Technical Yearbook 2024

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. 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. 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 w as 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

Bleached canes associated with Phomopsis cane and leaf spot. The black dots are fruiting bodies that produce and release spores.

Green shoot with black-brown lens-shaped lesions associated with Phomopsis cane and leaf spot.

Blackening at the node associated with Phomopsis cane and leaf spot.

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. 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.

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TECHNICAL YEARBOOK 2024