Climate change and global warming are reshaping how wine is made. Winters have grown shorter and warmer, and budbreak now arrives earlier in the season. As a result, damage from late frost and hail has become more widespread. Shifts in rainfall patterns increase disease pressure while also making irrigation a necessity. Summer heat brings sunburn and poor color development. Within these environmental changes, a more serious problem is surfacing. It is the possibility that phylloxera could return on the scale of the catastrophe that destroyed Europe’s vineyards in the mid-1800s.
What Phylloxera Actually Is
Phylloxera (Daktulosphaira vitifoliae), also called the grape louse, is an insect native to North America that measures less than a millimeter in length. It parasitizes the leaves and roots of the grapevine, forming galls and feeding on the sap to reproduce. As the galls multiply, root function declines, and the vine eventually dies.
Phylloxera reached France in the 1860s and spread unchecked. The result was devastating damage to the wine industry across the whole of Europe. It then continued to expand its range worldwide, reaching Japan in 1882.
Life Cycle and Control
The life cycle of phylloxera is complex. Some of the individuals that parasitize and multiply on the leaves migrate to the roots, where they begin to reproduce. In time, sexually reproducing individuals emerge among the adults on the roots. These are winged (alate) females, and the two types of eggs they lay hatch into males and females. This generation does nothing but mate. When the resulting eggs hatch, those individuals parasitize the leaves and mature into adults.
In other words, the population on the leaves is a mixture of individuals born by parthenogenesis (asexual reproduction) and individuals born by sexual reproduction. Infection begins on the leaves, and the individuals that build up there migrate to the roots. Populations living on the leaves and on the roots move back and forth between the two, and each adopts the form suited to its location.
Because phylloxera can move between the leaves above ground and the roots below it, treating only one of the two accomplishes nothing, and chemical control is regarded as effectively impossible. More than a century after the first damage appeared, no countermeasure other than the use of rootstocks has been found.
What Control Through Rootstocks Really Means
The use of rootstocks is not an absolute defense against phylloxera, and this point needs to be understood. Rootstocks did stop the vines from dying. But that does not mean phylloxera has disappeared from the roots and leaves. It still parasitizes both. The value of the rootstock lies elsewhere: even when the roots are infested, it prevents fatal escalation of the damage and holds the vine in a stable enough state that it does not die.
The grapevine’s response to phylloxera is divided into three stages. The first is susceptible: the vine is infected easily and suffers severe damage from root injury and deformation together with secondary infection. The second is tolerant: the vine is infected, but growth is not seriously affected, and it can cope with a certain level of infestation. The third is resistant: defense mechanisms limit or block reproduction, and the host is spared damage.
The European species Vitis vinifera is classified as susceptible. The American species used as rootstocks, by contrast, are classified as tolerant or resistant. The important point is that the cultivars used as rootstocks are not necessarily classified as resistant. SO4 and Kober 5 BB, for example, are rootstock cultivars still in common use, yet their classification is tolerant.
Phylloxera resistance trials of these cultivars were carried out in Germany. On a 1-to-9 scale where 1 denotes the highest resistance, SO4 scored 6.1 on the leaf and 3.3 on the root. Kober 5 BB scored 5.8 on the leaf and 3.5 on the root. Both cultivars show relatively high resistance in the roots. Their leaf resistance, on the other hand, is comparatively low, which indicates that they are readily parasitized on the foliage.
Why the Risk from Phylloxera Is Rising
In recent years, the risk from phylloxera has been described as growing, for several reasons. One is that a warming climate makes the reproductive cycle of phylloxera both earlier and more frequent. The insect reproduces sooner and in greater numbers, so populations rise and the concern over expanding damage grows.
A further factor is the reduction in pesticide use, driven in part by the push for organic farming policy. As pesticide levels fall, stress on the insect’s habitat eases, and there is concern that phylloxera may become more active. The increase in abandoned vineyards is cited as a source of the same kind of adverse effect.
Another important factor is the evolution of phylloxera biotypes. Because phylloxera on the leaves and roots multiplies by parthenogenesis, only genetically identical types are maintained. From within that population, however, a sexually reproducing group emerges. In this group, genetic recombination occurs, which raises the possibility that the number of biotypes will increase.
As already noted, some of the cultivars currently used as rootstocks fall into the tolerant category. In cultivars that lack full resistance, a change on the side of the insect could make that resistance impossible to sustain. Such change includes a change in the trait itself through a shift in genotype. It also includes the cumulative growth of damage that comes simply from rising population numbers. In fact, there are reports of phylloxera damage expanding in the mother-vine blocks where rootstocks are grown. Even the resistance of existing rootstock cultivars is coming under threat.
