- Author: Vanessa Ashworth
- Author: Mary Lu Arpaia
- Author: Philippe Rolshausen
Department of Botany and Plant Sciences, University of California, Riverside
An unusual population of avocado trees may soon suffer the same fate as many commercial orchards elsewhere in California: its water supply will be cut off and the trees fed to a wood chipper. And yet these trees (Fig. 1) potentially hold a key to the avocado's future: they are the cornerstone of scientific research at the University of California, Riverside, aimed at unravelling the genetic underpinnings of agricultural traits and at placing avocado breeding on a molecular footing.
It is well known to plant breeders that the traits observed in a promising selection are rarely transmitted to its offspring. This is because the so called phenotype (what you see or measure) is a poor predictor of the genotype (the underlying genetic machinery). Unfortunately, for breeding to make any progress, phenotypic traits need to have a genetic basis. Traditional breeding, which cannot distinguish between phenotype and genotype—only works because it starts our with a large pool of trees and, by chance, ends up with a few selections that show promise as future cultivars. In avocado, the selection efficiency of traditional breeding is in the order of 0.1-0.2%; in other words, only 1–2 promising selections are recovered for every 1000 trees that have been laboriously (and expensively) screened over the course of a minimum of 5–10 years. Clearly, a better understanding of the relationship between phenotype and genotype would make the breeding process more efficient.
Recognizing this need, Professor Michael Clegg of (then) UC Riverside established a carefully designed experimental population of avocado trees, later known as the Clegg Collection. It consisted of over 200 progeny from a single cultivar Gwen mother tree, and each progeny tree was clonally propagated four-fold, taking the total number of trees to ca. 800. The seedlings were grafted to a uniform Duke 7 rootstock to further reduce the impact of non-genetic variability. Between the fall of 2001 and spring of 2003 half of the trees (two clonal replicates of each unique genotype) were planted out at UC Riverside and the other half at South Coast Research and Extension Center, Irvine.
In this experimental design, every tree genotype is represented twice at each of two locations. Any variation between the clonal replicates at the same location sheds light on how much of a trait is environmental and how much of it is genetic. Only the genetic component is useful for breeding purposes. The environmental component is the “noise” that misleads breeders and, regrettably, often has a large influence on agriculturally relevant traits.
Since 2003 the Clegg trees have been put to good use. First, a quantitative genetic study was initiated to address the mismatch between genotype and phenotype and, specifically, to determine whether certain vegetative growth characteristics are amenable to breeding. This work (Chen et al. 2007) revealed that about 30% of the total phenotypic variation in growth rate and flowering was genetic in origin and thus amenable to breeding.
While the initial value of the Clegg experimental population arose from its utility in teasing apart genetic and environmental effects on a phenotype, the trees soon acquired additional roles. Genetic markers offer the opportunity of placing phenotypic measurements in a molecular framework. Microsatellite markers were used to determine the pollen parent of each ‘Gwen' progeny tree, revealing that approximately three quarters of the genotypes had been pollinated in roughly equal proportions by ‘Bacon', ‘Fuerte', and ‘Zutano', with the remaining quarter sired by miscellaneous cultivars or rogue pollen sources. What better opportunity than to examine genetic variation of traits in the context of the pollen parent. An interesting finding from this line of study was that ‘Gwen' ´ ‘Fuerte' progeny had significantly wider canopies and shorter stature than their half-sibs sired by ‘Bacon' or ‘Zutano' (Fig. 2). The fact that tree width and height is amenable to breeding is an encouraging result in the context of high-density planting such as that commonly practiced in apple.
At a time when avocado was gaining cudos as a healthy fruit with excellent nutritional qualities and beneficial effects in the treatment of high cholesterol and cancer, the Clegg Collection was next harnessed in a study on fruit nutritional composition. Data was gathered on fruit nutrient content in each genotype. Again, taking advantage of the experimental setup, the environmental “noise” associated with each measurement was stripped away to extract the
genetic portion that proved to be appreciable (Calderón-Vázquez et al. 2013).
The next step was to connect this data with a new type of molecular marker. These markers—so called SNP markers (Single Nucleotide Polymorphisms)—were developed using gene sequences from a subset of the Clegg trees. They were designed to reside in genes known to control the accumulation of particular fruit nutrients. Statistical analyses revealed that beta-sitosterol contents were being tracked by one of the SNP markers: in other words, the presence of this marker in an individual was indicative of high beta-sitosterol levels in its fruit.
