By Dr. Thomas T. Yamashita
Occasionally, almond trees will bloom early, sometimes as much as one to two weeks ahead of normal emergence. This is especially true in certain varieties, such as the Carmel, Sonora, and Butte, which sometimes feature unusually high bud drop.
What are the major considerations for growers regarding early bloom? What are the underlying causes of bud drop, and how can it be mitigated or avoided? There are several salient considerations:
- Discrepancies between emergence of different varieties will minimize bloom overlap and opportunity for effective pollination.
- Early bloom emergence may coincide with unusually cool days and evenings, minimizing both bee activity and the pollen germ tube growth rate that are necessary for successful pollination.
- Nut sets will have an increased chance of encountering a killer frost.
- When there have been multiple years of rains and moist conditions, the inoculum levels of pathogens will be particularly high, and thus it will be important to provide maximum disease protection from the outset of the season.
- What is the cause of the premature bud drop in varieties such as Carmel, Price, and Wood Colony?
Point 1: Minimal Varietal Synchronization of Bloom
Early varieties such as NePlus or Sonora are oftentimes far into their bloom before Nonpareil flowers emerge. On the other hand, Carmel emergence is sometimes far behind that of Nonpareil. Thus, in years were the opportunity for cross pollination is minimized, it is especially cost-effective to practice controlled pollination, supplementing the pollination process by placing collected pollen at the exit holes of beehives or blowing collected pollen into the tree canopy.
More than 30 years ago, I began developing a highly effective pollen-processing methodology. One of the problems I observed in existing technologies was the erratic and oftentimes low viability of finished pollen products. The pollen products on the market at that time were diluted with other flower parts, reducing their effectiveness.
Thanks to my understanding of the factors that contribute to high pollen viability, I was able to process pollen into a finished product that was almost pure pollen grains, which had the appearance of gold dust. Tests for viability indicated a near perfect germination percentage. This pollen was combined with several key additives and used for pollination.
To test this experimental pollination product, we selected a 240-acre contiguous block of almonds, which entirely lacked pollinators.
Merced variety trees had long before been planted on the site to serve as the sole pollinators for the Nonpareils which now dominated the block. However, the Merceds has degenerated to the point of total elimination. The next closest almond block was more than 5 miles away.
Pollen was placed in bee inserts, beginning at 5% to 7% bloom and continuing for 4 consecutive days. The pollen was diluted 1:1 with key enhancement materials. Eight grams of the diluted pollen were placed into each insert every morning for 4 consecutive days (3 hives per acre, with a total of about 48 grams per acre of pure pollen). The worker bees responded to this mixture with notable excitement. After placing the pollen at the mouth of the hive, they would immediately emerge, roll their bodies in the pollen dust, and then fly off.
At harvest, this block averaged over 2,500 pounds of almond meats per acre. This was quite impressive, as the yields were derived entirely from Nonpareil and Mission varieties, which constituted two-thirds of the block. The take home message was that the technology worked. While there is no substitute for excellent placement of pollinators in an almond planting, certain years may demand pollen supplementation, and this technology does the job.
However, while the quality of your pollen is important, so is the quality of your tree flowers. For example, in many years, trees in the block we tested would produce thousands of blooms. However, a close inspection of the flowers showed that as few as 10% were well-formed and truly viable. In many cases, the pistil (female organ) of a flower was partially or entirely degenerated. Thus, while the added pollen was providing a true supplementation to natural pollination, a low population of truly viable flowers had kept the technology from showing its full potential.
Point 2: The Problems with Cool Ambient Air Temperatures Experienced During Bloom
From the time a flower is completely expanded and open, there is a narrow window of about 72 to 96 hours of receptivity to pollination and fertilization. During this period, if overcast days and cool ambient temperatures hover around 65 degrees Fahrenheit or below, several impeding factors begin to materialize. Namely, bee activity is minimized, and the growth rate of pollen germ tubes is markedly reduced. For effective pollination to occur, a viable pollen grain must be placed on the female organ (the stigma) and within 72 to 96 hours grow downwards through the ovary to initiate nuclear fusion and successful fertilization.
As discussed above, certain practices can be instituted to excite and induce bee activity. However, let’s focus on defining the formidable obstacles that ensue during a cold, wet, and overcast pollination period, and the farm practices which can be manipulated to help overcome these obstacles.
Low temperatures produce less than ideal conditions for pollination. If you have done all you can to facilitate transfer of pollen to viable flowers, then the obstacle to effective pollination becomes pollen germination and the rate of pollen tube growth. In most living organisms, growth rates are directly correlated to temperature effects on a metabolic process known as ‘respiration.’ This is a process which utilizes carbohydrates and converts them to energy, which is used to build molecules used for growth and development.
