2. The rootstock must take into account orchard characteristics such as tree density and canopy size. 3. The rootstock must exhibit good graft compatibility with the scion and this should be well known by both the nursery and grower. 4. The rootstock must be well adapted to the soil conditions, such as calcareous soils, drainage/asphyxia, etc, regardless of the presence of soil pathogens. 5. Finally, once the rootstock has met the above criteria it must be able to tolerate or resist soil pathogens, such as nematodes. Some may have a different view to the process mentioned above, in particular they may place a heavier weighting on characteristics like vigour, or decide soil adaptation is more important than soil pathogens. Regardless, all these issues must be thought of at some stage in the decision process. Does increased genetic vigour equate to increased yield? I would like to highlight one thing about vigour – it’s not directly correlated to high yield, particularly when there are other limiting factors such as soil pathogens. However, when soil adaptation and soil pathogens are non-limiting, vigour is a good indication of resource efficiency and this could achieve greater yield when limited by resources such as water or fertiliser. This may result in more consistent yields from one season to next. Some growers this season have observed plantings with Peach x Almond hybrid rootstocks (generally regarded as high vigour) with a better return bloom than equivalent Nemaguard plantings after a heavy crop last year. Whether this equates to more yield, consistent cropping and less biennial bearing, time will tell. Nemaguard on the other hand, widely considered as moderately vigorous, still performs well and has done for a very long time. The CT Trial for example was planted on Nemaguard at 245 trees/ha, had to contend with a poor pollinator in Ne Plus, yet Nonpareil and Carmel still cropped at approximately 4T/ha at five years of age and averaged approximately 4T/ha from three years of age to 11 years of age. Is it worth taking the risk on rootstocks with RKN susceptibility? I would also like to make a comment on vigour outstripping other limitations, in particular soil pathogens such as nematodes. I regularly hear comments that “(inherent) vigour or extra fertiliser will compensate for a nematode infestation”. The rootstock commonly referred to in this discussion is GF677. Inherent or genetic vigour doesn’t always mean increased yield, but even if it did, vigour is also a function of abiotic (nonliving) and biotic (living) factors. GF677 is genetically a vigorous rootstock but it is also very susceptible to RKN and I do not subscribe to the view that genetic vigour or extra fertiliser will compensate for RKN infestation. Even if RKN infestation could be overcome with extra fertiliser, I find this counterproductive. Figure 1 is a sample of the first roots to be dug up from two random three year old almond trees of the same block in an almond replant site, previously planted with almond seedling rootstock. The galls on the GF677 are RKN infestation and you will notice the
Rootstock characteristics Ideally rootstock selection should consider your specific orchard site characteristics and the results of several years of local field evaluation, but this is not always possible. In the absence of local data, growers will have to rely on overseas field evaluations, original selection criteria of the rootstock breeding programs, other international trials and possibly generic traits of rootstock parentage (Table 1). Table 1: Generalised attributes of parent material used in almond rootstocks
Easy to vegetatively propagate from cuttings. More tolerant of waterlogging and asphyxia. Sometimes has high RKN resistance.
Some graft incompatibility.
Can sometimes produce suckers from below the soil surface.
Susceptible to drought. Difficult to vegetatively propagate from cuttings.
• • • • •
Tolerant of calcareous soils.
Tolerant of drought.
Susceptible to RKN.
High RKN resistance.
Good graft compatibility.
Susceptible to calcareous soils.
Nursery friendly, easy to propagate.
When deciding on a rootstock it is going to be rare, if not impossible, that it’s the “perfect” rootstock. One may therefore ask why bother trying an alternative rootstock? Well the answer is, rootstock characteristics can overcome certain limiting factors (e.g. calcareous soil, nematodes, etc) and this will make a difference in the orchards economic viability. Table 2 provides a summary of characteristics for many of the rootstocks currently available in Australia and should provide a good basis to begin deliberations. It should be highlighted that the data has been produced following the interpretation and results of local and international literature and field trials, and there are always exceptions to the rule. For example RKN tolerance: this assessment may be based on a selection of pathotypes from the country of origin but increased virulence of another local pathotype may exist and the generic claim of tolerance may not hold. Nevertheless, this table is a very good starting point. Rootstock selection process When reviewing Table 2 it may still be difficult to try and decide which rootstock to choose. Selecting a rootstock should involve a systematic objective process and is simply an exercise of elimination. I’ve included five sequential steps below to try and assist the process. 1. The rootstock (and scion) must begin with a high health status from a budwood repository with a record of virus assessment.