Markers that are highly predictive of desirable traits and are relatively easy to measure in young seedlings are the nuts-and-bolts of marker-assisted selection, a breeding method that draws on molecular tools. Consequently, a third project was initiated that harnessed the SNP marker that predicted high fruit beta-sitosterol contents. Progeny from trees of the Clegg population were screened using the marker. Out of an initial pool of over 600 seedlings 73 seedlings (12%) were identified that had the desirable form (allele) of the marker, and 12 seedlings (2%) were eventually planted out. The selection intensity of marker-assisted selection therefore is at least 10-fold higher than under traditional breeding.
A loss of the Clegg Collection would surely represent an opportunity lost. Many more projects could be envisaged that address the genetic determination of a trait, its association with SNP markers, the influence of the pollen donor, or the utility of a marker for marker-assisted selection. The Collection has also been the nucleus of a genetic mapping project. Significantly, the SNP markers developed for these trees are also relevant for studies beyond fruit nutrient content because their biosynthetic pathways intersect with those underlying plant stress and disease responses. This property makes the candidate genes equally relevant for studies on pathogen or salinity tolerance and a key resource that could help secure the future of avocado production in California during turbulent times.
Chen, H., V. E. T. M. Ashworth, S. Xu, and M. T. Clegg. 2007. Quantitative genetic analysis of growth rate in avocado. J. Amer. Soc. Hort. Sci. 132 (5): 691–696.
Calderón-Vázquez, C., M. L. Durbin, V. E. T. M. Ashworth, L. Tommasini, K. K. T. Meyer, M. T. Clegg. 2013. Quantitative genetic analysis of three important nutritive traits in the fruit of avocado. J. Amer. Soc. Hort. Sci. 138 (4): 283–289
The Clegg Collection: the trees shown here are growing at UC Riverside
- Author: Ben Faber
For the first time since the great freeze of '89-90, we have experienced a little more than minor damage to our crops. Compared to the San Joaquin Valley, Ventura country escaped without major damage; although there were some areas harder hit like the Ojai Valley and some canyons near Santa Paula. Many parts of the SJV were hard hit.
As in the freeze of 1990, your trees must be cared for in the same way during this post freeze period. In 1990, advice was issued to the grower about the rehabilitation of their trees, both citrus and avocado. We would like to review that information for you at this time. How can we best aid tree recovery so that tree growth and yield will proceed most rapidly?
Citrus and avocado leaves appear wilted or flaccid during periods of low temperature. This is a natural protective response to freezing temperatures and does not mean the leaves have been frozen. Leaves will be firm and brittle and often curled when frozen. Leaves become flaccid after thawing, and if the injury is not too great, they gradually regain turgor and recover, leaving however, dark flecks on the leaves. Seriously frozen leaves collapse, dry out, and remain on the tree. Foliage form recent flushes are most susceptible to this damage. If twigs or wood have been seriously damaged, the frozen leaves may remain on the tree for several weeks. If the twigs and wood have not been damaged severely, the leaves are rapidly shed. Trees losing their leaves rapidly is often a good sign and is not, as many growers believe a sign of extensive damage.
Cold damage to the twigs appears as water soaking or discoloration. In older branches and trunks it appears as splitting or loosening of bark where the cambium has been killed. Bark may curl and dry with many small cracks. Dead patches of bark may occur in various locations on limbs and trunk.
Sensitivity to frost is dependent upon many variables. In general, mandarins are the most cold hardy followed by sweet orange and grapefruit. Lemons are very frost sensitive with Eureka decidedly more sensitive than Lisbon. For avocados, Hass is about as cold tolerant as lemons, while Bacon is more cold tolerant. Limes are the least cold hardy. Healthy trees are more tolerant than stressed ones. The rootstock also imparts sensitivity onto the scion.
Injury to the foliage and to young trees may be immediately recognizable but the true extent of the damage to larger branches, trunks, and rootstocks may not appear for on to four months following the freeze. No attempt should be made to prune or even assess damage from the frost until spring when new growth appears.
The only treatment that should be done rapidly after a freeze is whitewashing. Often the most sever damage following a freeze results from sunburn of exposed twigs and branches after defoliation. Avocados and lemons are the most susceptible to sunburn, oranges not as much; but, if the tree has been defoliated, applying whitewash would be precautionary. Temperatures do not have to be extremely high to cause sunburn.