We know that most plants grow 2.5 to 3 times faster at 75 degrees Fahrenheit than they do at 45 degrees. Thus, if a pollen grain germinates and sends forth a germ tube to traverse 10 mm of floral tube and ovary in 3 days at a temperature of 75 degrees, we can project that the same process would take 9 days at 45 degrees. Herein lies one of the greatest obstacles to effective fertilization of almond trees, as it is not unusual to experience suboptimal weather during the bloom period. How can a farmer mitigate or overcome this environmentally induced deterrent to effective pollination?
During my studies with pollen processing technology, I conducted various lab studies examining factors affecting pollen germ tube growth. The factors which consistently stood out as necessary to overcome the obstacle of cold weather were:
- Carbohydrate constitution
- Calcium levels
- Boron levels
- B vitamin levels
I found that all four factors work cooperatively to increase the rate of pollen tube growth. But in addition, it was apparent that another parameter was of utmost importance: not just how quickly pollen germs tubes grew, but also how straight they grew. When supporting factors are in a deficient state, pollen germ tubes curve unnaturally, never connecting with the ovary. Instead, growth is arrested as the tube collides in a destructive fashion with the outer walls of the floral tube (pistil). This often gives rise to what is known as ‘dummy set.’
While all four factors above contribute to straight and true growth of the tube, sometime later I learned of tracer studies which showed that the leading edges of pollen germ tubes are replete with boron and calcium. This didn’t come as a major surprise, as it was previously well known that these two elements have a significant role in ensuring the strength and integrity of many plant tissues.
In other studies, we had observed the consequences of a common practice— ‘slugging’ trees with the majority of their yearly nitrogen in December and January. We ultimately found that inconsistencies in set and yield could be traced back to this practice. This is because about 8,200 Calories of energy are required to assimilate 1 pound of nitrogen. In the case of slugged trees, this assimilation occurred when trees weren’t undergoing photosynthesis, and so the needed energy was supplied by reserve carbohydrates.
When the spring push of blossoms came, they were fueled by a diluted supply of carbohydrates. Because of insufficient nutrition, these flowers would experience malformed pollen germ tube growth, and consequently substandard set and yield rates.
This understanding led to the development of refined soil and foliar nutritional regimes to maximize pollination and fruit set. While there isn’t space here to describe these regimes in great detail, the generalities can be summarized as follows:
- Maintain soil calcium levels in excess of 500 ppm for light soils and 650 ppm for heavier soils.
- Maintain available soil potassium levels in excess of 150 ppm for light soils, and 250 ppm for heavier soils.
- Build soil microbial populations and activity, which enhances the consistent release and availability of various key minerals, while enhancing a variety of chemical and physical characteristics of the soil.
Foliar programs which have proven to have an impact on pollination and set include incorporating into the pink bud and bloom sprays:
- A slow-release nitrogen
- A balanced carbohydrate
- A chelated calcium foliar
- A silicone-based surfactant
In addition, during the pink bud and bloom spray period, it’s important to avoid the use of metal-containing sprays, such as those which include zinc, copper, manganese, iron, and aluminum. The reason for this is most metals have the ability to displace calcium from plant cell walls and membranes, which disrupts a variety of critical biological processes.
It should be noted that the foliar program described above has been shown to extend the active life of flowers. This is largely due to the use of calcium, which has been shown to slow the onset of aging. Carbohydrates accomplish much the same, which is why many florists add a small amount of sugar to vases of water, extending the lifespan of bouquets.
Point 3: The Threat of a Killer Frost
Getting a good bud set is only one of many steps in the process of achieving high yields. Blooms that are a week or two early raise the risk of frost damage. Besides the usual ground preparation and moisture regimes designed to combat frost, growers must pay close attention to the nutritional status of the developing crop.
With early bloom, growers should focus on pushing the early development of crops, so that nuts won’t be susceptible to frost damage. It should be noted that when plants are allowed to grow during cool spring weather, the tendency is not only to grow slowly, but to develop low-integrity tissues that are more prone to frost damage. Tissues with higher density and firmness are better able to cope with sudden changes in temperature.
Freezing is not just a matter of temperature. It is a physical-chemical phenomenon which can, in part, be influenced by the number of particles dissolved in a liquid. For example, 1.6 ounces of glucose dissolved in a pint of water will lower the freezing point by 1.7 degrees Fahrenheit. The relationship between the quantity of dissolved glucose the change in freezing point is proportionate. Thus, twice as much glucose, 3.2 ounces, will lower the freezing point of a pint of water by twice as much, 3.4 degrees.
A tree with low vigor and health may typically contain the equivalent of about 0.6 ounces of glucose per pint of moisture. A healthy tree with high vigor may have about 2.4 ounces of glucose per pint of moisture. Thus, a weakened tree may have the potential to resist freezing down to about 31.4 degrees, while a healthy tree may resist freezing down to 29.4 degrees.
