Hello Monica, Thank you for your reply. I think you problem is different from mine but maybe this helps: The term 'gnats' is difficult to translate for me, I looked it up and maybe it is comparable to an infection with Euopean Sciaridae insects. One variant of these tiny insects causes damage in moist conditions, the larvae feed on tender roots and destroy seedlings this way. In seed trays or propagators they find the right conditions: moist and warm. In the past they used to be treated with insecticides but with little success, mainly due to quickly developing resistance. We now treat them with Steinernemia feltiae nematodes or Bacillus thuringensis israelensis. I tried the nematodes with very good result but not the Bacillus. I use the Bacillus on all kinds of caterpillars with excellent results. Both are not harmful to humans or pets or useful insects and are certified for ecological agriculture and are available commercially in Europe. Some of the 'gnats' are also part of the Sciaridae family so maybe these treatments might also work for you. I will further consider Dylan's advice on checking the compost moisture of stored dormant tubers next winter. Maybe it is a fungus that enters the tubers before they go dormant, Begonias have lots of fleshy stems that often turn into a mush when the plant goes dormant. The plants shed these dying shoots but maybe the damaging agent is already in the tuber before the shoot is shed. Thank your for your advice and ideas about this topic Uli.