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The plastic response during cyclic loading of crystalline materials and the methods for determining individual components of the cyclic stress are reviewed. Two basic approaches, the representative volume element and statistical description of the deformation of microvolumes with different critical internal stresses, are compared. Using the theory of thermally activated deformation, the behavior of a single microvolume during loading and unloading is analyzed. Basic features of the statistical theory of the hysteresis loop are described, and thermally activated processes are incorporated. The statistical approach leads to the rapid decrease of the effective stress during unloading and a slow buildup of the macroscopic stress in reverse straining. The analysis leads to a new interpretation of the unloading experiment and the evaluation of the effective stress component as approximately double the value obtained by the classical method. Using the statistical approach in modeling the plastic stress-strain response is emphasized.

Sharka caused by Plum pox virus (PPV) is the most harmful disease of stone fruits in Europe and elsewhere in the world. There is no highly PPV resistant cultivar of plum. Biotech approach has led to the development of resistance through genetic engineering. In this study, we evaluated a transgenic plum Prunus domestica L., clone C5 (cv. HoneySweet), where the PPV resistance is based on RNA interference (RNAi). Resistance in C5 plums has been evaluated for PPV, Prune dwarf virus (PDV), and Apple chlorotic leaf spot virus (ACLSV) in a regulated field trial in the Czech Republic for 12 years (2002–2013). Even under high and permanent infection pressure introduced through graft inoculation of the viruses, PPV has been detected in C5 trees only in several leaves situated close to the point of inoculum grafting in the first nine years. Mild symptoms of PPV disappeared year by year. No PPV symptoms were observed in the last three years and results of ELISA detection were negative. Similar results were obtained, when RT-PCR was used for PPV detection. No natural infection of PPV by aphids was recorded in C5 plums, even if PPV infected plants were growing in the same experimental field in close vicinity. Co-infections of PPV with PDV and/or ACLSV had practically no influence on stability of resistance in C5 trees. Twelve years of field testing in the Czech Republic clearly demonstrated the high level of resistance of C5 either to PPV infection by graft inoculation or natural infection through aphid vectors.