Researcher assesses white rot problems in SA vineyards

While Europe only has to cope with one wood rot fungus causing the white rot symptom of the vine disease esca, at least ten species are found in South African vineyards. Three of these species were unknown to science until plant pathologist Dr Mia Cloete recently traced and described.

Dr Cloete receives her doctorate in plant pathology this week during the March graduation ceremony of Stellenbosch University. Her study leaders were Dr Lizel Mostert of the SU Department of Plant Pathology and Dr Francois Halleen of the Agricultural Research Council (ARC).

To the untrained eye it is quite hard to spot the presence of esca in a vineyard. Farmers find that their vines are declining and do not look their best. In some cases, tiger-like stripes appear on vine leaves just before autumn. Only when the vines are sawed open can one see the clear discolouration or rotten areas in the wood tissue.

According to a recent article in the British daily The Telegraph, at least 13% of all French vineyards are affected by esca, and it has already cost their industry about 1 billion Euro. It also occurs in Australia, the USA and South America, and is transferred by a variety of airborne fungal species found in these countries.

“No monetary value has yet been placed on the impact that esca is having on South African vineyards, but farmers certainly do not achieve the 30-year age and bearing capacity of their plantings that they used to,” says Dr Cloete.

Her research forms part of a larger initiative launched by the ARC in the early 2000s to investigate the incidence of esca in South African vineyards, and the fungal complexes that cause them. Samples have since been collected from different vineyards across the country and analysed.

In the process Dr Cloete has described three new fungal species that cause wood rot. Two of these species are found widespread in Western Cape vineyards, while the third occurs in vineyards in the summer rainfall areas of the Northern Cape and Limpopo.

Dr Cloete suggests that most wood rotting fungal species found in South African vineyards occur naturally in the indigenous environment and are conveyed to vines as host plant.

“It is recommended that farmers focus on protecting pruning injuries as a control measure,” she advises.

Dr Cloete has also conducted further tests to ascertain the disease-causing effect of white rot species in two cultivars. All ten species caused white rot in Shiraz and Mourvedre. Mourvedre was furthermore found to be generally more susceptible to white rot than Shiraz.