- Editor: Ben Faber
Vanessa Ashworth and Philippe Rolshausen, Department of Botany and Plant Sciences, University of California Riverside.
Have you ever wondered where your favorite avocado variety came from? Not the nursery where it was purchased but the long, tortuous path that led to its selection. How are different varieties related? Did the expert tell you that your avocado is a “Guatemalan x Mexican” but you were afraid to ask what that means? Has your carefully nurtured seedling raised from a ‘Hass' pit morphed into a tree bearing unconvincing fruit? If so, read on.
Avocado breeders refer to the different types of avocado as varieties or, more correctly, as cultivars. The ‘Hass' cultivar is by far the best known, but several hundreds of named avocado cultivars have been bred in the USA alone since avocado was first introduced here. None was developed by the big commercial seed companies; Instead, avocados were patiently selected, originally by indigenous cultures of Mesoamerica, much later by growers/enthusiasts, and fairly recently in avocado breeding programs. Almost never do we know the precise pedigree of a cultivar. We are sometimes told the maternal parent, but older cultivars (including ‘Hass') typically lack a record of parentage, and any thought of fitting today's cultivars into a concise family tree is hopelessly optimistic. In any case, prior to the mid-19th century very little is known about the plant material that was imported from abroad, with at best an indication of geographic provenance. And yet it would be far more efficient if we could arrange cultivars in a hierarchy or a series of related assemblages, instead of just looking at a random scatter.
In order to understand how today's cultivars are related we need to dig deeper. Going back in time, we know that indigenous civilizations in Mesoamerica recognized the value of the avocado's wild ancestor(s) and were actively selecting superior forms for thousands of years, which eventually led to a semi-domesticated avocado. Evidence of selection by human hand as far back as 8,000 years before present is preserved in archaeological sites in Puebla State, Mexico. At the time of European contact, written records indicate that there already existed three distinct types of avocado, each from a separate geographic center of origin. Today, we refer to them as botanical races, and they represent the “primeval soup” that gave rise to modern avocado cultivars.
Here is what we know about the three botanical races of avocado, respectively called (1) the West Indian (formerly known also as the South American), (2) the Guatemalan, and (3) the Mexican (also known as the “criollo”): Each exhibits a characteristic suite of traits that includes differences in leaf chemistry, peel texture and color, and sources of tolerance (diseases and salinity). The races were domesticated in separate geographic regions, the “West Indian” race in lowland coastal Mesoamerica (possibly Yucatán), the Guatemalan race in upland Guatemala, and the Mexican race in highland Mexico. The Guatemalan and Mexican races remained fairly local, so their names reflect their respective centers of domestication, but the “West Indian” race seems to have been spread far and wide by indigenous cultures in Meso- and South America and was, incorrectly, named for a much later destination. The explorations of the 15th and 16th centuries kicked off the worldwide distribution of (mostly West Indian race) avocados, reaching Spain in the early 17th century, Jamaica in the mid-17th century, and Indonesia by the mid-18th century. It wasn't until the mid- to late 19th century that the three races of avocado found their way to the United States, primarily Florida and California.
After the avocado was introduced to California and elsewhere, there followed countless rounds of selection, generally resulting in hybrids among the botanical races. The selection process consisted of growing out seedlings from the seeds of “good” cultivars and screening them for chance seedlings with promising characteristics. However, in the same way that children are not identical to their parents, seedlings grown from the pit of a fruit are not identical to the tree the fruit came from. Each seedling represents a reshuffled version of its parents' genomes. The only procedure that preserves an identical genome is clonal propagation. Budding and grafting techniques that, today, ensure clonal propagation and keep cultivars “true to type” were not used until the first half of the 20th century.
Contrary to many major crops, most avocado cultivars we have today are bursting with so much genetic diversity that breeding is actually rendered difficult. When we grow out seedlings we get a huge number that look (and taste) nothing like their parents and most are discarded. The poor selection efficiency (an estimated 0.2%) has to do with the large variability caused by multiple domestication centers and a long history of open-pollination. There is no immediate danger of a genetic bottleneck, but breeding is slow and outcomes are unpredictable.
