- Author: Vanessa Ashworth
- Author: Philippe Rolshausen
Department of Botany and Plant Sciences, UC Riverside
Advances in molecular tools already have led to the development of large numbers of genetic markers distributed across the avocado genome. These markers are being put to good use, many forming the basis for projects to inventory avocado germplasm repositories and, in our own case, to create a pathway toward marker-assisted selection. Most recently, the first avocado genome was sequenced, marking the beginning of a new era. Genome annotation entails adding functional information about genes and other compartments of the genome, aided by comparisons with model organisms. The challenge is that the avocado occupies a highly divergent position on the evolutionary tree of life, close to such plants as laurel and magnolia, near the base of the flowering plants lineage and far removed from convenient model organisms (such as Arabidopsis) for which annotations are most advanced and comprehensive. Therefore, genome annotation in avocado is likely to be slower. While there is great potential for exploring the genome for interesting gene regions, the procedures will not be insignificant and will require bioinformatics and experimental verification before breeding targets can be identified. Additionally, quantitative traits such as yield-related traits are not controlled by a single gene but, instead, by many genes of small effect distributed across the entire genome. Even when the genome becomes thoroughly annotated, it is still only one genome and will not include all the variation present in the large pool of avocado germplasm. In comparison, efforts are underway to sequence 1 million human genomes. Clearly, additional avocado genomes will allow broader comparisons and will lead to an acceleration and broadened scope of breeding efforts.
Figure 1: Diversity of shape, size, skin color and surface texture of fruit picked from trees sharing the same maternal parent. Some of these fruit were assayed in the QTL analysis for fruit nutrient content.
A major goal of molecular breeding is to pinpoint which genes are responsible for a trait of interest and to make this relationship accessible for experimental manipulation. In avocado, many genetic markers1 have been developed in recent years and used widely to characterize scion and rootstock germplasm collections. However, few studies have attempted to examine how genetic markers are related to specific traits and how they can be used to improve breeding material using marker-assisted selection. In a recent scientific article, we reported our findings from a genetic study to identify genes and markers controlling various yield-related and nutritional traits in avocado. It centered on a procedure called Quantitative Trait Locus (QTL) analysis2 and presents a high-density linkage map3 for avocado useful for molecular breeding. Our study included quantitative traits (tree height, canopy diameter, and trunk diameter, and contents of vitamin E, beta-sitosterol and carotenoids in the fruit flesh) and a qualitative trait (flowering type) (Figure 1 and 2). Here are some of the major findings of our research.
Flowering type: Avocado flowers exhibit either A- or B-type flowering, a mechanism designed to prevent self-pollination and that increases fruit set. We found that a tight cluster of markers on chromosome 10 showed a very strong signal for flowering type. It appeared that a single gene on that chromosome is likely responsible for controlling which flowering type a tree will have when it reaches maturity. In orchards devoted to ‘Hass', with A-type flowering, inter-planting with B-type pollinizer cultivars is the norm to boost pollination and fruit set. Currently, most pollinizers are green-skin cultivars (especially ‘Bacon', ‘Fuerte', and ‘Zutano') whose fruit do not fetch a good price compared to ‘Hass', reducing overall market value of the orchard yield. Therefore, when breeding new cultivars for ‘Hass'-like taste and appearance, it would be advantageous to be able to include an early screen for B-type flowering: this would ensure that any promising breeding material with ‘Hass'-like attributes would also be usable as B-type pollinizers that produce marketable fruit. The screen would involve a routine lab procedure performed on DNA extracted from young leaves of the seedlings in a breeding program: the strategy would be to only keep those seedlings that have the particular marker that coincides with B-type flowering and to discard the rest. All seedlings progressing through subsequent tiers of the breeding program would have B-type flowering and there would be no wastage of time and resources by having to cull mature trees once they have been revealed as A-type flowerers.
