Wineland Article July 2008: Impact and extent of phytoplasma disease still a grey area

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Impact and extent of phytoplasma disease still a grey area
special report: Prof Johan Burger
The occurrence of Aster Yellows phytoplasma infection in a vineyard in the Slanghoek region has recently been confirmed at Stellenbosch University, linking it to disease symptoms in vineyards of the Olifants River Valley.

Typical symptoms associated with Aster Yellows infection in grapevine: yellowing of leaves, incomplete ligninfication; short internodes and abortion of immature bunches.

This, according to Prof Johan Burger of Stellenbosch University’s Genetics Department, who said that these were the first reports of the occurrence of the Aster Yellows phytoplasma in South African vineyards.
“The importance and impact of this discovery remains to be determined,” added Burger.
In a special report for WineLand, he said that grapevines throughout all the major viticultural regions of the world suffered from one or more of the various manifestations of Grapevine Yellows disease and one specific type had ”been reported to cause massive yield losses in European wine producing countries”.
“Until recently, South Africa was a rare exception with no grapevine phytoplasma diseases reported at all.”
WineLand reported in our previous edition that Plant SA had confirmed in a press release that researchers at the US had diagnosed a phytoplasma disease in grapevines in South Africa that causes the grapevine disease Aster Yellows. It also stated that the Directorate of Plant Health of the National Department of Agriculture had emphasised that the presence of this disease held no danger to man, animal or the environment. “In cases where control measures are applied effectively, vineyards can recover completely and the disease will have no effect on the grapes or the quality of the wine.”

Grapevines throughout all the major viticultural regions of the world suffer from one or more of the various manifestations of Grapevine Yellows disease ... Until recently, South Africa was a rare exception with no grapevine phytoplasma diseases reported at all.

Prof Burger and Mandi Engelbrecht of the Department of Genetics have worked together with Jeff Joubert of VinPro to monitor vineyards with typical phytoplasma symptoms during the past two growing seasons.
Burger reports as follows:
Detection of phytoplasma in grapevines in the Western Cape
Early during the 2005 growing season, unfamiliar disease symptoms were observed in vineyards of the Olifants River Valley. Researchers at the Genetics department of Stellenbosch University have now linked these to infection by an unusual new pathogen of grapevine in South Africa.
What are phytoplasmas?
Phytoplasmas are unculturable, wall-less, eubacteria that are obligate parasites of most crop plants, and can cause devastating economical losses. Most phytoplasma infections result in characteristic ‘yellowing’ or ‘witches broom’ symptoms, but are not limited to these. Phytoplasmas are transmitted by phloem-feeding insects, predominantly by leafhoppers. Traditionally, phytoplasmas have been classified according to their host range and the symptoms that they cause, but current classification is based on the genomes of these organisms, and specifically on their 16S ribosomal RNA gene sequences; with 20 major phylogenetic groups that have been proposed.
Grapevines throughout all the major viticultural regions of the world suffer from one or more of the various manifestations of Grapevine Yellows disease. The three major grapevine phytoplasma diseases are Flavescence Dorée, Bois Noir and Grapevine Yellows. Especially Flavescence Dorée has been reported to cause massive yield losses in European wine producing countries. Until recently, South Africa was a rare exception with no grapevine phytoplasma diseases reported at all.
Situation in the Olifants River Valley
Grapevines displaying symptoms consistent with phytoplasma disease were noticed by Jeff Joubert during routine inspections of vineyards in the larger Vredendal region of the Olifants River Valley. Symptoms included yellowing of leaves, incomplete lignification of shoots, shortening of internodes, and the abortion of growth tips and immature bunches.
Symptomatic grapevine material from the red varieties Merlot, Shiraz, Cabernet Sauvignon, Ruby Cabernet, Pinotage, and Corinth and the white varieties Chardonnay, Colombar, Chenin Blanc, Sauvignon Blanc, Sultana and Regal were collected for phytoplasma screening during the early growing season (November) of 2006 and 2007 from several farms in the Olifants River Valley.
Molecular diagnostic tests
Phytoplasmas are notoriously difficult to detect because of low concentrations and the uneven distribution of these organisms in different tissues of host plants. Grapevine samples were therefore tested using the very sensitive polymerase chain reaction (PCR). DNA was extracted from all samples and analysed using two rounds of PCR. Since the PCR test was designed to detect all phytoplasmas, the identities of all positive reactions were determined by DNA sequencing.
Of the 32 samples tested, 22 were positive, however, only one phytoplasma species, Aster Yellows, was detected in all 22 samples. This phytoplasma is unrelated to the typical grapevine phytoplasmas such as Flavescence Dorée, Bois Noir and Grapevine Yellows. Diagnostic results from the two seasons correlated well, but a few vineyards that tested negative during the 2006/7 season tested positive during 2007/8, probably indicating spread of infection.
Future research
Observations of the South African Grapevine Yellows disease over the past three seasons suggest that the disease is spread by a biological vector; probably a phloem-feeding insect. A research project to find this vector as well as to determine whether the disease occurs in other wine producing regions of the Western Cape, has already begun.
Researchers from Stellenbosch University and the ARC, supported by Winetech, collaborate in this project. Aster Yellows phytoplasma infection in a vineyard in the Slanghoek region has recently been confirmed at Stellenbosch University. – by prof Johan Burger, with contributions by Mandi Engelbrecht (SU Genetics) and Jeff Joubert, VinPro Vititicultural Consultation Services.

Limited incidence in the Breede River
WineLand attended a VinPro post-harvest information session in Worcester where Jeff Joubert (VinPro) and Roleen Carstens addressed the matter of phytoplasma in the Breede River region. EDO HEYNS reports.
Phytoplasma has been detected in the Breede River valley, but this area has not been affected to the same extent as the Olifants River area.
Joubert said that phytoplasma is similar to leafroll and Shiraz disease in the sense that it is virus-like, but doesn’t show the yellowing symptoms throughout the vine.
He says that the industry does not have all the answers on how to manage the disease, because the vector has not been identified yet. The following precautionary steps were, however, suggested once the vector has clearly been identified:

Removal of infected vines
Management of host weeds
Monitoring of leafhoppers
Use of pesticide
Joubert warned producers to be careful when considering the use of pesticide, since this might worsen the situation if done before proper research on the vector and possible pesticides has been done.
Carstens said that phytoplasma has about 200 - 300 known hosts, including several vegetable crops.
When referring to grapevines, Chardonnay seems to be the most vulnerable although most varieties, including home-grown Pinotage, have already been affected. Rootstock varieties are also susceptible and pose a threat as they do not show symptoms and function as a latent host of the disease.
Phytoplasma can also spread through grafting, but plant material can be cleared by warm water treatment. She suggested that nurseries should evaluate their mother blocks to prevent further infection.

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