Viruses Viruses are small particles (1.0-2,000 tun in size) that infect rtikaryoles as obli gate intracellular parasites. They reproduce by invadiny and (Ain), over other cells as they lack own metabolism and the machinery for sell reproduct ion (Nienhaus 1985a). Typically, they carry either DNA or RNA surrounded by a coat of protein or protein and lipid. Plant viruses penetrate the shoot, leaf tissue and root via wounds or they are transferred by vectors I aphids, ci-cadas, nematodes, among fungi: Sphaerotheca lanestris (Erysiphales) on oak I. Partial bleaching of chlorophyll results in angular, circular (mosaic) or dif-fuse chloroses. Leaf damage, dwarfing or growth inhibition, distorted growth, and necrotic areas or lesions can occur, that is, virus infection can reduce the tree growth. Over 1,000 virus diseases of plants are described for Europe. Virus diseases in forest trees have been summarized e.g., by Nienhaus and Castello (1989) and Cooper and Edwards (1996)
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