In the absence of accurate breeding pedigrees, we have come to describe avocado cultivars in terms of their resemblance to one or several botanical races, based on their combination of traits. For example, ‘Hass' is considered to be a Guatemalan x Mexican (G x M) hybrid because it has the thick, rough skin of the Guatemalan race but the high oil content of the Mexican race. Cultivar ‘Gwen' is also called a G x M hybrid, but is possibly a little more Guatemalan than Mexican and certainly more Guatemalan than ‘Hass'. Cultivar ‘Fuerte' is often called a G x M hybrid or sometimes Mexican which makes it more Mexican than ‘Hass' and a lot more Mexican than ‘Gwen' but not as Mexican as ‘Mexicola'... What these examples show is that description of avocado cultivars in terms of botanical race composition has its limitations and we are most likely dealing with a continuum of blending among the three botanical races. Can we improve on this? ¡Sí, se puede!
Enter the modern tools of genomics: molecular markers and new analytical approaches are emerging that can peek inside ancestral genomes and discover hidden patterns within genetic information. The new approaches track the progress of tiny nuggets of genetic information (markers) by comparing their distribution across a large numbers of cultivars. Several studies have revealed that today's cultivars continue to harbor the genetic footprints of the three botanical races and a lot more besides. In this instance, having cultivars bursting with genetic diversity is a good thing. Eventually, given a large enough dataset (markers and cultivars), we will be able to place cultivars into assemblages that go beyond first-pass assignments to one or more botanical races.
A working framework of cultivar assemblages confers predictive information that helps guide cultivar choice and breeding decisions. In a slow-growing tree crop such as avocado where years elapse before many traits are available, a marker-guided, predictive framework represents huge savings in time and resources. A simple example illustrates this point: In the event of an epidemic there is no time to start breeding new cultivars from scratch. Instead, the first line of defense is to explore tolerance present in existing cultivars, and having access to a framework helps prioritize among hundreds of cultivars. Avocados of Mexican ancestry are known to exhibit better disease tolerance than Guatemalan and West Indian stock, so material that contains a Mexican-race footprint would be a good choice for early screenings for tolerance. Epidemics such as grape phylloxera or potato blight are well known examples where the mainstream cultivars shared too uniform a genetic base and where a cultivar monoculture permitted disease pressure to attain dangerous proportions. Consequently, today's dominance of ‘Hass' should be viewed with some trepidation.
In fact, it is possible that we are facing a new epidemic right now: Fusarium dieback (FD) has impacted many tree species, especially the avocado, in southern California since its introduction in 2013. The Fusarium fungus is transmitted by a beetle, the polyphagous shot hole borer (PSHB), and fungus and beetle act in partnership to breach the defenses of their plant host, leading to wilting, branch dieback, and fruit losses. Moreover, we know that ‘Hass' is highly susceptible to the disease. It is time to look for sources of tolerance and to revisit the cultivars that have lost ground to ‘Hass', such as ‘Bacon', ‘Fuerte', and ‘Reed', and to take advantage of germplasm collections that contain material of older vintage, often dating back to the start of the 20th century, if not before. A major germplasm collection is maintained at the University of California South Coast Research & Extension Center in Irvine, and additional material is grown at UC Riverside's Agricultural Operations. A good digital resource to study the diversity of cultivars available in California is the UC Riverside Avocado Information website http://ucavo.ucr.edu/avocadovarieties/VarietyFrame.html.
Is the consumer ready to embrace new cultivars? Preliminary evidence is promising. There are few opportunities today to come face-to-fruit with the more unusual cultivars because they have largely been banished to back yards or live sheltered lives in today's germplasm collections. A notable exception is the UC Riverside avocado breeding program. Headed by Dr. Mary Lu Arpaia, the program runs a monthly avocado tasting session where participants record their views on visual (external) fruit characteristics and on fruit sensory qualities (flavor). These tasting sessions have shown that participants are drawn to novel fruit shapes/sizes and value the taste of many cultivars, not just of ‘Hass'. There is also considerable interest in learning more about existing cultivars and about the history of avocado breeding and domestication.
Some of the avocado cultivars featured on the UC Riverside Avocado Information website
Clearly, to place avocado cultivars into a workable framework that reflects their interconnections as well as the footprint of the three botanical races will be a valuable addition to the tools available to breeders and will benefit our knowledge of avocado diversity. For now, however, we are unable to give concise answers to the questions of the introductory paragraph, but it is safe to say that your favorite cultivar is probably a hybrid between at least two botanical races of avocado, contains genetic footprints left by ancient Mesoamerican breeders, capriciously gives rise to highly promising seedlings, and has a murky pedigree yet to be laid bare.
Water woes are probably not going to go away, so readup on how to best manage water at this new blog.