Nutrient content of the fruit flesh: We also found that the content of alpha-tocopherol, a form of vitamin E, is strongly associated with a group of markers on chromosome 3. This finding opens up the possibility of breeding avocado for enhanced vitamin E concentrations and to further elevate its status as a nutritious fruit. Selection for a suitable marker in young seedlings would avoid the long wait until the seedlings have produced fruit, which can take many years. Though weaker, a signal for marker association with beta-sitosterol was also detected on a short section on chromosome 1. This plant sterol has been shown to have anti-oxidative properties and to reduce blood cholesterol levels in humans. The fact that vitamin E and beta-sitosterol are controlled by genes on different chromosomes is a practical advantage because it means that breeding for one nutrient can be performed independently of the other nutrient. It is noteworthy that both vitamin E and beta-sitosterol have been cited as targets for biofortification in other crops.
Avocado production in California, the main avocado-producing state in the US, cannot keep pace with consumption, and the market is supplemented by imports from Mexico and many other countries. For now, almost all fruits imported are of ‘Hass' or “Hass-like” cultivars, yet taste panels at UC Riverside suggest that consumers are open to new tastes and visuals. Supermarket offerings in the form of a startling abundance of different apple and pear cultivars are in stark contrast to those for avocado, which is essentially synonymous with ‘Hass' alone. The time for customized breeding may be ripe. As a nutritious and tasty fruit crop, avocado has acquired a strong culinary following and is prized for its healthy attributes. Having shown that vitamin E content is amenable to marker-assisted selection, future breeding could focus on generating nutrient-enriched cultivars, and future elucidation of similar trait-marker associations could create opportunities to generate high-value avocados that would coexist alongside the mainstream crop. Currently, health-conscious consumers and avocado fans are being short-changed, and molecular breeding can play a role in developing new and interesting material.
Figure 2: Tree height—a quantitative trait—at different locations in identical genotypes (clonally replicated). The white stick in each photo measures 1 m in length.
1 Genetic markers: specific locations on the DNA sequence that can be readily identified by molecular methods. They are also known as molecular markers.
2 QTL analysis: infers which markers on a linkage map influence a trait of interest. Results are shown in a chart with all markers on the map plotted against their (statistical) contribution to the trait.
3 Linkage map: a map showing the order of markers and genes along each of an organism's chromosomes (i.e., how they are linked). It is also called a genetic map.
The very fact that avocados can be grown in hard to get to places means that the trees are also in areas that are subject to wildfire damage. Recently several hundred acres of avocado burned in the foothills. The fire was fanned by high winds and low humidity.
Every year there are avocado trees that burn, either through careless attention to early morning fires that pickers build, wildfires or car accidents. A grower needs to be patient and observant to bring the trees back into production.
Although injury to foliage and young growth is visible within a few days of the fire, the full extent of the damage may not be known for several months or possibly the next growing season. In the case of severe injury, die-back may continue to occur for several months after the fire. New growth that occurs after the fire may suddenly collapse the following year when the growth is tested by Santa Ana conditions.
The important rule to follow after a fire is to do nothing - don't prune, don't fertilize and maybe don't water. Or rather, water very carefully. Dry winds may have sucked the water out of the ground and may need to be replenished. The fire may have burned the irrigation lines and need to be replaced.
The need to immediately replace irrigation systems will depend on the time of year, the extent of damage, the soil moisture after the fire, the weather forecast, labor availability and many other considerations. It is not terribly expensive to replace hoses, but if PVC sub mains have burned, it will be a big job and it may not be necessary to jump on their replacement. Check the soil and the tree status before rushing to replace an irrigation system. It is on the list of things to do, but maybe not tomorrow.
In the meantime, if the tree has been defoliated by the fire, it has lost its ability to transpire water. Watering a tree with no leaves will set up those conditions that are conducive to root rot. Until the tree begins to leaf out, watch soil moisture to decide how much water the trees are pulling out of the soil. The emitters should be capped or plugged on some leafless trees. Then as the tree puts on new growth, shallow, infrequent irrigations should start. This may mean replacing the 10 gph microsprinkler with a 1 gph dripper if only a portion of the orchard has been burned and the rest of the trees need their usual amounts and frequency of water.
