Phylloxera, the great plague of Europe is back! The current rootstocks are not, or only partially, resistant to new phylloxera populations. A new crisis seems to be imminent and new rootstocks with a better resistance are desperately needed. Genetic studies seem to offer a solution in the development and realization of rootstocks with a higher resistance to phylloxera.
Phylloxera – also called Daktulosphaira vitifoliae, Phylloxera vastatrix, Viteus vitifoliae or Phylloxera vitifoliae – has a strong preference for the Vitis vinifera vines and affects both the roots and the leaves (Figure 1). This weakens the plant, leaves die off, and roots are deformed. In particular, the damage to the roots causes the grapevine to eventually die within a few years. Phylloxera is native to North America and was at the end of the 19th century accidently introduced in Europe. This caused an almost complete destruction of the wine-growing areas in Europe. However, it was noted that North American Vitis species were not sensitive to these insects. Therefore, to re-establish the vineyards in Europe, these resistant North American vines were used as rootstocks on which the European Vitis vinifera grape varieties (e.g. Merlot, Chardonnay, Pinot Noir, Riesling, etcetera) were grafted. To date, this is used in almost all vineyards in the world to prevent root damage caused by phylloxera.
Figure 1. Phylloxera (Daktulosphaira vitifoliae, Phylloxera vitifoliae). A side view of an adult winged specimen (A), larvae on the roots (B) and eggs from the phylloxera laid in leaf galls (C).
CBG Photography Group, Center for Biodiversity via CC0 (A) and Joachim Schmid (1) (2) via CC BY 3.0 DE (B and C).
Since the 19th century phylloxera has adapted. The widespread use of rootstocks has put a selection pressure on phylloxera, and new – genetically slightly different – phylloxera populations developed that were able to feed and reproduce on the North American rootstocks. To date, seven phylloxera populations – called biotype A to G – are known. Biotype A is the biotype as first characterized in Napa Valley that performs especially well on Vitis vinifera roots. This biotype performs poorly on the AxR1 rootstock (a crossing of Vitis vinifera with the North American Vitis rupestris) and all crossings of the North American Vitis berlandieri and Vitis riparia, e.g. the SO4, 125 AA, 5BB Kober and 420A rootstocks. However, the use of these rootstocks resulted in the emergence of biotype B (in California) and biotype C (in Europe) that are able to feed and reproduce on these rootstocks. This led to replanting most of the vineyards in the 1980s in California due to the failure of the AxR1 rootstock. As a result of these newly emerging phylloxera biotypes, the currently used rootstocks are only partially resistant. This means that adapted phylloxera biotypes can still live on the roots and leaves of the plant. They affect the health of the plant, but rarely cause enough damage to kill the grapevine. However, a major disadvantage of these persevering phylloxera populations is that newer biotypes may emerge and the resistance to phylloxera of current rootstocks will disappear (even further). Therefore, new rootstocks are needed that have a higher, or preferably complete, resistance to the phylloxera biotypes that are present in the geographical area of the vineyard.
Resistance to phylloxera
Researchers try to identify the pieces of DNA in the grapevine rootstocks that are responsible for their resistance to phylloxera. This is useful when making new and more resistant rootstocks. The DNA of a new rootstock can immediately be checked to see if the rootstock contains resistance to phylloxera. This is much faster than field studies that have to show whether resistance is indeed occurring in each generated rootstock. Previous research has already shown that a gene (a piece of DNA) located on chromosome 13 in the Börner rootstock (Vitis cinerea x Vitis riparia) provides an increased resistance to phylloxera of (probably) biotype C, which is the predominant phylloxera biotype present in Europe. This gene originates from Vitis cinerea and was named RESISTANCE DAKTULOSPHAIRA VITIFOLIAE 1 or in short RDV1.
More research into resistance to phylloxera
In Australia, in contrast to Europe, biotype A is the most common phylloxera population found in the vineyards. The North American Vitis cinerea is also resistant to biotype A, but it is not known if this is also due to RDV1 or due to another gene in the grape genome. To investigate which part of the grapevine DNA may provide resistance to phylloxera biotype A, an Australian research group crossed Vitis cinerea C2-50 with Vitis vinifera Riesling and infected the resulting plants with phylloxera biotype A. These plants – the so-called F1 generation – received one copy of their DNA from Vitis cinerea C2-50 and the other copy from Vitis vinifera Riesling. Only the plants that inherited the right piece of DNA from the Vitis Cinerea C2-50 can be resistant. Vitis Vinifera Riesling has no resistance whatsoever and is very sensitive to all phylloxera biotypes. Riesling is therefore an ideal “background” to view the effect of the inherited Vitis cinerea DNA on phylloxera resistance.
Figure 2. The association between DNA of Vitis cinerea C2-50 and resistance to phylloxera. An overview of the entire genome of the Vitis cinerea C2-50, with the peak on chromosome 14 demonstrating the association with resistance to phylloxera (A). A LOD (“logarithm of the odds”; a statistical value) of more than 2.89 indicates that the association is significant. An enlargement of the significantly associated region, the RDV2 gene is located just to the right of the highest peak (and to the right of SNP S14_4196799) but is not shown here (B).
Smith, 2018 via CC BY 4.0
The piece of DNA responsible for resistance to phylloxera biotype A was found by looking at which parts of the DNA were inherited each time resistance occurred in the F1 plants, and were not inherited in the phylloxera susceptible plants. Figure 2 shows this analysis in which a clear relationship can be seen between the DNA on chromosome 14 of the Vitis cinerea C2-50 and the resistance in the F1 plants. The same analysis, but compared to the other parent, the Vitis Vinifera Riesling, does not show any significant associated parts of the DNA (no image shown). As expected, the resistance to phylloxera comes entirely from the DNA of the Vitis cinerea C2-50 and not from the Riesling grape. The piece of DNA that causes the resistance is named (very originally) RESISTANCE DAKTULOSPHAIRA VITIFOLIAE 2 (RDV2). So now there are two known genes – pieces of DNA from the grapevine – that regulate the resistance to phylloxera biotype A (RDV2) and biotype C (RDV1).
How does the winegrower benefit from this science?
Eventually the discovery of these genes (RDV1 and RDV2) may save their (new) vineyard. Upcoming studies will investigate whether these genes also regulate resistance to the other phylloxera biotypes, and whether there are more genes present in the grapevine genome that can provide resistance. What really benefits the winegrower is the cultivation of new more resistant rootstocks by using this knowledge. By selecting new vines for the presence of RDV1 and RDV2 (and possibly other resistance genes) in their DNA, new rootstocks can be grown that are resistant to multiple phylloxera biotypes. Because the resistance genes are known, a genetic technique called “marker assisted breeding” can be used. This technique uses markers to determine if e.g. RDV1 and RDV2 are present in the DNA of the new plant that is therefore resistant to phylloxera. This radically shortens the breeding process of new resistant rootstocks, which may be needed faster than expected. Nobody wants to experience another wine-growing crisis as it happened in Europe in 19th century or in California in the 1980s.
Smith HM, Clarke CW, Smith BP, Carmody BM, Thomas MR, Clingeleffer PR, Powel KS. (2018) Genetic identification of SNP markers linked to a new grape phylloxera resistant locus in Vitis cinerea for marker-assisted selection. BMC Plant Biology 18; 18 (1): 360. https://doi.org/10.1186/s12870-018-1590-0