Winetech Technical Yearbook 2022

and only in specific vineyards. Phaeoacremonium scolyti was also detected during this period in the Slanghoek rootstock nursery. No correlation could be found between spore release and specific weather conditions (i.e. rain fall), and therefore farmers must be aware that spores are also released during dry periods. Previous studies conducted in the same Rawsonville and Stellenbosch vineyards identified numerous arthropod species carry ing Petri disease pathogen spores that could vector these to neighbouring plants (Moyo et al ., 2014). The practice of establishing new vineyards in close proximity to old vineyards may lead to early infection during wounding and faster disease development in young vines as spores are aerially dispersed. In the current study, spore traps were also placed in newly established vineyards adjacent to the 24 year-old vineyard in Rawsonville and 30 year-old vineyard in Paarl. In these vineyards, pathogens detect ed in the old vineyards were also trapped in the young vineyards, including Pa. chlamydospora , Pm. minimum , Pm. sicilianum and other trunk disease pathogens. Many Phaeoacremonium species also have wide host ranges, many of which are planted in close proximity to vineyards, and these act as additional inoculum sources (Spies et al ., 2018). This emphasises the need to adopt wound protec tion and control strategies as early as possible. The patho genicity trial conducted with nine Phaeoacremonium spe cies found in South African vineyards for the first time on pruning wounds showed that all the Phaeoacremonium species were pathogenic (Baloyi et al ., 2018). This is also in line with a previous study where Phaeoacremonium species were shown to be pathogenic when inoculated on grapevine trunks and pruning wounds (Halleen et al ., 2007). CONCLUSION Fruiting bodies of Petri disease pathogens were found in several vineyards, as well as rootstock mother blocks in the Western Cape. Spore trapping studies showed that spores are released throughout the year. The high species diversity and frequency of spore release in vine yards and rootstock mother vine nurseries coinciding with traditional pruning practices emphasise the need to develop effective wound protection strategies to avoid in fection of unprotected pruning wounds. The occurrence of Pa. chlamydospora and six Phaeoacremonium species in rootstock mother vine nurseries, highlights the risk of pathogen spread through infected nursery material. Pruning wound protection in rootstock mother blocks and sanitation practices during the propagation process are therefore highly recommended together with similar practices in vineyards. ABSTRACT Petri disease, caused by Phaeomoniella chlamydospo ra and Phaeoacremonium species, is one of the major grapevine trunk diseases affecting young vines in newly established vineyards. Prior to this study, no information was available on the availability of inoculum within South African vineyards and rootstock mother blocks. Not only was viable inoculum sources found within vineyards and rootstock mother blocks, but a positive correlation was found between spore release and susceptibility periods during pruning and suckering activities when spores infect wounds. These results highlight the importance of sanitation practices in nurseries and vineyards, as well as the necessity to protect wounds. ACKNOWLEDGEMENTS AND REFERENCES https://www.wineland.co.za/petri-disease-when-and how-do-western-cape-vineyards-get-infected/

Example of an artificially inoculated Cabernet Sauvignon pruning wound several months after being inoculated with a Pheoacremonium spore suspension. Notice the brown to black streaking in the xylem originating from the wound.

Phaeomoniella chlamydospora fruiting bodies (pycnidia) forming in aggregates on grapevine bark (left). A closer view of the globose fruiting bodies (right).

Phaeoacremonium flask-shaped fruiting bodies (perithecia) with long necks. The fruiting bodies are sometimes hidden in deep cracks and crevices and therefore produce long necks to release their spores (left). The spores are released in sticky droplets that form at the neck opening on the tip (right). This is also an adaptation for arthropods to pick up spores and to vector these pathogens to new hosts.

Vaseline-coated microscope slide spore traps affixed to arms of infected vines (left) and visually healthy rootstock mother vines (right). The slides were replaced weekly and fungal spores were retrieved from them, cultured, counted and identified. No tice all the old pruning wounds with cracks and crevices. These are typical hiding placed for fungal fruiting bodies that produce and release spores into the air to infect nearby plants.

For more information, contact Francois Halleen at halleenf@arc.agric.za.

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WINETECH TECHNICAL YEARBOOK 2022