- Author: Rachael Long
Guest post from Rachael Long, UC Cooperative Extension Farm Advisor, Yolo County
The Yolo County Flood Control and Water Conservation District (YCFC) is an agency that supplies water to farmers in northern California. The agency is at the forefront of innovative efforts aimed at banking groundwater by diverting flood waters into their unlined canals. This gives flood waters time to infiltrate soils and recharge groundwater.
Using a water right permit that they recently obtained from California's State Water Resources Control Board, flood waters from recent storms are being captured from Cache Creek as it enters the Sacramento Valley. YCFC recently opened their lateral gates, allowing the flood waters to...
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Microbes can form large, jelly-like mats that lead to well failure from what is known as biofouling. Biofouled wells can be both expensive and technically challenging to repair. There are even times that repair is not possible and replacement is the only option. In Washington State, for example, researchers have encountered well pipes completely clogged by mats of bacteria....
California's Sacramento-San Joaquin River Delta region, commonly referred to simply as the Delta, is often described as a unique part of the world. Although it is located between two big urban centers – the greater Sacramento and San Francisco Bay areas – the Delta can feel like another world altogether.
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The California drought has shined a spotlight on stories of people and communities living without water. Unfortunately, lack of access to clean and affordable water is not a new issue. Water security has been an enduring challenge across the state in wet and dry years alike, particularly for disadvantaged communities. Trying to meet concerns about water availability and affordability with pragmatic action is where things get both complicated and interesting.
One approach that the state has invested a great deal in exploring is known as integrated regional water management. While it is a complex topic, the basic idea is that there are multiple needs for water throughout the...
Street-side stormwater facilities are turning runoff once seen as a nuisance into a resource. Also known as bioretention areas, rain gardens, and bioswales, these small stormwater facilities provide a decentralized approach to alleviating peak stormwater runoff and subsequent flood damages. These are particularly critical functions in cities like San Francisco where the storm and sanitary sewer systems are combined because they help managers to prevent dreaded “combined sewer overflow” events. As a bonus, stormwater facilities have also proved useful in promoting groundwater recharge and filtering pollutants as water percolates through soils.
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This is a sad time to be an avocado. Winter's gone and temperatures are just ripe for flowering and the trees are going bust. So much so, that those sad leaves that have accumulated salts over the last year are being dropped and only flowers might be seen, especially on young trees. This is time for a little shot of nitrogen to encourage some new vegetative growth. Not a bunch, but a nudge. Several pounds per acre, something less than 10-15 pounds of N for a mature orchard and even less for a new orchard.
A commonly held belief is that if you apply nitrogen at the wrong time, it will push resources into vegetative growth at the expense of flower and fruit. This is somewhat true for annual plants that get most of their nutrients from outside sources (soil, air, fertilizer, water), but trees have a huge buffer in their storage organs (roots, stems, leaves, etc.). Most growth in trees occurs from this storage source and most importantly from photosynthesis and the sun. The more sun captured the more energy for flowering and fruit production.
So it is this competition for photosynthates that becomes the most limiting factor. When there is not enough to go around, the tree sheds fruit. If you see fruit dropping off a tree after applying a slug of fertilizer, it's a salt effect. Too much salt and it causes a water competition and the tree is stressed. It's not the nitrogen, but too much salt. With fertigation this is not so likely to happen as when dry fertilizers were applied and someone got too aggressive with the application
In fact a dose of nitrogen fertilizer is a good idea at this time when there are lots of flowers. This can encourage a flush of leaves that will protect the fruit that does set from sunburn and damage that would cause fruit to drop. A bit of nitrogen to encourage leaf replacement is a good approach to dealing with persea mite damage that occurred the previous season.
For further reading about the competition between vegetative and reproductive growth as affected by nitrogen (or little affected in fruit trees by nitrogen), D.O. Huett wrote a wonderful review of past research on this topic:
Also, if the trees have really defoliated, it might be time to do some whitewashing, south and west sides of branches, to prevent sunburn.
Avocado defoliated and ones in a balanced bloom
Drought Induced Problems in Our Orchards
Abiotic disorders are plant problems that are non-infective. They are not caused by an organism, but through their damage, they may bring on damage caused by organisms. Think of a tree hit by lightning or a tractor. The damage breaches the protective bark which allows fungi to start working on the damaged area, eventually leading to a decayed trunk. It was the mechanical damage, though that set the process in motion.