The avocado has a tremendous ability to come back from fire and frost damage. However, the tree will tell you where it is coming back. It will start pushing growth where the tree is still healthy. It may take 3 to 6 months for this growth to occur.
Delay pruning until the tree clearly shows where it is going to regrow. By waiting, you save the expense of having to return sometime later to remove more wood and also will be able to save the maximum about of tree.
An activity the grower can perform is whitewashing. The defoliated tree can be further damaged by sunburn after it has lost its protective cover of leaves. The upper surface of horizontal limbs and the south sides of exposed trunks are the most affected. The whitewash can delay the appearance of new growth, but it does not affect total growth. There is usually no value in applying the whitewash to small limbs.
There are various commercial whitewashes on the market. The easiest to prepare is the cheapest white latex paint on the market mixed with water to the extent that it will go through a sprayer.
Avocado trees have a great ability to recover after fire damage. Even trees killed below the bud union will frequently develop into good trees if they are rebudded and given good care. Trees which do not put out vigorous sprouts should be removed. Interplanting avocados would rarely be advisable because of their rapid recovery.
For another version of fire recovery, go to: https://www.californiaavocadogrowers.com/cultural-management-library/post-fire-grove-recovery
Photos from Kevin Ball, firefighter/farmer
Laurel Wilt Disease has been on the radar since 2002 when it became apparent that trees related to avocado were dying in the Georgia area. The Lauraceae family comprises a major portion of the evergreen tree species in the southeast. It's a fungal disease spread by the red bay ambrosia beetle. Since its introduction, the pest/disease complex has spread throughout the Southeast US, causing significant death and economic loss in forested areas. Since about 2011, it has been causing significant damage to the avocado groves in the Miami area. A massive amount of energy has been put into studying the control of the disease, but as yet, there are no clear solutions once a tree is infected. Tree removal is the answer to contain the spread.
This is a grower letter describing the frustration of dealing with Laurel Wilt Disease in Florida and an alert to California growers to get prepared for it.
DLT Farms LLC
October 27, 2019
Open Letter to Avocado Producers:
I grow tropical avocados commercially in Homestead, Florida. That's been my passion for the last 16 years, The last five years have been a frustrating nightmare caused by Laurel Wilt. This morning I marked three more trees for extraction in addition to the 10 trees I marked three days ago. I lose about 20-25 a month and it's on the rise.
The largest avocado-producing areas are free of the problem at the moment. In South Florida, tropical avocado is a very small industry with little economic and political influence, as a result not much attention is paid to this crises. In the Homestead, Florida avocado region the pathogen and vectors are everywhere. There are orchards that disappear from one year to the next. Unfortunately, we have been on the front lines of this disease.
This creates an opportunity for the rest of the avocado producing areas, so far free of this disease to do research in this area. There is a lot of research to be done and as time goes on we gain more experience. Here, there is access to adult trees in the field for testing and research. This is not the case for areas in which the disease has not made its appearance.
I only mention some investigations that could be pending:
Some trees die in a week, however there are trees that look resistant to Laurel Wilt, they have tested positive from root to canopy, proven on multiple occasions and methodologies, yet they are still alive and show no symptoms of the wilt. I know of two, one in particular I call 9-7, is positive since January 2019 and continues alive and recovering. Personally I think this tree is talking to us, however there are no funds to see what it tells us.
Early detection in the process is vital to early removal of diseased trees, before other trees acquire the disease. It is essential to determine whether the tree is contaminated with little pathogen, to extract it before it contaminates adjacent trees. There is no data on the time between when a tree is contaminated and when the first sign of "sadness" becomes visible. Less than a week ago I detected a little sadness in a branch. When cutting it, it looked positive for wilt, I continued to check three more trees on the same row and all three tested positive at various levels without visible symptoms. Root contamination moves unnoticed for weeks or months. This type of root contagion remains an observable presumption of farmers and has not been investigated.
