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HLB Resistant Rootstock!!!
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)
hlb resistant rootstock
Qustions and Answers about Avocado Varieities and Rootstocks
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:
avocado fruit 6 pounds
Citrus Rootstock Knowledge Base from Florida
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
rootstock effect
Which Lemon Rootstock?
On Thursday, October 17, 2019 the University of California Cooperative Extension (UCCE) held a Lemon Rootstock and Scion workshop and field tour in Santa Paula, California. Results from Citrus Research Board (CRB) sponsored field trials evaluating several lemon rootstocks and scions were highlighted at this meeting. This event was well attended by over 100 grove managers, citrus growers and industry representatives. Speakers included CRB-funded researchers Ashraf El-kereamy, Ph.D., Mikeal Roose, Ph.D., Tracy Kahn, Ph.D., and Glenn Wright, Ph.D. The CRB has been funding research on several single site and multi-location lemon field trials, some of which have been ongoing since the late 1980's. Mikeal Roose, Tracy Kahn and Glenn Wright provided updates on the results of previous rootstock trials and the multi-location lemon rootstock and scion trials at Santa Paula, Lindcove Research and Extension Center and desert locations. Attendees learned about the various yield, packout and fruit quality of several lemon selections being studied in addition to new, non-lemon citrus cultivars to be released in the coming years. Following the presentations, attendees were invited to visit one of the lemon rootstock field trials.
Limoneira Company has been extremely helpful in much of this lemon research over the years. We thank them and also the Master Gardeners who helped in organizing the event held at the Limoneira meeting center. Read about a sideline at the event HERE.
Presentations from the workshop are available on the UCCE Ventura website HERE.
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glenn wright
ashraf
Tracy Kahn
What We Know About Lemons Workshop
Lemon Scions and Lemon Rootstocks Workshop & Field Tour
October 17, 2019
8:00am-12:00 PM
Visitors Center, Limoneira Co., 1141 Cummings Rd, Santa Paula, CA 93060 View map/directions here
Contact: Ben Faber, 805-645-1462, bafaber@ucanr.edu
Which Way Lemon Industry?
What are Lemon Rootstock Choices?
What are Lemon Scion Choices?
How are they Complementary?
What are their Effects on Yield? Fruit Quality? Canopy Growth? Soil Compatibility? Tree Health?
HLB Tolerance? Maybe We Know a Little.
How do Scions and Rootstocks Compare in the Desert, Valley and Coast?
Come Learn this and more. Tour a Scion/Rootstock Trial
Speakers:
Mike Roose, UC Riverside Citrus Breeder
Tracy Kahn, UC Riverside Curator Citrus Collection
Glenn Wright, University of Arizona Citrus Extension Specialist
Ashraf El-Kereamy, UC Riverside Citrus Specialist
citrus rootstock pictures
