- Author: Ben Faber
For those who missed it and for those who missed catching all the fine and coarse details of the hybrid zoom/in-person meeting on avocado varieties and rootstocks, here it is - the Shoot. Also, there is a survey on avocado rootstocks that a group of researchers headed by Patricia Manosalva at UC Riverside would like you to complete in order to help direct their breeding program.
California Avocado Growers Seminars Series 2022
From the:
August 17
Live Seminar and Hybrid Zoom
Avocado Varieties Panel Discussion
A big thank you to our Speakers
Speakers:
Click here for the August Seminar Recording
Also tell us about your rootstock breeding interests:
https://surveys.ucanr.edu/survey.cfm?surveynumber=38885
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- Author: Ben Faber
ARS Citrus Rootstocks: A Success Story
The United Nations General Assembly designated 2021 as the International Year of Fruits and Vegetables. This issue of Down on the Farm takes a look at one of ARS's major contributions to fruit production: citrus rootstocks.
Remember that old commercial that declared, "A day without orange juice is like a day without sunshine"? Thanks to the Agricultural Research Service (ARS), consumers can enjoy "citrus sunshine" whenever they like. Begun by USDA more than a century ago, the citrus research program has helped to ensure a bounty of not only oranges, but also grapefruits, mandarins, lemons, and more.
Orange tree leaves with symptoms of Huanglongbing (HLB), also known as citrus greening disease. (Photo by Tim Gottwald, D2259-1)
But that bounty was severely threatened in 2005 with the appearance of a new and destructive disease. Citrus greening, or huanglongbing (HLB), has caused Florida citrus production to plummet around 70 percent in the 15 years since the disease hit U.S. citrus groves. HLB, which causes low yields, yellowed leaves, and bitter-tasting fruit, is caused by a bacterium, Candidatus Liberibacter asiaticus. So far, there is no cure.
Like other crops, citrus crops are susceptible to a variety of diseases and pests. One reliable way to fend off those threats is to graft the fruit-producing part of a tree (the scion) to the lower trunk and root system (the rootstock) of a different tree that has been bred to resist the disease or pest. Rootstocks are also used to obtain specific tree sizes, yields, and fruit quality, among other goals.
A 6-year-old Owari Satsuma Mandarin tree on US-942 rootstock developed by ARS. In this trial, US-942 was the highest yielding rootstock, averaging more than 300 pounds of fruit per tree. (Photo by Jake Price, University of Georgia, D4632-1)
With ARS's long history of helping growers keep their groves healthy and productive, the agency had the expertise required when HLB appeared. To quickly address the problem, the ARS citrus breeding project was refocused in 2005 partly to develop new, HLB-tolerant, highly productive citrus rootstocks.
Led by Kim Bowman, a plant geneticist in the ARS Subtropical Insects and Horticulture Research Unit in Fort Pierce, FL, the team released 12 new HLB-tolerant citrus rootstocks between 2007 and 2018. Before and after the releases, Bowman conducted dozens of field trials to evaluate and validate the rootstocks' performance, providing the scientific data needed to demonstrate their potential and gain industry acceptance. These rootstocks, all with the prefix “US,” have since become a key component in the survival of the Florida citrus industry.
ARS plant geneticist Kim Bowman in front of 5-year-old Valencia orange trees on HLB-tolerant rootstocks he and his colleagues developed. (Photo by Diane Helseth, D4631-1)
Not surprisingly, demand for the rootstocks was extremely high, and growers also needed assurances that they'd be getting the real deal. Bowman arranged for the plant material to be certified disease-free by the Florida Department of Agriculture, paving the way for the rootstocks to be commercially propagated on a large scale.
Bowman and his colleagues have also done a great deal of research on rootstock propagation. Even though most common citrus rootstocks can be grown uniformly from seeds, it takes several years for a young tree to produce a lot of seeds, and the seeds of many new rootstocks don't grow into true-to-type plants. The scientists have shown that using plant cuttings or tissue culture is an acceptable alternative to starting new rootstock trees from seed, and it's a much faster way to create hundreds of thousands of plants.
The use of these alternative methods has dramatically increased propagation for some of the new rootstocks, so that nurseries are not limited by seed supply.
From 2018 to 2020, the HLB-tolerant "US" rootstocks were used to produce nearly 3 million new citrus trees, or about 37 percent of all trees propagated in Florida. These rootstocks have also proven effective in areas affected by other diseases besides HLB. The rootstock "US-942" demonstrated the most consistent outstanding performance in field plantings and was the most popular rootstock in Florida from 2018 to 2020, with about 1.8 million trees propagated during that 2-year period, or about 22 percent of all propagations.
For more information, visit Citrus Rootstocks.—By Sue Kendall, ARS Office of Communications.
