Viruses in Forest Trees

Abstract

Many forests around the world have been changed from complex mixtures of woody and herbaceous species to intensively managed plantations consisting of only one or a few species. Many forests, especially in Europe, are no longer wild places. As forests become monocultures of one or several species, and as they suffer from mismanagement, past cutting practices, planting of species off site, etc, they become more vulnerable to the impacts of biotic and abiotic stresses. Declines, caused by abiotic and biotic factors, have been reported in more or less restricted areas for many years (37, 41, 5 1 , 83, 1 08, 1 32). In contrast, in Europe widespread forest deterioration has been reported since the 1 970s (25, 50, 1 55, 1 56, 1 74). Forest decline has become a popular issue, especially in connection with air pollution emanating from industrialized areas. Natural factors, i.e. drought, frost injury, insect damage, and disease, have been considered of minor importance. However, the interaction of interchangeable predisposing, inciting, and contributing factors may lead to gradual or sudden decline ( 1 1 3). Recent findings of viruses and virus-like diseases in forest trees have stimulated us to review the extent and role of viruses in forest ecosys­ tems. We begin with a summary of the viruses reported in forest ecosystems and recovered from forest tree species. Viruses of shade and ornamental species and unsubstantiated reports of virus-like symptoms are not discussed. The