Too much or too little water can also predispose a plant to disease. Think of Phytophthora root rot or even asphyxiation that can come from waterlogging or too frequent irrigations.
Salinity Effects from Lack of Water
Lack of water and especially sufficient rainfall can lead to salinity and specific salts like boron, sodium and chloride accumulating in the root zone. This happens from a lack of leaching that removes native soil salts from the root zone or the salts from the previous salt-laden irrigation from the root zone. These salts cause their own kind of damage, but they can also predispose a tree to disorders, disease and invertebrate (insect and mite) damage.
Lack of water and salt accumulation act in a similar fashion. Soil salt acts in competition with roots for water. The more soil salt, the harder a tree needs to pull on water to get what it needs. The first symptom of lack of water or salt accumulation may be an initial dropping of the leaves. If this condition is more persistent, though we start to see what is called “tip burn” or “salt damage”. Southern California is tremendously dependent on rainfall to clean up irrigation salts, and when rain is lacking, irrigation must be relied on to do the leaching
As the lack of leaching advances (lack of rainfall and sufficient irrigation leaching) the canopy thins from leaf drop, exposing fruit to sunburn and fruit shriveling.
Leaf drop and fruit shriveling in avocado.
In the case of sensitive citrus varieties like mandarins, water stress can lead to a pithy core with darker colored seeds, almost as if the fruit had matured too long on the tree.
Total salinity plays an important factor in plant disorder, but also the specific salts. These salts accumulate in the older leaves, and cause characteristic symptoms that are characteristic in most trees. Boron will appear on older leaves, causing an initial terminal yellowing in the leaf that gradually turns to a tip burn.
Often times it is hard to distinguish between chloride, sodium and total salinity damage. It is somewhat a moot point, since the method to control all of them is the same – increased leaching. There is no amendment or fertilizer that can be applied that will correct this problem. The damage symptoms do not go away until the leaf drops and a new one replaces it. By that time hopefully rain and/or a more efficient irrigation program has been put in place.
The Impact of Drought on Nutrient Deficiencies
Salinity and drought stress can also lead to mineral deficiencies. This is either due to the lack of water movement carrying nutrients or to direct completion for nutrients. A common deficiency for drought stressed plants is nitrogen deficiency from lack of water entraining that nutrient into the plant.
This usually starts out in the older tissue and gradually spreads to the younger tissue in more advanced cases.
The salts in the root zone can also lead to competition for uptake of other nutrients like calcium and potassium. Apples and tomatoes are famous for blossom end rot when calcium uptake is low, but we have also seen it in citrus. Low calcium in avocado, and many other fruits, leads to lower shelf life. Sodium and boron accumulation in the root zone can lead to induced calcium deficiencies and increased sodium can also further lead to potassium deficiencies. Leaching can help remove these competitive elements.
Drought Effect on Tree Disease
Drought and salt stress can also lead to disease, but in many cases once the problem has been dealt with the disease symptoms slowly disappear. They are secondary pathogens and unless it is a young tree (under three years of age) or one blighted with a more aggressive disease, the disease condition is not fatal. Often times, in the best of years, on hilly ground these diseases might be seen where water pressure is lowest or there are broken or clogged emitters. The symptoms are many – leaf blights, cankers, dieback, gummosis – but they are all caused by decomposing fungi that are found in the decaying material found in orchards, especially in the naturally occurring avocado mulch or artificially mulched orchards. Many of these fungi are related Botryosphaerias, but we once lumped then all under the fungus Dothiorella. These decay fungi will go to all manner of plant species, from citrus to roses to Brazilian pepper.
Another secondary pathogen that clears up as soon as the stress is relieved is bacterial canker in avocado. These ugly cankers form white crusted circles that ooze sap, but when the tree is healthy again, the cankers dry up with a little bark flap where the canker had been.
Drought Effect on Pests
Water/salt stress also makes trees more susceptible to insect and mite attack. Mites are often predated by predacious mites, and when there are dusty situations, they can't do their jobs efficiently and mites can get out of hand. Mite damage on leaves is often noted in well irrigated orchards along dusty picking rows
Many borers are attracted to water stressed trees and it is possible that the Polyphagous and Kuroshio Shot Hole Borers are more attracted to those trees.
And then we have conditions like Valencia rind stain that also appears in other citrus varieties. We know it will show up in water stressed trees, but we aren't sure what the mechanism that causes this rind breakdown just at color break. Could it be from thrips attracted to the stressed tree or a nutrient imbalance, it's not clear?