A 6-year-old Owari Satsuma Mandarin tree on US-942 rootstock developed by ARS. In this trial, US-942 was the highest yielding rootstock, averaging more than 300 pounds of fruit per tree. (Photo by Jake Price, University of Georgia, D4632-1)
- Author: Ben Faber
Avocado Varieties and Rootstocks
CAS/UCCE/CAC Webinar June 10, 2020
These are Answers to Questions from Chat and Q&A from this webinar provided by:
Mary Lu Arpaia, Herve Avenot, Ben Faber, Eric Focht, Rodrigo Iturrieta, Petr Kosina, Patricia Manosalva, Themis Michailides
1. Where can we get the old illustrations of the fruit?
Mary Lu Arpaia: They are available on line to download free at https://usdawatercolors.nal.usda.gov/pom/about.xhtml. This takes you to the general page and then you can search for avocado. I spent hours looking at the avocado pictures, but they have a lot of other fantastic prints as well of many other fruit crops.
2. What is California's Hass volume production or acreage in 2019 compared to 2005? Petr Kosina: here are some stats: https://www.californiaavocadogrowers.com/industry/industry-statistical-data. Hass is estimated to be 95% of the acreage in California.
3. What exactly is classified as a calcareous soil.
Ben Faber: It is a soil that is dominated by calcium carbonate, which is lime. Lime controls the pH at above 7, usually around 7.5 and higher up to 8.3. Avocado likes a pH below 7, in the range of 6 or so. A high pH causes problems with iron up take primarily and to a certain degree with zinc, manganese, and copper
4. When do you anticipate Orondis to be available?
Mary Lu Arpaia: I was told at a January 2020 meeting that it is most likely summer 2021.
5. What is the best way to treat root rot in a mature grove with unknown rootstocks? Ben Faber: Same as with known rootstocks: phosphonates, gypsum and mulch AND make sure you irrigate correctly.
6. We have been told that we have “root rot on steroids” in San Marcos. We lost new trees planted on Dusa. Will the new chemical help us with new trees? Dan Grant came and look and said it was root rot death. Soil is good - not clay. We are spraying every month now with Phosgard. He told us that with the type of root rot we have that Dusa did not stand a chance without aggressive treatment. Wish we had heard that before we lost half our new trees.
Mary Lu Arpaia: There are other factors beyond root rot that can cause tree death. Even if your soil is “good” the trees can be waterlogged. Waterlogged trees can have similar symptoms to root rot (die back and water-stressed looking trees). Salinity can also influence the response of the tree to root rot and poor soil drainage. So, you can have several things going on in your orchard. Dusa tends not to be very tolerant of this type of conditions. If it really is just root rot, we probably need to get some isolates from you. As for the “new chemical”, I think you are referring to the pending registration of Orondis. This material is very effective and so could possibly help you once it is registered. Contact Patty Manosalva through your local farm advisor, Sonia Rios (sirios@ucanr.edu) to collect a root sample to see what isolate(s) of Phytophthora you have and then we can understand its virulence.
7. In relation to dry matter - as a measurement of ripeness & quality, as opposed to oil content?
Mary Lu Arpaia: Oil and dry matter are highly correlated, so yes, high oil will give you a higher dry matter reading. If you want to know more check out a paper by SK Lee and CW
Coggins from 1982 on www.avocadosource.com. The paper was published in the Journal of the American Society for Horticultural Sciences. This work formed the foundation for the current dry weight standards we have in California. It is a highly readable paper and the graphs are easy to understand.
Eric Focht: Dry matter is a measure of avocado fruit maturity: the percent of the fruit that is NOT water; high dry matter usually means high maturity.
8. Will the new varieties be released to the public or will they be limited to commercial growers?
Mary Lu Arpaia: Great question. I reached out to Joyce Patrona, the licensing officer for avocado in the UCR Office of Research. This is what she replied, “In California, they will be available for licensing to eligible (CAC member) avocado grower and will have licensing terms (TBD).” If you need more information, please contact Joyce Patrona at joyce.patrona@ucr.edu.
9. Do all the new varieties have black skins?
Mary Lu Arpaia: The four selections that Eric discussed all turn color when ripening. The two early selections, UCR02 and UCR03 turn more of a dark olive green/black when ripe.
10. What is the pounds per acres one can expect per acre of GEMs?
Mary Lu Arpaia: Great question, I think you could expect conservatively with good management 20,000 pounds on average. GEM does alternate bear but its alternation in our experience is not as severe as Hass.
