The aim of the current study was to identify Petri disease pathogen inoculum sources in vineyards and rootstock mother blocks and to determine if, and when, spores are released and whether spore release events coincide with susceptible periods in vineyards and mother blocks.
Petri disease is a serious problem in grapevines affecting grape production worldwide including South Africa. The disease is caused by xylem inhabiting fungi Phaeomoniella (Pa.) chlamydospora, Phaeoacremonium (Pm.) minimum and several Phaeoacremonium species and usually affects young vines in newly established vineyards. Symptoms include stunted growth, shortened internodes and dieback. Internal symptoms include black to brown spots (in cross-sections) and brown streaking in the xylem tissues (in lengthwise cuts). Petri disease pathogens spread within vineyards as aerial inoculum, which originates from fruiting bodies. Pruning wounds are known host ports of entry for aerial spores of these pathogens. However, knowledge is lacking on the occurrence of these fruiting structures within South African vineyards, as well as its possible contribution towards aerial inoculum. At the onset of the current project, only limited information was available from studies conducted in France, California and Australia.
Materials and methods
Grapevine wood pieces were investigated for fruiting bodies. Spore trapping studies were conducted in six 24 – 40-year-old vineyards (two each in Stellenbosch and Paarl, and one each in Rawsonville and Durbanville) and two rootstock mother blocks (19-year-old in Slanghoek and 17-year-old in Wellington) over two seasons. Pathogenicity studies with newly found Phaeoacremonium spp., or species that have not been subjected to pathogenicity studies in local vineyards, were conducted on field-grown Cabernet Sauvignon vines by inoculating pruning wounds.
Results and discussion
During the study Pm. minimum perithecia and Pa. chlamydospora pycnidia were found on trunks and cordons of vines in several vineyards, as well as the “heads” of mother vines in rootstock mother blocks. Perithecia were found in wood crevices and old pruning wounds, while pycnidia were found in crevices and cracks, as well as on bark and old pruning wounds.
The spore trapping studies, which were conducted from mid-March to the beginning of December over two seasons, revealed for the first time the presence of aerial spores associated with Petri disease in South Africa. Phaeomoniella chlamydospora and a total of 14 Phaeoacremonium species were identified, including Pm. australiense, Pm. griseo-olivaceum, Pm. griseorubrum, Pm. inflatipes, Pm. iranianum, Pm. italicum, Pm. minimum, Pm. parasiticum, Pm. prunicola, Pm. scolyti, Pm. sicilianum, Pm. subulatum, Pm. venezuelense and Pm. viticola. Of these, only Pa. chlamydospora, Pm. minimum and Pm. inflatipes have been reported as aerial inoculum within vineyards in other countries. This is by far the greatest diversity in Petri disease pathogens ever recorded in vineyards worldwide.
The highest number of pathogen species found in a single vineyard was eight and six in a rootstock mother block. Two pathogen species, Pa. chlamydospora and Pm. minimum, were trapped in all the vineyards and rootstock mother vine nurseries during both seasons of the study. One of these two pathogens was the predominant species detected in all vineyards, except in Rawsonville where Pm. sicilianum was the predominant species. Similar to previous isolation studies conducted in South Africa, Pm. parasiticum appears to be the most common Phaeoacremonium species associated with grapevine after Pm. minimum, as this species was detected in four and six vineyards, respectively, during the two spore trapping seasons.
Petri disease pathogens were detected throughout the trapping periods in all the vineyards. Spore release coincided with winter and spring (i.e., suckering) pruning activities. Phaeomoniella chlamydospora and/or Pm. minimum spore release events occurred during the week of pruning or within four weeks after pruning, and rootstock cane harvesting in all the vineyards and rootstock mother blocks. Grapevine pruning wounds remain susceptible to Petri disease pathogens for four to sixteen weeks after pruning. Phaeoacremonium sicilianum and Pm. parasiticum spores were also detected after pruning in Rawsonville and one vineyard in Paarl, respectively, in both seasons. Phaeoacremonium subulatum, Pm. scolyti (Rawsonvile), Pm. prunicola (Stellenbosch), Pm. iranianum (Paarl) and Pm. inflatipes (Durbanville) were also detected during the susceptible period, but during only one of the seasons 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., rainfall), 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 carrying Petri disease pathogen spores that could vector these to neighbouring plants.
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 detected 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. This emphasises the need to adopt wound protection and control strategies as early as possible.
The pathogenicity trial conducted with nine Phaeoacremonium species found in South African vineyards for the first time on pruning wounds showed that all the Phaeoacremonium species were pathogenic. 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.
Black vascular streaking associated with Phaeomoniella chlamydospora wound infection in a grapevine rootstock (left). In a cross-section, the occluded xylem vessels can be seen as black spots (right).
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. Notice 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.
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 vineyards and rootstock mother vine nurseries coinciding with traditional pruning practices emphasises the need to develop effective wound protection strategies to avoid infection 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 is therefore highly recommended together with similar practices in vineyards.
Petri disease, caused by Phaeomoniella chlamydospora 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 were 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.
The authors thank Winetech (Project WW06/41), ARC Infruitec-Nietvoorbij, National Research Foundation and THRIP (Project ID TP2009072900007) for financial support.
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– For more information, contact Francois Halleen at firstname.lastname@example.org.