“1) “”Reduction in yield or quality caused by an accumulation of pathogens and pests in planting material due to successive cycles of vegetative propagation.””

2) “In vegetatively propagated crops, pathogens tend to accumulate if planting material is drawn from within a crop population over multiple generations, resulting in significant quality and yield losses.””

References 

 

“1) Thomas-Sharma, S., A. Abdurahman, S. Ali, J. L. Andrade-Piedra, S. Bao, A. O. Charkowski, D. Crook, et al. 2016. “Seed Degeneration in Potato: The Need for an Integrated Seed Health Strategy to Mitigate the Problem in Developing Countries.” Plant Pathology 65 (1):3–16. https://doi.org/10.1111/ppa.12439

2) Thomas-Sharma, S., Andrade-Piedra, J., Carvajal Yepes, M., Hernandez Nopsa, J. F., Jeger, M. J., Jones, R. A. C., et al. (2017). A Risk Assessment Framework for Seed Degeneration: Informing an Integrated Seed Health Strategy for Vegetatively Propagated Crops. Phytopathology 107, 1123–1135. https://doi.org/10.1094/PHYTO-09-16-0340-R“