When planning their pest management programmes, South African viticulturists typically assume that different types of microscopically small grapevine mites are to blame for the disease symptoms they notice in the buds or on the leaves of their vines. That's not quite so, concludes a recent study in the journal Diversity. Yes, different species of mites are found across a grapevine, but these are by no means location specific, as was previously thought to be the case. These insights come from pest experts of the Department of Agriculture, Land Reform and Rural Development (DALRRD), and researchers from the Department of Genetics at Stellenbosch University and the Agricultural Research Council (ARC).
The grapevine bud mite (Colomerus vitis) is a pest found in vineyards worldwide. At between 0,1 mm and 0.2 mm in size, adult grapevine bud mites are much too small to see with the naked eye. Their eggs and larvae are even smaller.
Lead author Dr Davina Saccaggi who worked for the Plant Health Diagnostic Services of DALRRD while conducting the research, and now works for Citrus Research International.
She says their findings have practical implications for how producers react to grapevine mite infestations. Most tend to only intervene when they notice symptoms in the buds, because the resulting stunted growth of the plant or even bud death could have economic consequences for the harvest going forward.
“The damage is caused when mites feed in the buds. The symptoms only come to light once these start to bud," explains one of the co-authors, Dr Ellenorah Allsopp of the ARC Infruitec-Nietvoorbij, further.
Adds Dr Saccagi: “In the past producers assumed that there's one type of mite that causes bud symptoms and galls, and others that cause symptoms of leaf curling and blisters on the leaves. The general advice was therefore to treat these separately, as being caused by different strains.
“However, our genetic studies have now shown that although there are indeed different types of mites, they are not limited to specific parts of a grapevine. You will find all of them everywhere on the plant, all at once. If you only spray when you see bud symptoms, but ignore blisters on leaves, those same mites could reappear in the buds the next season," she says.
“The different symptoms are therefore all flags for the same problem. Producers should treat them all as part of the same thing, until we know more," adds Dr Barbara van Asch of the SU Department of Genetics.
She says that their genetic studies have shown that there are at least five different species of grapevine bud mites to be found worldwide, with three present in South African vineyards, They also found that all three of these mites can be collected from all parts of the plants.
“No one species can therefore be assigned to a particular grapevine symptom."
Dr van Asch says that it is very likely that further studies will show that there are even more species to be found elsewhere in the world.
“Future study should include the species found on wild grapevine species too," she recons.
The current project was funded in tandem by DALRRD and Winetech.
Dr Saccaggi and Dr van Asch both agree that they would not have been able to gain the insights shared in the Diversity paper if it were not for the mutual support and sharing of genetic and morphological skills between the collaborators.
The project started a decade ago when mites of unknown species were first noticed on grapevine cultivars being imported. Dr Saccaggi was at the time working in DALRRD's Plant Health Diagnostic unit. She started doing the first basic genetic sequencing work. With only a light microscope at her disposal, she struggled to investigate the minute mites' physical properties in detail. At the time, she worked extensively with Dr Charnie Craemer of the Agricultural Research Council, Biosystematics, who is an expert in this group of mites.
“The more we looked, the more complicated things became," she remembers the first frustrating years of the project.
The project gained momentum when she turned to experts at the Department of Genetics at Stellenbosch University.
The expertise of Dr James Wesley Smit of the Electron Microscopy Unit of the Sefako Makgatho Health Sciences University was also roped in. He subsequently produced what is probably currently the best electron microscopy images available of both sexes of microscopic grapevine mites of different species.
The end result is greyscale images of what looks like an armadillo with two pairs of legs placed next to a snout. There are bristle-like empodia or “feather claws" on the front part of each leg.
*Other researchers involved in the Diversity paper were Ms Palesa Maboeei and Ms Chanté Powell of the SU Department of Genetics and Ms Nompumelelo P. Ngubane-Ndhlovu of the Plant Health Diagnostic Services of DALRRD.