The reports of expanding damage are not confined to rootstock cultivars. The same trend has been confirmed in plots planted with disease-resistant cultivars and traditional cultivars. And much of this damage, it is noted, occurs on the leaves.
The Demand for New Cultivars
To counter a phylloxera threat that shows every sign of expanding, what is now called for is the development of new rootstock cultivars. The emphasis falls in particular on fully resistant cultivars that carry high resistance in both the leaves and the roots.
Research to date has established that leaf resistance and root resistance are conferred by different genetic loci. Among the rootstock cultivars already commercialized, the only fully resistant one carrying both is said to be Börner, registered in 1991. In 1998, two sister cultivars of Börner were registered, but their strong effect on vine vigor made them less favored, and their distribution is limited.
As successors to these, in 2024 two cross-bred cultivars derived from Börner were registered with the German Federal Plant Variety Office (Bundessortenamt) as rootstock cultivars with high phylloxera resistance. In evaluation trials so far, these two, Libero and Vinto, show higher phylloxera resistance than SO4 and Kober 5 BB. At the same time, they match or exceed the existing cultivars on the other criteria as well. On that basis, they have been judged fit for practical use as replacement cultivars.
Why Full Resistance Is Being Demanded
Viewed as a cultivar in its own right, a vine that carries high resistance at every site phylloxera might parasitize is desirable, and the reasoning is easy to follow. It matters in particular for suppressing damage in mother-vine blocks.
Viewed as a rootstock, however, a question remains. A rootstock is grafted to a scion before it is planted out. As a rule, all of the leaves and bunches come from the scion, and the only parts of the rootstock put to use are the roots and a short length of trunk. The roots, which are used as they are, therefore need high resistance. But there are no leaves, so leaf resistance should not be needed.
Moreover, even if the rootstock carries leaf-resistance genes, those genes do not transfer to the scion and express resistance there. Once grafting is done, the rootstock’s own leaves are no longer used. If there is no chance for phylloxera to parasitize them either, there is no need to insist on leaf resistance, at least on the surface. In practice, conventional cultivars such as SO4 and Kober 5 BB tended to show high root resistance but weak leaf resistance.
That line of thinking, however, is itself increasing phylloxera damage. The reason is that infection by phylloxera begins on the leaves in the first place.
The mother-vine block is the typical case. But there are also cases elsewhere where shoots growing from the rootstock portion are left unremoved and allowed to extend, putting out foliage. Because those shoots grow close to the ground surface, they are physically near the roots. As a result, the sexually reproducing individuals born on the roots reach the leaves more easily. And the individuals that reproduce there raise the risk of reaching the scion’s leaves higher up.
The spread of infection on the leaves, it is pointed out, raises the risk of a further airborne infection cycle and of a collapse in resistance. In keeping with this, the spread of infection has been reported in plots of existing cultivars. There are reports, too, that disease-resistant cultivars are highly susceptible to leaf infection. In light of the recent direction of cultivar selection, suppressing leaf infection has become important.
In Summary: The Risk of Looking Only at the Scion
More than 150 years have passed since the phylloxera catastrophe drove the world’s wine regions to collapse. Throughout that time, control of the damage through rootstocks has worked well. That long stretch of calm appears to have dulled people’s awareness of how dangerous phylloxera is.
Some regions prohibit own-rooted planting of grapevines by law. Even so, there are still growers who, when filling gaps in a planting, make use of a shoot extended from a neighboring vine. There are also regions where own-rooted planting is not legally prohibited at all. And there are producers who plant own-rooted vines on land that has suffered phylloxera damage in the past.
The drive for diversity in clones and cultivars is confined to the scion. Voices calling for attention to the rootstock are rarely heard at all.
Behind that, however, the risk that phylloxera is building its numbers is steadily growing. Within the areas surveyed, no marked increase in biotypes has yet been confirmed, according to the reports. But there is likewise no guarantee that the same holds outside the surveyed range.
In growing grapes and making wine, diversity in scion cultivars and clones is certainly important. It is understandable that interest is rising in cultivars suited to recent climate change and warming, or in cultivars that show higher resistance to disease. But if a phylloxera catastrophe were to return, all of those efforts would come to nothing. Whatever scion is planted, the outcome is the same once the vine dies from the roots.
Looking at the scion matters. But the recent trend shows that looking at the rootstock matters even more. At the same time, the irresponsible practices of the past need to be reconsidered and corrected. The climate, the environment, every condition around us is changing. In the middle of that, the assumption that what was safe until now will stay safe from now on no longer holds.