11. I am replacing old rootstock with GEMs on 7 acres. What type of challenges can I anticipate?
Mary Lu Arpaia: The question is why you are replanting and where are you located. We do not have a good understanding on how GEM responds to salinity. I cannot tell you if it is more salt tolerant than Hass. So, if salinity is an issue for you now, it may be an issue after replanting. You do not mention which rootstock you will be using. In our experience with high soil pH and salinity, GEM on Dusa can become quite chlorotic. At the UC South Coast REC in Irvine, we have dealt with this by applying iron chelate in early summer and doing periodic leaching irrigations. For the most part this takes care of the problem for both GEM and the other varieties that we have. Another challenge could be the commitment that your packer has to a variety other than Hass. You need to discuss this with them. The GEM is still a new variety and we need to build market acceptance.
12. What rootstock do you recommend for GEMs? Dusa rootstock seems ill-matched because DUSA rootstock grows more vigorously than the GEMscion.
Mary Lu Arpaia: We have data from the Butler trial with 9 rootstocks. Most of our trials have been with Dusa and on high pH soils. We have had a lot of chlorosis on this rootstock requiring the use of iron chelates. We do not have data with Toro Canyon. If you contact me later, I can share the data from the Lloyd-Butler trial since we now have just collected the 2020 yield data.
13. What pH should I target for GEM?
Mary Lu Arpaia: I think avocado prefer a soil pH between 5.5 and 6.5 although they grow under many other situations. But the GEM I think is more dependent on this range to thrive.
14. How does the Dusa compare to the VC801 overall-especially interested how on it works on the GEM variety?
Mary Lu Arpaia: We have no data so I cannot give you any advice on this. It would be great to establish a large rootstock trial with GEM that includes the VC rootstocks.
15. Do you think Maluma could do well in Florida?
Mary Lu Arpaia: Good question for which we do not have an answer. The variety is being grown in Israel with good results as well as in Peru. The trees also perform well in the warm, humid conditions of South Africa, so I think it is worth exploring. I would contact Andre Ernst with this question. His email is andre@allesbeste.com.
16. Do we have a comparison between Maluma & GEM for heat tolerance?
Mary Lu Arpaia: No, since we do not have trials in California yet for Maluma. From what I have seen in South Africa it appears to have heat tolerance but compared to GEM, cannot really say except for the fact that both varieties are being grown in the same areas.
17. What is the salinity tolerance of the GEM variety--is it all about the rootstock selection?
Mary Lu Arpaia: I think GEM is slightly more salt tolerant than Hass. Salt tolerance is dependent first on the rootstock but also, we need varietal material that is also tolerant since there are different mechanisms for salt tolerance. So, if a tolerant rootstock takes up salt and the salt then moves up into the variety, you will need tolerance in the variety. But it is primarily about the rootstock selection.
18. And with the reduced sun on flowering areas, what is the effect on ultimate fruit bearing potential per tree? How do they react to high-density reduction of lateral access to sunlight?
Rodrigo Iturrieta: Great points, Yes, we want the fruits to be "inside" to minimize sunburned fruits. Having said that, what is actually occurring is more related to what non-fruiting shoots do next or around the fruiting ones (so they are basically outgrown by non-fruiting shoots and they end up being "inside" the canopy).
Sunlight responses is something we need to consider. We are keeping our eyes open for avocados that react less to shade (instead of more to sunlight). If shoots laterally branch despite being more shaded (less branching bias) then you still keep the complexity and some parts of the shoot will have a chance to harvest light under high-density planting.
19. Ease of harvesting and less pruning would be other important qualities in a trellising situation.
Rodrigo Iturrieta: Thank you for pointing that out and it has been duly noted. I have been thinking about mechanization but defining what is needed to reach that goal should have the consequence of facilitating harvest as we do it today. Less pruning is a must, they need to have growth habits that requires fewer interventions and/or, if you go in, would need fewer pruning cuts.
20. Which varieties are being trellised besides Maluma?
Rodrigo Iturrieta: Personally, I have only observed and gathered data with Maluma on trellis. This focus happened because the Ernst family at Allesbeste (South Africa; owners of the variety) have jumped into a really close to commercial scale trellising system, so is beyond a small trial and the data I got is from that site.
THESE are only half the questions and answers. Read the rest at:
- Author: Ben Faber
A recent University of Florida-Institute of Food and Agriculture Sciences (UF-IFAS) blog has a review of the last 60 years of rootstock trials and the lessons learned from those trials. It also has links to the results of those trials and a summary of the results of those trials.
What's also interesting is the “Expert System” that can be used to select a rootstock based on the horticultural traits (height, fruit size, etc), soil properties (pH, salinity, wetness, etc) and/or disease resistance.
The data is based on 21 characteristics of 48 rootstocks. It encompasses both the UF and USDA rootstock trials in Florida. And this last word is a key to all the data. It's based on the Florida condition and on many rootstocks that have not be trialed in California and aren't available commercially here. It is, however, a wonderful resource (Bibliography also included), learning tool and good starting point for reviewing potential rootstocks before ordering trees and planting an orchard. It's best to learn of potential problems before planting than having to learn how to correct those problems once the trees are in the ground.
