The effect of hot water treatment (50°C for 45 min) on fungal pathogens in propagation material and nursery vines
Objectives and Rationale
The aim of the study was to determine the effect of Hot Water Treatment (50°C for 45 min) on the occurrence of fungal pathogens associated with grapevine propagation material and nursery vines in order to ensure complete eradication by these harmful pathogens and to prevent re-colonisation.
1) To determine the effect of HWT (50°C for 45 min) on the survival of fungal pathogens (in vitro).
2) To determine the effect of HWT on the eradication of fungal pathogens inside propagation material.
3) To determine the effect of HWT on the eradication of fungal pathogens inside dormant grafted vines.
Twelve grapevine trunk disease pathogens were subjected to HWT (50°C for 45 min) in vitro to determine mycelial growth and conidial germination inhibition. Dormant, detached rootstocks (“entlote”) were vacuum inoculated with the most heat tolerant isolates, subjected to HWT and incubated in plastic containers. Dormant grafted nursery vines were subjected to HWT, where after isolations were conducted from the roots, basal end of rootstocks, rootstocks and graft unions. Glasshouse pot trials were conducted with rootstock plant shoots (“plantlote”) vacuum inoculated with heat tolerant pathogens (Pa. chlamydospora, Pm. parasiticum, Pm. minimum, Ca. luteo-olivacea and Pl. richardsiae) and HWT’ed before planting.
In the in vitro assays pathogens associated with Petri disease were more tolerant to HWT (50°C for 45 min) compared to pathogens associated with Black foot disease, Botryosphaeria canker and dieback and Phomopsis dieback. Pa. chlamydospora germination and mycelial growth were inhibited at 51°C. Pm. parasiticum and Pm. minimum mycelia grew at 51-53°C, but growth was delayed, whereas spore germination was completely inhibited at 51°C. Ca. luteo-olivacea growth was inhibited at 52°C. Pl. richardsiae mycelia grew up to 60°C whilst germination still occurred at 57°C, although significantly lower. In a detached shoot assay Ramsey, Paulsen 1103, Richter 110, US 8-7 and 143B rootstocks (“entlote”) were vacuum inoculated with Pm. minimum, Pm. parasiticum, Ca. luteo-olivacea, Pl. richardsiae and Pa. chlamydospora, the most heat tolerant pathogens and subjected to HWT (50°C for 45min). Pa. chlamydospora and Ca. luteo-olivacea were not re-isolated from inoculated HWT’ed material. Pm. minimum and Pm. parasiticum were re-isolated from HWT’ed inoculated material, but Pl. richardsiae was the most heat tolerant pathogen. In the glasshouse trials, Pm. minimum and Pm. parasiticum were able to survive HWT of 50°C for 45min, but Pl. richardsiae was by far the most heat tolerant Petri disease pathogen. Regarding the dormant grafted vines obtained from grapevine nurseries, 21 rootstock / scion combinations (9 table, 10 wine and 2 raisin grape) representing 6 rootstock varieties from three nurseries were subjected to HWT and isolations made from 230 dormant nursery vines in total to determine if any fungal pathogens survived the treatment. Phaeoacremonium minimum and Pl. richardsiae were isolated from several HWT’ed combinations. The project was successfully completed.
Key Conclusion of Discussion
Results obtained through in vitro trials clearly show that Petri disease pathogens are more heat tolerant than other trunk disease pathogens. Pa. chlamydospora was inhibited at 51°C, Ca. luteo-olivacea at 52°C Pm. parasiticum and Pm. minimum 53°C and Pl. richardsiae up to 60°C. These results correlate with our observations made from the detached shoot and glasshouse pot trials where Pl. richardsiae, Pm. minimum and Pm. parasiticum were re-isolated form HWT’ed (50°C for 45 min) dormant rootstocks (“entlote” and “plantlote”), as well as HWT’ed (50°C for 45 min) dormant nursery vines where Pm. minimum and Pl. richardsiae (Petri disease) were isolated from several HWT’ed combinations. It is recommended that industry continue using (50°C for 45 min) which was introduced to combat Aster Yellows, if no alternative is practical. In Spain for example the current
recommendation is 53°C for 30 min. Petri disease pathogens infect mother vines through pruning wounds and grafting material throughout the propagation process. It is recommended that “plantlote” undergo HWT before establishing new rootstock mother blocks. Pruning wound protection of mother plants should be made mandatory and sanitation practices, together with HWT, should be recommended during the propagation process.
A new project is highly recommended to investigate the use of HWT in combination with fungicides to improve the efficacy of HWT. This is a strategy that is successfully used in the fruit industries (i.e. citrus) for the treatment of post-harvest fruit diseases. It is believed that heat treatment in combination with fungicides could increase penetration into fruit tissues. This effect could be beneficial to kill fungal trunk pathogen fruiting bodies that might be present on grafting material. Furthermore, a fungicide treatment might also be the only practical option to kill Pl. richardsiae, the most heat tolerant GTD pathogen in this study. The origin of Pl. richardsiae is unknown, although, from previous observations it is most likely a contaminant in nurseries at the grafting stage (Project WW06/33). From diagnostic observations Pl. richardsiae is present in 3.1 – 28.6% of diagnostic samples (ARC Infr-Nietv. 2005-2014) where it is mostly associated with black necrosis in severely affected graft unions.
Take Home message for Industry
HWT (50°C for 45 min) is effective in killing most trunk disease pathogens, and is therefore highly recommended for use by nurseries, but Petri disease pathogens are not completely eliminated by this treatment. However, through the use of an integrated strategy throughout the entire propagation process, including HWT (50°C for 45 min) of dormant grafting material prior to grafting, infections of these pathogens can be limited.