Damaged by Viruses And Bacteria
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).
Viruses occur in several gym-nosperms (Chamaecyparis, Cupressus, Larix, Picea and Pinus), angiosperms (Acer, Aesculus, Betula, Carpinus, Cormus, Corylus, Fagus, Fraxinus, Juglans, Populus, Prunus, Quercus, Rhamnus, Robinia, Salix, Sambucus, Sorbus and Ulmus) (Nienhaus 1989; Brandte et al. 2002), in bamboos and palms. Twig increase in horse chestnut (Butin 1995), and witches'-broom on beech and robinia are probably likewise due to the participation of viruses. Viruses have been detected several times in forest dieback sites (Parameswaran and Liese 1988; Winter and Nienhaus 1989; Gasch et al. 1991).
Viroids are infectious agents that consist of a single-stranded RNA. Viroids are smaller than viruses, lack a protein cover and are the smallest causal agents of plant diseases, like discolorations, chloroses and distorted growth, e.g., in coconut, cucumber, hop, potato and tomato (Schlegel 1992; Butin 1995; Nienhaus and Kiewnick 1998). About 33 species of viroids have been identified.
"The Prokaryotes" (Dworkin et al. 2005) is a comprehensive reference on bacterial biology.