Cryphonectria parasitica (Murr.) Barr. (Syn. Endothia parasitica (Murr.) And. & And.), the causal agent of chestnut blight, is a fungal pathogen of Castanea and Quercus species. After its introduction at the beginning of this century, C. parasitica virtually eliminated C. dentata Borkh. as an important forest tree species in the eastern United States. A similar result was anticipated when chestnut blight was found in Europe, but the unexpected appearance of transmissible hypovirulence–a unique natural biocontrol phenomenon–prevented the European chestnut (C. sativa Mill.) from succumbing to the blight (for reviews see 5, 51, 69, 92, 101, 103, 130).
Castanea sativa was introduced by the Romans from Minor Asia (79). This tree species was formerly of major economic importance in the mountainous areas of Southern Europe, in the southern foothills of the Alps from Italy into Hungary, and along the Black Sea. It provided timber, firewood, tannin, and litter bed on marginal land, and produced nuts for human consumption and animal forage. Chestnuts are traditionally grown as coppices with a rotation period of 15 to 30 years, as coppices with standards, in high forests, and in orchards. The latter are often formed of old-growth giant trees (100 years is not uncommon), grafted with the local varieties (Figure 1). Socioeconomic changes and the introduction of the devastating chestnut blight (C. parasitica) caused a rapid decline in chestnut cultivation in many regions after the second world war (54, 76).
Natural dissemination of hypovirulence has allowed many chestnut stands to recover. Renewed interest in this tree species and the economic importance of its nuts (3) led to a revival of chestnut cultivation in Europe. Selected nut-producing varieties of C. sativa are now cultivated in plantations.
This review concentrates on the epidemic of chestnut blight in Europe, the biology of hypovirulence, and its effect on the phytosanitary situation in European chestnut stands.

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