A SATI-funded research project tested disinfectants and fungicides in combination with hot-water treatment to control Petri disease in plant material. Petri disease is one of several trunk diseases resulting from fungal infections of the woody parts of grapevines. It usually affects young vines, leading to poor growth, gradual decline, partial dieback and premature death.
In South Africa, Petri disease is caused by Phaeoacremonium species, Phaeomoniella chlamydospora, Pleurostoma richardsiae, and Cadophora luteo-olivacea. “Phaeomoniella is probably the most common,” says Dr Minette Havenga, researcher at the ARC Infruitec-Nietvoorbij. “But during the last 25 years, Pleurostoma was often associated with graft unions of young diseased vines analysed by the Diagnostic Service at Nietvoorbij.”
Previous SATI- and SA Wine-funded research conducted by Prof Francois Halleen, then a researcher at the ARC Infruitec-Nietvoorbij, showed that Pleurostoma is more heat tolerant than other trunk-disease pathogens, surviving hot-water treatment at 50 °C for 45 minutes.
The new SATI-funded project was launched to investigate whether disinfectants and fungicides, in combination with hot-water treatment, could offer better Petri disease control. The project was led by Halleen and conducted by MSc student Leander Engelbrecht from 2021–23 and led by Havenga in 2024.
Choosing the right weapons
The first phase of the research aimed to identify effective fungicides and disinfectants against Petri disease pathogens. Based on the literature, their mode of action, and their spectrum, 17 fungicides and one disinfectant were selected for screening. As Pleurostoma is the most likely Petri disease pathogen to survive hot-water treatment, it was used as the guinea pig in these trials.
Engelbrecht inoculated the fungus on agar media containing different concentrations of each fungicide or disinfectant and measured the resulting fungal growth after 10 days. He also exposed Pleurostoma spores to different concentrations of each fungicide or disinfectant and evaluated spore germination after 24 hours. “We eliminated all products that weren’t effective at the registered dose and not registered on grapevines,” says Havenga. “We selected six products to conduct laboratory trials with detached shoots.”
The detached grapevine shoots were inoculated with Pleurostoma spores and immersed in 50 °C water for 30 minutes, followed by 30 minutes in cold water. Engelbrecht tested each of six fungicides or disinfectants in either the hot-water or cooldown bath and compared them with controls. After the treatments, he assessed the presence of Pleurostoma in shoots that were incubated for seven days. Pyraclostrobin and DDAC (didecyldimethylammonium chloride) in the hot-water bath and captan and carbendazim in the cooldown bath were associated with the lowest survival of Pleurostoma. Engelbrecht also tested DDAC and captan in a cold bath against the other Petri disease pathogens.
The best performers were either DDAC alone or in combination with captan. Field trials were conducted by Engelbrecht in 2022–23 and by Havenga in 2023–24. In both seasons, scion (Sweet Celebration™) and rootstock (Ramsey and 1103 Paulsen) material was subjected to different treatments, grafted and planted in a commercial nursery. After eight months, the young vines were lifted, the percentage take determined, and a sample of vines was analysed for root and shoot mass and fungal infections. A total of 5 760 treated nursery vines were evaluated in season 1 (Table 1).
In treatments where DDAC was added, the DDAC concentration was 50 ml per 100 ml of water. Table 2 shows the treatments for the second season, and in instances where DDAC was applied, the concentration was 100 ml per 100ml of water, except for treatment 12. Treatments 1 -10 were done on the same day using clean water in the hot water baths. Treatments 11 – 12 were done the following day using the dirty water from the previous day.
The results after two seasons
There were no significant differences between the percentages of certifiable vines or the root or shoot masses for the various treatments. Hot-water treatment with or without adding fungicides or disinfectants did not negatively affect the vines. Chemicals in either the hot-water or cooldown bath reduced Cadophora in season 1.
Very few Cadophora infections were found in season 2. “We relied on natural infections in the field trials, and Cadophora was the least prevalent Petri disease pathogen in season 2,” comments Havenga. In season 1, all treatments that included a hot-water bath reduced Phaeoacremonium.
However, in season 2, Phaeoacremonium was decreased only in the treatments where captan with or without DDAC was added to the cooldown bath. The highest prevalence of Phaeoacremonium was in the hot-water treatment using dirty water and the untreated control. This underscores the importance of frequently replacing the water in the baths. Hot-water treatment with or without fungicides or disinfectants controlled Phaeomoniella. “In previous studies, we isolated fungi from mother material and found Phaeomoniella chlamydospora to be the most prevalent in the canes,” says Havenga.
None of the treatments controlled the heat-tolerant Pleurostoma. In some cases, its prevalence was higher when fungicides or disinfectants were added to the hot-water treatment than when it was done with clean water only. “This project was created because hot-water treatment doesn’t control Pleurostoma,” says Havenga. “But Pleurostoma is not acting as we would expect it to.” She speculates that this could be because Pleurostoma infects graft unions during the grafting process, as previous results have shown that Pleurostoma is rarely present in propagation material.
Although hot-water treatment (50 °C for 30 minutes) with or without added fungicides and disinfectants failed to control Pleurostoma, hot-water treatment remains an effective weapon against most grapevine trunk diseases, including black-foot disease, Botryosphaeria, Phomopsis dieback, and Phaeomoniella – the most prevalent Petri disease pathogen. In addition, hot-water treatment (50 °C for 45 minutes) controls aster yellows.