And here is more from the USDA on citrus rootstocks: https://citrusrootstocks.org/
For an overview of California rootstocks for lemon, check out the presentations here
By Susanne von Rosenberg, UC Master Gardener of Napa County
Have you been thinking about adding a fruit tree or two to your garden? December and January are the best times of year to plant deciduous fruit trees. (Citrus trees are best planted in late spring, after the threat of frost has passed.) In December and January, local nurseries have a wide variety of bare-root fruit trees available. Bare-root fruit trees are less expensive than comparably sized trees in pots, and they are easier to plant. Here are some considerations for choosing the perfect tree or trees for your garden.
The first consideration, always, is what kind of fruit do you enjoy? I have more than 20 fruit trees, but no peaches. I don't like the fuzzy skin so I prefer nectarines.
The next consideration is whether you primarily want fruit for fresh eating, or whether you also intend to preserve a portion of your harvest. How much fruit you want will suggest how large a tree you need. Even a small tree (five feet in height and with a five- to six-foot diameter crown) can easily provide 50 or more fruits several years after planting. That may not sound like a lot, but it's enough for a small household for a month. Depending on the variety, a tree that is only a bit larger can provide as much as 100 fruits.
For most gardeners, it makes sense to keep trees on the small side. You can do that by pruning the tree regularly. Semi-dwarf or dwarf trees will need less pruning than standard size trees. Smaller trees are easier to care for because you won't have to get on a ladder, and they also require less water and fertilizer.
When you keep trees smaller, you have room for more trees. That means you can spread out the harvest period (for example, you can plant early, mid-season and late apples), or you can plant more types of fruit. With proper pruning, you can fit five or six trees into a 30 – by 7-foot space.
It's best to buy your trees from a local nursery. Fruit trees are typically grafted, with the fruit-bearing part (the scion) grafted onto a rootstock that is appropriate for a specific area. Different types of rootstocks are suited to different soils and have different kinds of disease resistance. When you buy a fruit tree locally, it is grafted onto a rootstock suitable for Napa County. Equally important, the trees in our local nurseries have been inspected to make sure that they are not harboring invasive pests.
The third consideration is your location. Different parts of Napa County vary in the number of annual chill hours they get, with Carneros receiving the fewest and the Lake Berryessa and Pope Valley areas having the most. Chill hours are the number of hours each year between 32°F and 45°F. Fruit trees require a minimum number of chill hours to produce well. The number of chill hours varies dramatically by variety. Granny Smith apples require 900 chill hours, while Anna apples require only 200 to 300.
The number of chill hours in all areas is projected to decline with climate change. Your trees will not suffer if they receive more chill hours than required, so look for varieties requiring fewer chill hours. You also need to verify that you have adequate sunlight; fruit trees need at least eight hours per day. If you are in a windy location, make sure that the variety you want to plant isn't sensitive to wind.
Next, find out whether the variety you're considering is self-fertile, semi-self-fertile or requires a pollinizer (another variety of fruit tree that provides pollen to fertilize the flowers). Self-fertile trees do not require a pollinizer. Semi-self-fertile trees will set fruit without a pollinizer but will yield more fruit with one nearby. For trees that require a pollinizer, the label will indicate which varieties will do the job.
If you don't have room for the pollinizer, see if someone in your neighborhood has the correct variety.
Depending on the type of tree, the pollinizer may need to be as close as 70 feet or can be as far away as 250 feet. Some labels provide this information. If not, check with the nursery or investigate online. As an alternative, you can ask someone who has the desired pollinizer to give you a few branches when they flower. Put the branches in a vase and prop them up in your tree. You can also graft the pollinizer variety onto your tree.
Keep these considerations in mind, and you'll be successful in picking the perfect fruit tree or two for your garden.
Up coming events: “An Apple a Day: Choose the Right Apple for You and Your Garden” on Thursday, January 2, from 7 p.m. to 8 p.m., at the Napa Library, 580 Coombs Street, Napa. The talk is free and no RSVP required.
“Rose Pruning and Winter Care” on Saturday, January 11, from 10 a.m. to 12 noon at University of California Cooperative Extension, 1710 Soscol Avenue, Napa. Join the UC Master Gardeners for this interactive forum and workshop. Reserve your spot for only $5 for reservation and more details go to http://ucanr.edu/2020rosepruning or call 707-253-4221.
The UC Master Gardeners of Napa County are volunteers who provide UC research-based information on home gardening and answer your questions. To find out more about upcoming programs or to ask a garden question, visit the Master Gardener website (http://napamg.ucanr.edu) or call (707) 253-4221 between 9 a.m. and noon on Mondays, Wednesdays or Fridays.