Since 2009 when the Coonawarra Rootstock Trial was planted, management by Wynns Coonawarra Estate has been homogenous across the rootstocks. This has allowed us to observe and measure the inherent differences of the rootstocks in the field over five climatically-different seasons of V15, V16, V19, V21 and V22.

Over this time we have noticed clear and consistent trends in both vigour and yield for the highest yielding rootstocks of 140 Ruggeri and 1103 Paulsen and the lowest yielding rootstocks of M6262, Börner and Own Roots.

This homogenous management does, however, have the downside of biasing the median rootstocks and exacerbating the extremes. Continuing this management style may negatively impact the long term health of these lower vigour rootstocks in particular and may be exacerbating Börner’s dislike for calcareous soils.

It also appears to be causing a decline in the yield rank of the highest vigour rootstocks in more recent seasons, when compared to the other rootstocks in the trial; perhaps indicative of years of excess shading of the fruit zone and also of too few bud numbers for the capacity of the vines under the current management regime. These highest vigour rootstocks were also the least preferred in the small lot wines made in 2019.

We have noticed that the yield rank for two of the three CSIRO Merbein-bred rootstocks – M5489 and M5512, now places them as the highest-yielding rootstocks in the trial, indicative that they are performing well in the Terra Rossa soil under the current management conditions.

Interestingly, the trial has also shown that the traditionally high yielding and high vigour rootstock, Ramsey, is performing differently in this soil type. With the soil structure of the Terra Rossa limiting the ability of the Ramsey root system to develop its traditional tap roots, this rootstock has become considerably contained. It is consistently showing low-moderate vigour and yield compared to the other rootstocks and is showing a decline in yield rank over time.

Given we feel we have a good handle on how the rootstocks in the trial perform under this homogenous management, and indicative of their various parentages, we are now trying to transition the trial into a period of differential management by rootstock, to look to maximise the potential of each rootstock.

In a randomised, replicated trial such as this, differential management will be difficult and we are not certain we will be able to achieve a result we are after. However, we have started our differential management phase by adjusting bud number to try to bring the highest and lowest vigour rootstocks into better balance.

For 140 Ruggeri and 1103 Paulsen, we are aiming to significantly increase bud number on four of nine replicates each and treat the remaining replicates as controls for comparison. For the lowest vigour rootstocks of M6262, Börner and Own Roots, we are aiming to reduce bud number on the same number of replicates, but without compromising spur positions. After two years of these treatments, results have been variable.

A current focus of the trial is also to ensure we are keeping a watching brief on the integrity of the trial as the vines age, to ensure conclusions drawn from the trial by rootstock remain valid. We have recently mapped the incidence of observed virus, Eutypa and dead arm by vine across the seven trial rows and will repeat this study to monitor spread over time. While some interesting observations were noted, these were only from a single baseline year.

Results showed large variation in incidence of Eutypa by rootstock (4% to 32%), with Own Rooted vines showing the highest incidence. This result is worthy of further investigation and evaluation.

There was also a difference noted in visual virus incidence by rootstock across the trial rows with the most vigorous rootstocks showed the higher levels of incidence.

A meeting between the trial partners at the end of this season will thoroughly evaluate all findings and establish a plan for next season to ensure we are continuing to learn all that we can about the rootstocks and their management.

For more information about the Coonawarra Rootstock Trial click here.