, thereby permitting the favourable evolution of the epidemic and so ensuring the survival of the trees themselves.
INK DISEASE
Ink disease gets its name from the inky black colour of the diseased roots and the visible seepage at the base of the trunk and is, together with canker blight, the most damaging disease of the chestnut tree. The disease has been studied since the last century but the actual pathogenic agent (a fungus) was only identified in 1917 by Lionello Petri, an Italian pathologist.
Phytophthora cinnamomi and P. cambivora, which tend to thrive in humid and poorly drained soils, are the agents of the disease.
The disease manifests itself with rot extended to the roots and the base of the trunk, defoliation, premature drying out of the foliage, unripe husks which remain on the bare branches, leading to the gradual withering and death of the tree.
To prevent the spread and attacks of the disease it is very important to avoid stagnant water areas and to keep the trees well managed and trained.
The control of the diseases involves the use of Euro-Japanese chestnut hybrid rootstocks which however do not always appear suited to the pedo-soils or are not genetically compatible with local varieties. Searching and selection of resistant rootstocks in the European chestnut germplasm may be an alternative solution to the problem, while chemical disease control with systemic fungicides has many economic and ecological drawbacks.