Water and salt stress can have all manner of effects on tree growth. It should lead to smaller trees, smaller crops and smaller fruit. The only way to manage this condition is through irrigation management. Using all the tools available, such as CIMIS, soil probes, soil sensors, your eyes, etc. and good quality available water are the way to improve management of the orchard to avoid these problems.
Scroll down for Images
Tip Burn, notice sun burn bottom right hand fruit
Endoxerosis with dried out core
Blossom end rot
Bot gumming in lemon
Black Streak in Avocado
Citrus red mite
Polyphagous Shot Hole Borer damage on avocado
Valencia Rind Stain
Nutrient availability from organic sources has been considered “slow release” by many growers and advisers. This may be true in environments are colder and especially soils are cooler. Organic nutrients are dependent on microbes to break down materials and release those nutrients, and when soils are cold, microbes can't do their thing. Soils in much of agricultural California tend to be warm and lack the freezing conditions that occur in many soils in the continental US. Imagine how much microbial activity occurs in the Mid-West when soils cool down to 32 deg F at a four inch depth and deeper. The top layers of soil are where organic matter accumulates and where most microbial activity occurs. When soils cool below 50 deg F, nitrogen leaching becomes less common, because less activity is occurring which also coincides with much less plant growth.
Soils in coastal California rarely fall below 50 deg F in the surface layers, so microbial activity is ongoing, all year long. So the question is, how “slow acting” are organic fertilizers? A recent study by Tim Hartz, Richard Smith and Mark Gaskell looked at release rates of injectable organic fertilizer and found that much of the nutrient release occurs within about a week after application depending on the formulation and temperature during the study. The results conform to another study that they did where they evaluated the nitrogen release rates of dry formulations of organic fertilizers – compost, manures, feather meal, etc.
Aside from the issues of the higher costs of these materials and their potential clogging, there is the issue of application timing. In the case of avocados and citrus, adequate levels of nitrogen are needed in the trees going into to fruit set in order to optimize set. And then after fruit set, in order to maintain growth into the fast growth period, again nitrogen needs to be adequate. Using organic fertilizers with a rapid conversion to useable forms of nitrogen, means that application timing should coincide with these critical periods in tree phenology or growth cycle.
Using information on organic nutrient management based on work from cold soil climates needs to be carefully evaluated before applying it to California soils. One of the most common problems in organic production is nitrogen management. Part of the problem is the cost of supplemental nitrogen amendments, but also learning to anticipate when that applied nutrient becomes available to the plant. Developing better estimates for local release rates and patterns will better help manage organic nutrient sources.
Summary: Limited soil nitrogen (N) availability is a common problem in organic vegetable production that often necessitates additional N fertilization. The increasing use of drip irrigation has created a demand for liquid organic fertilizers that can be applied with irrigation. The N availability of three liquid organic fertilizers was evaluated in an incubation study and a greenhouse bioassay. Phytamin 801 contained fishery wastes and seabird guano, while Phytamin 421 and Biolyzer were formulated from plant materials. The fertilizers ranged from 26 to 60 g·kg−1 N, 8% to 21% of which was associated with particulate matter large enough to potentially be removed by drip irrigation system filtration. The fertilizers were incubated aerobically in two organically managed soils at constant moisture at 15 and 25 °C, and sampled for mineral N concentration after 1, 2, and 4 weeks. In the greenhouse study, these fertilizers and an inorganic fertilizer (ammonium sulfate) were applied to pots of the two organically managed soils with established fescue (Festuca arundinacea) turf; the N content of clippings was compared with that from unfertilized pots after 2 and 4 weeks of growth. Across soils and incubation temperatures, the N availability from Phytamin 801 ranged from 79% to 93% of the initial N content after 1 week, and 83% to 99% after 4 weeks. The plant-based fertilizers had significantly lower N availability, but after 4 weeks, had 48% to 92% of initial N in mineral form. Soil and incubation temperature had modest but significant effects on fertilizer N availability. Nitrification was rapid, with >90% of mineral N in nitrate form after 1 week of incubation at 25 °C, or 2 weeks at 15 °C. N recovery in fescue clippings 4 weeks after application averaged 60%, 38%, and 36% of initial N content for Phytamin 801, Phytamin 421, and Biolyzer, respectively, equivalent to or better than the N recovery from ammonium sulfate.