Some producers believe that there are two or more strains of the fungus that causes the disease, (Raffaelea Lauricola), one much more aggressive than others. A full DNA study is complicated and costly. As I understand it, it hasn't been done and there are no plans.
Other producers began doing thermal treatments on infected and stumped trees. I tried on 8 of my trees. All are alive and doing well, the two oldest are 17 months old and continue to grow. On the other hand, the eight have tested positive for Laurel Wilt, post-treatment. There is no scientific data to understand what is going on and what opportunities this offers.
What attracts vectors and what can farmers do? Many growers are gaining experience of their own. I stopped injecting phosphorous acid to treat Phytophthora because I have noticed that these trees have a higher frequency of vector inoculation than others not injected. I suspect why, but we don't have a scientific foundation to know what's going on.
Can trees be vaccinated with some form of vaccine to increase resistance or defense against the pathogen?
And so on, I could mention a lot more.
Looking back we knew it was coming, but we didn't prepare enough, partly because it wasn't always easy to test in the field and there were a lot of restrictions on conducting test in greenhouses and we just didn't believe it or wanted to invest funds in research. Some thought it would never come.
We all see these bark beetles moving around the world with little or no restriction. One day any area can wake up to the bad news.
I can tell you this: you can imagine this disease in your grove, more or less we know what it does, you can read articles, watch YouTube videos, after you finish your imagination exercise, I can tell you as a grower dealing with this every day, MULTIPLY IT BY 10 !
Please, support research in the area where it can best be done. Get ahead of it.
Carlos de la Torre
Keep abreast of Laurel Wilt Disease!
You can see presentations that have been made by a University of Florida research contingent which spoke to avocado growers this summer, 2019:
You can also hear a webinar about the pest/disease complex by UCR's Monique Rivera:
Photos: Symptoms of Laurel Wilt Disease in avocado. Distribution map over time of Laurel Wilt Disease in the Southeast. Image of Georgia forest affected by Laurel Wilt Disease.
Boom, just when you think everything is going fine, you run across a reminder that there are little things going on in the orchard that are out of our control. In Goleta, I just ran across these crick-side, crook neck fruit that are a reminder that we just had about 3 weeks of foggy weather (in the fall, when it's supposed to be in the spring) and then a week of hot weather which is just the condition to create this calcium imbalance and distorted fruit. It's no big big deal, unless it was all over tree. Just more of an oddity. Harvest time is past this year so didnt expect to see it on this new fruit.
At avocado harvest time, growers are in the orchard checking things out a little more closely and to see what is going into the bins…..and they see some unusual shaped fruit. Here's what's been popping up and some possible explanations.
Crick-side - First described by Dr. J. Eliot Coit as kink-neck and later by Horne (1931) as kink-side. Finally, the name crick-side (Horne, 1934) was adopted. It is characterized by a definite depression on one side between the stem end and the larger portion of the fruit causing a distortion. In some cases, the area of depression turns black and the fruit drops. In other cases, the fruit grows and matures but the distortion remains. Crick-side is usually found on trees carrying a heavy load of fruit. It has been suggested that high temperatures or temporary water-stress may relate to the occurrence of crick-side, but no definite determination as to its cause has been made.
Carapace Spot - First described by Horne (1929), the name carapace-spot was chosen because of the resemblance to a turtles' back. This external blemish is corky and usually cracked into somewhat regular, angular divisions. The flesh under the carapace spot is undamaged, but exterior appearance is such that the fruit is reduced in grade. Slight rubbing or brushing of tender young fruit on leaves or stems appears to cause this corky growth to start. Fruit on trees exposed to strong winds are more apt to develop the trouble. Windbreaks should reduce injury in windy areas.
Photo: Avocado thrips damage, carapace damage and greenhouse thrips damage.
Sunblotch - This is a viroid that can affect fruit, leaves, and stems with a yellow or reddish streaking, cause a compacted growth and willowy growth habit. The streaking in the fruit is usually depressed and doesn't extend the length of the body.