Pruning should be carried out to prevent secondary pathogens and wood decay organisms from slowing tree recovery. Again, however, there should be no rush to prune. Premature pruning, at the very least, may have to be repeated and, at the worst, it can slow tree rehabilitation. It should be remembered that when pruning, all cuts should be made into living wood. Try to cut flush with existing branches at crotches. Do not leave branch stubs or uneven surfaces. Tools should be disinfected in bleach or other fungicide before moving on to the next tree.
The extent of pruning is dictated by the amount of freeze damage:
|Light Damage||Medium Damage||Severe Damage||Extreme Damage|
|Where only the foliage and small twigs are injured,pruning is not required||Where a considerable part of the top has been killed but the trunk and main crown limbs show little damage, branches should be removed back to living wood above vigorous sprouts||
Where the top and crown limbs are severely damaged but there are sprouts above the bud union, the tree should be cut back to the uppermost sprout
Where trees are killed to the bud union or the rootstock has been girdled, the trees should be removed and replaced with new trees
Irrigate carefully! Remember that when leaves are lost, obviously evaporation from leaves is greatly reduced, and, therefore the amount of water required is also greatly reduced. A frost-damaged tree will use the same amount of water as a much younger or smaller tree. Over irrigation will not result in rapid recovery. Instead, it may induce root damage and encourage growth of root rotting organisms. This is particularly true for avocados. Irrigation should be less frequent, and smaller amounts of water should be applied until trees have regained their normal foliage development.
Fertilization of freeze-damaged trees should be carefully considered. There is no evidence to indicate that frozen trees respond to any special fertilizer that is supposed to stimulate growth. If trees are severely injured-with large limbs or even parts of the trunk killed-nitrogen fertilizer applications should be greatly reduced, until the structure and balance of the tree become re-established. Trees should be watched for evidence of deficiencies of minor elements. Deficiencies of zinc, manganese, copper, and iron are most likely to develop. For citrus, these materials should be applied as sprays, and they should be used as often as symptoms are observed. Two or more applications may be required the first year.
- Author: Ben Faber
Gordon Frankie a bee biologist at UC Berkeley and I are doing a study to ultimately identify what plants could grown in avocado orchards to attract more honeybees, as well as other pollinators and potential biocontrol agents. There are five orchards in Ventura and Santa Barbara where we have been monitoring flower visitation by different insects in order to get a baseline of what is there before introducing potential pollinator/biocontrol attractants. The numbers are finally in for last spring avocado bloom. Virtually no honeybees, but significant numbers of syrphid flies. They are also called flower flies or hover flies. They superficially resemble bees. They are predatory as larvae, primarily feeding on aphids. Then they become important pollinators as adults. They are not as efficient as honeybees because they lack all the hairs on their bodies where pollen gets stuck and carried to female flowers. The cause of the honeybee decline has many causes, but the most likely one is the drought in the avocado growing areas. There just aren't any plants in the foothill areas to provide pollen and nectar year round.
Images of egg, larvae (going after aphids) and adult syrphid flies
- Author: Sabrina Drill
As discussed in previous issues of this newsletter, polyphagous shot hole borer (PSHB; Euwallacea sp.) is a new pest/fungal complex attacking a wide variety of host trees in Southern California, from avocado to common residential and street trees and native oaks and riparian species. The PSHB is morphologically identical to the tea shot hole borer, E. fornicatus, but a genetic analysis confirmed that this is a new species of ambrosia beetle. PSHB has been found to carry several symbiotic fungi, including new fungal species Fusarium euwallacea, andan undescribed Graphium species.It was first identified in 2003 in Whittier Narrows, an undeveloped riparian area in Los Angeles County, and was officially first linked to tree injury and mortality in a residential avocado in 2012. Since then, the pest complex, also referred to as Fusarium die-back, has spread throughout Southern California and is now present in Los Angeles, Orange, San Bernardino, Western Riverside, and San Diego counties. The population in San Diego County is a different genotype from that found in the rest of the region, possibly indicating a separate introduction. The pest/disease complex has caused significant impacts to the avocado industry in Israel, and is now spreading in commercial groves from Escondido to Fallbrook. It has caused injury and mortality of hundreds of ornamental trees in the urban areas, impacting roadways, botanical gardens, parks, and private residences, and impacts all major native riparian trees species. The official list of reproductive hosts, meaning trees that can support growth and reproduction of the beetles and fungi, includes 35 species with several having been confirmed in just the past few months.
