Farmers and home growers don’t always want the same qualities in an avocado tree. But both value efficiency.
A farmer wants to get many pounds of avocados out of a given area of land. Likewise, a home grower wants a lot of avocados out of the precious yard space that is dedicated to an avocado tree.
At last week’s seminar of the California Avocado Society, Mary Lu Arpaia reported on a research project involving different varieties and rootstocks that has been going on for a dozen years at the Jim Lloyd-Butler Ranch in Saticoy.
I think that the findings will interest home growers as much as farmers:
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Man, I love avocado research! So much work over time to achieve results that are significant OR non significant. Science values either, of course.
I have been running my own amateur backyard research project, spraying very dilute urea (a teaspoon of urea in two quarts) every evening on flush leaves of a Hass, a Gem, and a Fuerte and getting interesting results. Even slightly higher urea concentration damages leaves, but they don’t die except on the tips. I have two new avocado saplings – a Pinkerton and a Lamb – that I spray every day. They just keep flushing! And I have observed gigantism in certain leaves. I am having so much fun for so little money. I don’t need no USDA grant.
You’ve been doing this dilute urea spray for so long that you must be the expert now. You need to write up your methodology and observations soon!
I’m still seeing new phenomena such as gigantism on select leaves. As we know from the literature, the thick cuticle on adult leaves prevents absorption of urea. It looks as though certain leaves prolong their young flush permeability by rapid growth. This creates a “virtuous circle” of additional uptake and continued expansion of leaf area.
Also seeing “endless” flushing in two young trees delivered in May and flushing all summer. Lacking a thick cuticle, the new leaves are open to force feeding.
But maybe they would have performed similarly without the urea? Alas, true research requires control populations and measurements!
I respect Mary Lu a ton, but viewing avocado trees as “factories” is a bit disturbing to me. I don’t think we should be competing with everyone else. I believe we should produce the best quality and most healthy avocados that we can. Those trees seemed to be lacking in mulch to me!
Hi Nick,
There are definitely other things to consider than production efficiency. I would like to have it all in my trees, too: highest quality fruit grown in the most healthy and natural way possible, and lots of it per tree (size).
Yes to more mulch!
The factory metaphor can be taken too far, for sure. But here in Southern California we’re living on expensive land with limited, expensive water, which makes me feel (even as a home grower) that I can’t afford many trees that don’t earn their keep. So I do appreciate the efficiency aspect of Mary Lu’s message.
Just to mention one other thing to consider besides yield efficiency: harvest season. Reed may not match GEM in efficiency, but Reed remains the best avocado for the summer season, after GEM’s harvest season is finished. So they are not even competitors in that sense.
I hear you. My initial criticism was a bit harsh.
No, I think the angle of your comment is an important one to keep in mind. Plants aren’t just widgets, and we need to remember that too.
Hi Nick,
I agree 100% that we need to produce the best quality and most healthy avocados. This has been a major research and extension activity of mine since I came to Riverside. We need this so we can compete with the onslaught of imported fruit. We are just a small part of the US market these days and in order to maintain a viable industry we need to have the highest quality possible going into the market. But to pay the bills we need more efficient trees. Thus the aim and goal of the varietal breeding program: equal or exceed Hass quality with new varieties but beat Hass in terms of environmental inputs and response to plant stress.. This is what i was trying to get at with the angle that we need to view the avocado tree as a factory.
This is super interesting! Thanks for posting! As a statistician, I automatically wondered if she mentioned the interaction between variety and rootstock at some other point in the presentation. Was it statistically significant? In other words did the effect of variety on yield efficiency differ significantly by rootstock? (Equivalently, you could ask whether the effect of rootstock on yield efficiency significantly differed by variety.) If so, I wonder what combinations of variety and rootstock produced the greatest yield efficiency? It appears that she only reported the effect of variety averaged across rootstocks and the effect of rootstock averaged across varieties (i.e., the main effects of variety and rootstock, in statistical terms) in the segment of the presentation you posted.
Hi Holly,
This is exactly what I wondered when I first heard Mary Lu report on this trial in 2018. When I asked her at that time, she said she saw no significant differences. But she didn’t talk about this aspect in the above presentation and I didn’t ask about it again.
But on this point, an Israeli researcher named Ben-Ya’acov did many studies related to specific rootstocks and scion varieties (and even scions taken from certain individual trees), and he found interesting differences, such that at one time he wrote that the best rootstock for the Pinkerton variety was VC66, for example, whereas other rootstocks were preferred for other varieties.
Here is one of Ben-Ya’acov’s reports: https://www.avocadosource.com/CAS_Yearbooks/CAS_59_1975/CAS_1975-76_PG_122-133.pdf
Thanks very much, Greg!
