South Africa Wine Technical Yearbook 2025

DECEMBER

Grapevine-associated Aspergilli By Klaas Coetzee

There is a wide range of fungi that can infect grape berries and cause a resultant reduction in wine quality. Botrytis cinerea , black Aspergilli , Penicillium spp., Colletotrichum spp., Greeneria uvicola and Phomopsis viticola are examples of such fungi. Following the launch of the European Green Deal on 19 December 2019, a greater focus was placed on alternative crop protection to reduce the use of chemical agents. Aspergillus is considered one of the most significant diseases due to its substantial impact on weight loss for producers, as well as quality losses on wine cellar side. The loss in wine quality can be attributed to the black spores that remain suspended in settling tanks, resulting in lower recoveries. The production of ochratoxin A, and also Aspergillus’ association with Gluconobacter, which can convert glucose to acetic acid, is problematic due to the legal limits products like juice and wine must adhere to. This article focuses on black Aspergilli. The species grouped under this genus due to their black appearance, which is also visible in vineyards, include Aspergillus niger , Aspergillus tubingensis , Aspergillus welwitschiae , Aspergillus uvarum , Aspergillus awamori , Aspergillus aculeatus and Aspergillus carbonarius. Although Alternaria spp. and Rhizopus do not form part of the Aspergillus genus, black spores are also characteristic of the presence of these fungi. The Aspergillus genus occurs in various climatic regions worldwide. An Italian priest and biologist, Pier Antonio Micheli, recorded having observed the fungus under his microscope as early as 1729. The form of the fungus or mould reminded him of the aspergillum (holy water sprinkler) used in the Roman Catholic Church. Aspergillus is mainly asexual, with approximately one-third of the species that can reproduce sexually. Asexual reproduction in Aspergillus creates favourable conditions for rapid germination and distribution of spores. Aspergillus species grow under conditions of high osmotic pressure, for example, where there is a high sugar concentration and sufficient oxygen. The fungi also have an affinity for starch and often cause contamination of bread and potatoes. Aspergillus oryzae is used to break down starch to sugar that is fermentable to alcohol, as in the case of Saki preparation. A further challenge of black Aspergilli for wine production is the ability of the fungi to produce ochratoxin A. Ochratoxin A is a possible carcinogen and is controlled under the European Commission Regulation 2023/915. This regulation sets a maximum permissible level of 2 μg/kg for grape juice and wine produced for human consumption.

Aspergillus on Sauvignon blanc.

Black Aspergilli on the vine and bark.

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TECHNICAL YEARBOOK 2025