Sunburn - Fruit exposed to full sun may be injured by sunburn. This occurs when trees defoliate, or partially defoliate, from any of several causes, leaving the fruit exposed. It is normally most severe on fruit on the south and southwest portion of the tree. Sunburn shows as a pale yellowish area on the exposed side of the fruit. Often the center of this area turns brown to black and may wither.
Ring Neck - This trouble has been observed occasionally, particularly with Hass. The cause is unknown but is believed to be related to soil-plant water deficiency at a critical time. A ring of tissue on the pedicel just above the attachment to the fruit dies, turns black and peels off. If only superficial, the fruit remains on the tree. Growth may be retarded because the restriction impedes movement of nutrients and water outward to the fruit. Most severe in humid coastal areas.
Embossment - Occasionally, and particularly on Fuertes, a section of the surface will be raised slightly or be a darker or lighter color. This is referred to as a sectional chimera or genetic mutation.
Healed over damage - if fruit has mild damage that allows it to heal over (remember avocado fruit expand by cell multiplication not enlargement), then a scar is left, such as this likely amorbia feeding
Cuke - As in cucumber but not a squash. These are seedless fruit that can most often be seen from a fruit set in cooler weather or due to some hormonal stimulus. We don't know the reason, but seems to occur more commonly along the coast.
Double Fruit - In some instances there may be a normal shaped fruit with a single cuke attached ot in some cases there is a double ovary and two fruit are attached.
Woody Avocados - For some unknown reason, avocado fruit will form into a grotesque woody structure hardly resembling an avocado. The cause is genetic and non-transmissible.
Sources: R.G. Platt - California Avocado Society Yearbook 1972-73 and Reuben Hofshi and M.L. Arpaia Yearbook 2002.
The California Avocado Society held its 104th Annual Meeting recently and acknowledged an individual's contributions to the avocado industry with its Oliver Atkins Award. This award is presented in honor of nurseryman Oliver Atkins who went beyond what was required or expected, benefitting the avocado industry and its growers. His devotion to the industry was noted in his day-to-day activities and to the changes that he brought to the industry. The award was made to Mr. Pablo Rodriguez.
The award was presented by Nurseryman Rob Brokaw and the following notes were made during the address:
Pablo Rodriguez was born in Mexico in 1950, one of eight kids. He attended a private school on scholarship from ages 8-12, finishing his formal education at 8th grade. He came to the US in order to support his parents and siblings at age 20 in 1971 and worked up and down California for two years before landing at an avocado/citrus nursery in Ventura county.
The nurseryman at the time made a point of “keeping an eye on Pablo”, and Pablo soon rose to be the nursery manger, developing an expertise in grafting. He bought a home in Santa Paula, raising a family there. He gained US citizenship in 1996. He and his family also have a company that performs topworking which is now managed by son Robert.
Readying a rootstock for a graft
As the nursery became involved in working with partners overseas, Pablo's expertise was in demand. He has worked with collaborators in Mexico, Peru, and Dominican Republic, Chile, Florida, Hawaii, Spain, South Africa and more.
His skills and expertise are evident in the millions of avocado and citrus trees that have been produced here in California under his watchful eye, but the greater impact of his efforts resonates around the world.
Pablo and Samuel Garibay showing South Africans the Brokaw Way
Today Pablo is in semi-retirement. In his case, that means he works just as much as he ever did. He's constantly operating in the background shoring up processes and ensuring smooth operations. On Sundays, he can be found wandering the orchards – “just making sure the graftwood is good for harvest this week!” He isn't asked to do this; he just does it because it needs to be done.
But the real story about Pablo, apart from his intelligence and his accomplishments, is his profound humility and humanity. Pablo's dealings with others are informed by a deep spirituality and morality. He seeks to raise others, preferring to remain in the shadows.
Pablo is a guy who, when asked if he can do the impossible, will think for a while, then shrug and say, “Well, it has to be done”. And then does it. This happens regularly.
To his family, to California growers and to the global avocado industry, Pablo has given selflessly of himself. We're all enriched by his presence in our community.
A Serious budman