Does hot water treatment against Xylella affect the vitality of grapevine propagation material?

Notwithstanding the pathogenicy tests on several cvs proved that grapevine is not host of X. fastidiosa. CoDiRO strain, since 2016 hot water treatment (according with EPPO protocol - Bulletin OEPP/EPPO Bulletin, 42(3), 490-492) was adopted as mandatory preventive measures (Commission Implementing Decision (EU) 2015/2417) for all grapevine nurseries in the infected area of Salento. Even if about 10 milions plants have been treated and commercialized without failings in 2016, several market problems occurred for the fear of viticulturists not of Xylella but of possible direct damages on plants. In order to prove the lack of damages a specific trial was started in 2015 comparing not treated material with 3 HWT protocols (the official one 50°C for 45' and 2 more severe 50° 1h and 55° 45') and using 6 different cvs and 5 rootstocks with 75 plants for each thesis. The preliminary results indicate the lack of direct damage in the two HWT thesis at 50° and reveal just a slight delay (about 1 week) in bud breaking. The treatment at 55° cause direct damages to the main buds and a remarkable delay in breaking of secondary buds. After the 2 recent findings of the Pierce disease's agent (X. fastidiosa. subsp. fastidiosa) in Germany and Spain and considering the efficacy of heat treatment against other pathogens and parasites, those results could suggest to spread the adoption of HTW as a safe and common technique in European grapevine nursery industry.