Abstract
This South African wine industry-funded study identified the causal organisms of Phomopsis cane and leaf spot and Phomopsis dieback in Western Cape vineyards. It also assessed the effectiveness of commercially available fungicides against the disease. Results from this research provide critical insights and strategies for managing Phomopsis, ensuring vineyard health and productivity.
Introduction
The increasing occurrence of symptoms of Phomopsis (‘streepvlek’) in Western Cape vineyards prompted the undertaking of this research project. The primary aim of this project was to identify the specific Diaporthe (= Phomopsis) species responsible for Phomopsis cane and leaf spot (‘streepvlek’) and to assess the efficacy of fungicides currently in use. The research sought to clarify the causal organisms and ensure that registered fungicides are still effective, even under favourable conditions for disease development. This article delves into the findings from the recently concluded, industry-funded research project.
Leaf symptoms associated with Phomopsis cane and leaf spot.
Objectives and methods
Identification of causal species
Detailed surveys were conducted in vineyards with typical ‘streepvlek’ symptoms on green shoots, bleached canes and dieback symptoms (Phomopsis dieback). The researchers conducted the surveys in vineyards known for high incidences of ‘streepvlek’ symptoms. Each vineyard had 100 grapevines visually inspected for symptoms.
During the 2016/2017 season, surveys were conducted in 36 vineyards across 18 regions, representing 33 cultivars in Western Cape vineyards. The focus was on identifying brown-black cracks that typically manifest on the basal internodes of green shoots. Subsequently, during the 2017/2018 and 2018/2019 seasons, surveys on bleached canes were carried out in 108 vineyards spanning 15 areas and 18 grape cultivars. Samples of symptomatic green shoots and bleached canes were collected for laboratory analysis. In addition to these surveys, samples were obtained from grapevine propagation material, dormant grafted nursery vines, and internal wood necrosis associated with declining vines (dieback) in vineyards. Diaporthe species were isolated and identified in the laboratory from all the samples collected using molecular techniques. The aim was to identify the responsible Diaporthe (= Phomopsis) species definitively.
Bleached canes associated with Phomopsis cane and leaf spot. The black dots are fruiting bodies that produce and release spores.
Alternative hosts survey
Other fruit and woody hosts around the vineyards were surveyed to determine their status as alternative hosts for Phomopsis. Examples of dieback observed in these hosts were collected and analysed in the laboratory to identify Diaporthe species, determining their potential as alternative hosts.
Pathogenicity confirmation
Pathogenicity studies were conducted to determine the pathogenic status of all the isolated species. This involved inoculating grapevine shoots with isolated Diaporthe species to observe lesion formation.
Fungicide evaluation
Based on the outcome of the surveys and pathogenicity trials, representative isolates were subjected to fungicide screenings to evaluate the effectiveness of commercially available fungicides in controlling the disease. Fungicides currently used in South Africa were first screened in vitro, after which field trials were conducted in two vineyards during the 2019/2020 season to test the selected fungicides under practical conditions.
Green shoot with black-brown lens-shaped lesions associated with Phomopsis cane and leaf spot.
Results
Species identification & pathogenicity
140 Diaporthe isolates were recovered from grapevines and 166 from 25 alternative hosts. A two-gene (ITS and TUB) phylogenetic analysis revealed nine species from grapevine, namely D. ampelina, D. ambigua, D. cynaroidis, D. eres, D. foeniculina, D. novem, D. serafiniae, Phomopsis sp. 5, as well as a new species described as D. nebulae. Notably, D. novem was reported for the first time in South African grapevines, and D. serafiniae and D. cynaroidis were identified in grapevines for the first time globally. Interestingly, D. foeniculina was found as frequently as D. ampelina in dying spurs, the pathogen traditionally associated with the disease.
The survey of typical ‘streepvlek’ symptoms (black-brown cracks on green shoots) conducted during the 2016/2017 season revealed that D. ampelina was associated with 100% of these symptoms. The survey of bleached canes during the 2017/2018 and 2018/2019 seasons revealed that D. ampelina was linked to every instance of bleached canes. This proves that D. ampelina is the culprit behind Phomopsis cane and leaf spot.
Pathogenicity studies conducted as detached shoot assays showed that D. ampelina formed the largest lesions, but D. novem and D. nebulae also formed moderate to large lesions.
Blackening at the node associated with Phomopsis cane and leaf spot.
Alternative host screening
Out of the 13 Diaporthe species identified from alternative hosts, most were recorded on these plants in South Africa for the first time. Interestingly, D. ampelina was not found on any of the sampled alternative woody hosts. However, several Diaporthe species present on grapevines were also found on other woody plants. Some species were exclusively found on these alternative hosts, and their potential threat to grapevines remains uncertain.
Fungicide efficacy
In vitro fungicide screenings determined the EC50 values for seven fungicides against D. ampelina. Field trials demonstrated that when applied preventatively and according to manufacturers’ guidelines, fungicides provided excellent Phomopsis cane and leaf spot control.
Blackening at the base of the shoot is associated with Phomopsis cane and leaf spot. These shoots become brittle and may break, especially if the wind blows.
Key conclusions from the results
Diaporthe ampelina is the causal organism of Phomopsis cane and leaf spot and bleached canes. It is found exclusively on grapevines. While other Diaporthe species have been identified in South African grapevines, they are linked to Phomopsis dieback, a different manifestation of the disease. This form most likely infects pruning wounds, leading to internal necrosis. Effective Phomopsis dieback management involves pruning wound protectants to prevent these infections. Commercially available fungicides effectively control Phomopsis cane and leaf spot in vineyards. However, they must be used as part of a preventative spray program and according to manufacturers’ guidelines.
Diaporthe life cycle and disease spread
Diaporthe ampelina overwinters in pycnidia (flask-shaped fruiting bodies) on canes, spurs, petioles and other dead or diseased grapevine parts. During wet conditions, spores are released from mature pycnidia and spread via rainwater to shoots or other susceptible tissues, leading to new infections. Bleached canes are formed when pycnidia, produced in abundance on these infected shoots, push through the cane surface, allowing air to enter.
Practical recommendations for the industry
Sanitation practices
Remove pruning debris from vineyards, especially bleached canes that harbour Diaporthe fruiting bodies, to reduce inoculum levels. Alternatively, chip the debris finely and incorporate it into the soil.
Management of alternative hosts
Remove dead wood to extend sanitation practices to surrounding woody hosts. This will help prevent Diaporthe species from infecting grapevines and causing Phomopsis dieback.
Fungicide application
Apply registered fungicides against Phomopsis cane and leaf spot as recommended, especially if it frequently rains during bud break and the early growth stages in spring.
Pruning wound protection
Treat Phomopsis dieback as a grapevine trunk pathogen, with pruning wound protection as a key, integrated strategy.
Conclusion
The research highlights the critical need for ongoing surveillance, effective sanitation practices, and the correct use of appropriate fungicides to manage Phomopsis in Western Cape vineyards. By understanding the specific Diaporthe species involved and their pathogenicity, vineyard managers can implement more targeted and effective control measures, ensuring the health and productivity of their vineyards.
Reference
Halleen, F., 2019. The occurrence and control of Phomopsis species causing ‘streepvlek’ symptoms and dieback in Western Cape vineyards. Winetech final report: P04000084.
For more information, contact Francois Halleen fhalleen@villacrop.co.za.
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