As a statistician in a not-so-well-funded field, my first guess was that there probably wasn’t enough replication to do much with an interaction in terms of identifying high-yield rootstock by scion variety combinations, only enough replication for interactions to make main effects uninterpretable.
My read of the Ben-Ya’acov paper is that scion source (individual tree) within cultivar had a large effect on production, in some cases greater than the main effect of rootstock. Using google scholar to find similar papers, I found other hints at budwood source nested within cultivars having large effects on production across rootstocks or sites, and strong interactions with sites (& possibly with rootstocks). So rootstock by scion cultivar effects will be very difficult to estimate, and the main interaction might be at the rootstock by lineage/tree within scion cultivar level, which requires unrealistically large field trials to test.
Thus, I see the lack of huge rootstock by scion interactions mentioned by Dr. Arpaia as a good thing! A set of clonal rootstocks can be tested against different soils & salinity using just 1 or 2 commercially dominant varieties to give commercial growers what they need, and we can at least approximately generalize rootstock by environment interactions to other budwood varieties. Brocaw reports their rootstocks with Gem or Haas: https://www.brokawnursery.com/ourproducts/avocado/rootstocks/:
As for the factory metaphor: I’m fine with thinking of trees as factories producing avocados. Avocados vary in taste & quality; cars vary in style and quality. But, I don’t think that efficiency should be in units of yield per m^3 canopy (although that might be a useful intermediate measurable metric).
With a bit of plant ecophysiology in my past, I’m partial to water use efficiency: either carbon fixed or fruit yield per unit water. But as Greg said, in practical terms fruit yield per area may matter more for commercial growers and those of us with small backyards.
The efficiency that matters to commercial growers is $$ in crop value vs $$ spent on land, water, growing, and selling. Converting everything to units of $$ accommodates the quality of the fruit, not just the volume. Most folks here put a higher premium on fruit quality than the average supermarket consumer, and we don’t have harvest, shipping, and marketing costs, so our “efficient” or nearly-optimal varieties and growing.practices will differ from those for most commercial growers. My attempt at Gem + Sir Prize on one tree and Reed + Sharwil on the other gets me quality fruit year round in a limited space, but would never work for a commercial grower.
Hi Tom, The trial was well replicated. We had 9 rootstocks, 5 cultivars with 10 replications per combination for a total of 450 trees. The trees were planted in a randomized block design. Through 2023 there were rarely any rootstock:scion interactions that were significant and when there were it was very slight with one rootstock changing order with another but no large impacts.
We also have some preliminary data, not from this site, on water use efficiency between different scion varieties using the Dusa rootstock, but you are correct, water use efficiency (and tolerance to low water quality) are traits that we need to increase attention and within my program, that is exactly what we are trying to do. Thanks for your lengthy comment.
Hi Greg: The video was super interesting. Mary Lu Arpaia gave an avocado education like drinking from a firehose; had to watch several times. Many questions. What happened to Zutano rootstock? I was amazed by the GEM factory. Is that why the GEM tree can be planted on10’ spacing as its canopy volume is so small (relatively). Do you think any of these trees would be available to the homegrower or mainly wholesale like from Brokaw? I would hope her talk and publication provokes growers to consider some of these new highly efficient strains. Lastly GEM production statewide is very low compared to HAAS. is this the market speaking on their preference or reflective of older habits on what works best (commercially). Thank you
Hi Joe,
Zutano is almost never included in rootstock trials because it is generally not used by farmers. Farmers mostly use clonal rootstocks. One reason for this is that they can get more predictable performance from clonals; seedling rootstocks vary in their performance more. Zutano seedling rootstock is the most common used for trees sold to home growers in Southern California, but UC researchers primarily orient their research to commercial growers (farmers).
GEM’s efficiency is impressive, but for some home growers, GEM still might not be the best option. It is a finicky tree to get started (as you know!). Small GEM trees can bloom too much, especially after a wet winter, and they can sometimes grow painfully slowly in their first years, even to the point of dying back. I’ve become convinced that the best way to grow a GEM is to topwork the variety to an older rootstock, as Gray Martin has been advocating. But this is not feasible for most home growers.
As for the availability of the clonal rootstocks in Mary Lu’s trial, it’s hard to come by them for the home grower. But unless you know that you’re dealing with a specific set of soil conditions such that you know which clonal rootstock will perform best at your site, then it isn’t profitable to seek a certain clonal rootstock. A given clonal rootstock does not perform the same in all soil conditions.
GEM production in California is a fraction of Hass production for many reasons. One is that Hass has been around so much longer so Hass is more widely planted and the trees are bigger and more productive. Another is that the middlemen are used to Hass and prefer to sell just one variety of avocado. Another is that most avocado eaters even in Southern California still haven’t even eaten a GEM. New varieties take a long time to gain a foothold in the market. Hass took decades to displace Fuerte. We’ll see if GEM does the same to Hass.