When an organism or cell is propagated asexually for any period of time it inevitably picks up somatic mutations. These are mutations in genes in cells other than germline cells and they are not passed on by sexual reproduction. These mutations may be silent (not obvious) or cause a change in the phenotype (the organism looks or behaves differently). Somatic mutations may cause cancer or variegation, for example. Many cancers are the result of the acquisition of many somatic mutations, each allowing the cell to escape from normal control mechanisms. I can only assume that John Bryan's 'clonal breakdown' is the actually accumulation of multiple somatic mutations that cause the desirable features of a plant to be lost, or undesirable ones gained. 'Clonal breakdown' cannot be fixed. In the case cited by JB he selected a single individual from the population with desirable (or in reality fewer undesirable) characteristics, put it into TC and propagated it thereafter. It will still have carried forward all the somatic mutations it had previously acquired, and would continue to acquire new ones. Clonal drift is the same process as above. It is a problem in many areas. For example human cells passaged in culture many times will sooner rather than later start behaving differently to the cells that were originally isolated. This has a big impact if those cultured cells are to be used in research because they no longer are representative of the cells that would be found in the body. Continued asexual reproduction, per se, does not cause a 'loss of vigor', that is pure bunkum! It is wrong to think that all viruses cause leaf streaking - infection can have many different symptoms or none at all that are visible. Humans carry many viruses that are silent for some, most or all of the time. One of the aging processes in cells other than stem cells is (as Boyce said) a gradual shortening of the telomeres (the caps at the end of chromosomes). Once they get past a certain point the cell detects this and can become senescent or enters a pathway of programmed cell death (apoptosis). In the devastating disease Progeria, which causes premature aging in humans, the telomeres become prematurely shortened. Thanks, J.