The plant pathology, IPM, environmental horticulture, and natural resource management teams working on this pest at UCR and UC Cooperative Extension in San Diego, Orange, Los Angeles, and Ventura Counties have put together several useful tools that can help you manage this pest. These include a new field identification card set, a decision tree for tree removal, a guide to managing infested plant materials, and information about how to report an infested tree and how to collect samples for lab identification, as well as a continuously updated web-based map. You can find these at www.pshb.org.
To determine if your trees are affected by this pest:
- Look for a small (tip of a ball point pen) round entry/exit hole surrounded by wet discoloration of the outer bark
- Follow the gallery to look for the beetle (may or may not be present)
- Look for other hosts (Castor bean, sycamore, maple, coast live oak, goldenrain, liquidambar) showing symptoms of the beetle/disease
- Report suspect tree infestations to email@example.com with the following information:
o Your contact info (name, city, phone number, email)
o Suspect tree species
o Description of suspect tree's location (and/or GPS coordinates)
o Description of suspect tree's symptoms
o Photos of suspect tree and close-up photos of symptoms
- If symptom photos and descriptions indicate it might be PSHB/FDB a field assessment may be needed or a sample can be submitted following detailed directions on how to collect and submit a sample for fungal confirmation at pshb.org.
To protect your trees, avoid movement of infested firewood and chipped material out of infested areas. Infested material should be chipped to under 1”, wrapped in clear plastic, and solarized on site for up to 6 mos (depending upon environmental conditions). Research is ongoing in the use of insecticides and fungicides, as well as exploration of the use of endophytic bacteria as a biocontrol agent. If you suspect that you have found this beetle or seen symptoms of the Fusarium dieback on your tree please contact the Ventura CE office, your pest control advisor or qualified arborist, or contact UC Extension Plant Pathologist Dr. Akif Eskalen by at firstname.lastname@example.org. For more information visit www.pshb.org or http://eskalenlab.ucr.edu/avocado.html.
- Author: Akif Eskalen
The Polyphagous Shot Hole Borer (PSHB), Euwallacea sp. #1, is an invasive beetle that carries three fungi: Fusarium euwallaceae, Graphium sp. , and Acremonium sp. The adult female tunnels galleries into a wide variety of host trees, where it lays its eggs and grows the fungi. The fungi cause the Fusarium Dieback (FD) disease, which interrupts the transport of water and nutrients in over 35 tree species that are suitable for beetle reproduction.
Once the beetle/fungal complex has killed the host tree, pregnant females fly in search of a new host.
A separate invasion was recently detected in commercial avocado groves and landscape trees in San Diego county. It has been determined that the damage has been caused by another closely related species of PSHB (Euwallacea sp. #2), carrying a new species of Fusarium and Graphium. The beetle in LA, Orange, Riverside, and San Bernardino Counties are morphologically indistinguishable, but genetically distinct from the beetle found in San Diego County.
Signs and Symptoms
Attack symptoms, a host tree's visible response to stress, vary among host species. Staining, sugary exudate (B), gumming, and/or frass may be noticeable before the tiny beetles (females are typically 1.8-2.5 mm long). Beneath or near these symptoms, you may also see the beetle's entry/exit holes, which are ~0.85 mm in diameter. The abdomen of the female beetle can sometimes be seen sticking out of the hole.
Sugary exudate on trunks or branches may indicate a PSHB attack (photos A-E). Note that exudate may be washed off after rain events and therefore may not always be present on a
heavily infested branch.
PSHB attacks hundreds of tree species, but it can only successfully lay its eggs and/or grow the fungi in certain hosts. These include: Avocado, Box elder, California sycamore, Coast live oak, White alder, Japanese maple, and Red willow. Visit eskalenlab.ucr.edu for the full list.
Fusarium dieback pathogens cause brown to black discoloration in infected wood. Scraping away bark over the entry/exit hole reveals dark staining around the gallery, and cross sections of cut branches show the extent of infection. Advanced infections eventually lead to branch dieback and death of the tree
How to report a suspect tree
Please report suspected tree infestations to UC Riverside (email@example.com).
Submit the following information:
•Contact information (name, city, phone number, email)
•Suspect tree species
•Description of suspect tree's location (and/or GPS coordinates)
•Description of suspect tree's symptoms
•Photos of suspect tree and close-up photos of symptoms (see examples)
Take photos of suspect trees from several distances. Include photos of:
1. the trunk or symptomatic branches;
2. the symptoms (close-up); and
3. the entry/exit hole, if visible, with a ballpoint pen for scale (remove exudate if necessary). If dieback is observed, take a picture of the entire tree.