However, frost hardiness in living tissues goes well beyond mere considerations of dissolved solutes. Typically, tissues from healthy plants have a tighter, more rigid framework of cells and membranes, which adds to frost hardiness. Recall that high integrity tissues are typically replete with calcium and boron in cell walls and membranes. Such characteristics impart resistance to freeze damage. Thus, healthy trees with frost-hardy tissues can withstand temperatures down to 27 degrees or less.
After promoting nut set with pink bud and bloom sprays, switch to more balanced nutrients which can accelerate development even more, such as 20-20-20, chelated calcium, boron, and micronutrient mixes (e.g. zinc and manganese). As you approach nut gelation, you may want to enhance further potassium treatments by switching from 20-20-20 to 9-15-30, or comparably balanced blends. Years with early bloom are sometimes typified by leaf buds emerging together, or more commonly, behind the flower buds. Such a scenario is further cause to step up nutrition, especially during the early stages of development.
Point 4: Multiple Successive Years of Wet Weather, with Buildup of Disease Inocula
In periods where there is extensive rainfall over multiple years, such as occurred in the mid-1990s, environmental conditions are conducive to the proliferation of disease. Many of these, such as brown rot and scab are easily transported by wind currents. Others, such as anthracnose and shot hole, tend to stay more localized, as the former is a wet sticky spore, while shot hole requires the rain-facilitated transport of spores. A key factor to remember is these pathogens tends to have multiple lifecycles during the season, as they’re capable of producing more than ‘crop’ of spores during the year. If moist and relatively mild temperature conditions prevail, these organisms will respond with growth and reproduction. One may have the cleanest orchard in the state, but can easily inherit the unsanitary, spore-laden conditions of neighboring fields, or in some cases, those of fields many miles upwind.
One of the most dramatic historical examples of wind-transported epidemics was the outbreak of coffee rust in Brazil in the late 1970s. It was known that coffee rust was indigenous to coffee plantations in Africa, but had never before been observed in South American plantations. When plant pathologists were brought in to investigate its introduction into Brazil, the disease was traced back to Africa. Apparently, rust spores were caught up in wind currents and funneled into a jet stream, which transported the spores more than 3,500 miles across the Atlantic Ocean, eventually dropping them in the lap of Brazilian coffee plantations.
This long-distance spread of disease or contaminants can occur in California, or any region for that matter. I once ran a study of the levels of cadmium in the San Joaquin Valley. Cadmium is a natural contaminant of automobile tires. Thus, the closer one gets to the highway, the higher the concentrations of cadmium that may be found. Cadmium was used as a model of particulate transport in the San Joaquin Valley. As the winds come off the Pacific Ocean through the Carquinez Strait near San Francisco Bay, they move east and hit the natural barrier of the Sierra Nevada. One half of the wind currents split northward, and the other half south. It is part of the reason why our predominant wind patterns are northwesterly in orientation. While cadmium levels begin to dilute and decrease as winds carry them southward, occasional areas can be found with very high levels of cadmium. These areas tended to reside in what I call “positive eddying zones,” where winds swirl and concentrate particulates. The swirling and concentrating of particulates is not unlike the accumulation of leaves and dust that occurs by front porches following a windstorm.
This same model must be kept in mind with wind-transported diseases such as brown rot and scab, and many others. For example, fruit growers around the foothills in Reedley, Kingsburg and Dinuba are especially careful in their disease programs, because they reside in a positive eddying zone. Unfortunately for them, they inherit spores from upwind and surrounding neighbors who practice lower levels of disease control.
The take home message is that a grower should not take shortcuts with disease control in years where there’s a great deal of wet weather. The key to disease control resides in taking a preventative approach. The preventative approach is especially important with deep-seated diseases such as anthracnose, which can persist in lesions and cankers in the wood. Growers who skimp on protective sprays early in the season invariably must contend with disease pressures extending longer into the season.
Point 5: The Premature Bud Drop in Certain Almond Varieties
In the late ‘90s, when I was still particularly active in the field, we had a season where several varieties of almonds experienced premature abscission of flower buds. Moderate to severe bud drop was observed in Sonora, Livingston, Wood Colony, Butte, and Carmel varieties. The Carmel was the most severely affected variety.
Examination of many affected orchards revealed the etiology of this malady, with the Carmel variety presenting a particularly effective model for the premature bud drop. Carmel is a variety with slightly lowered photosynthetic efficiency than that of Nonpareil, NePlus, Mission, or other cultivars. That is, the efficiency with which an almond tree can utilize solar energy and convert carbon dioxide to plant food appears to be relatively lower in the Carmel variety.
Most almond trees will harvest about 13 to 18 milligrams of carbon dioxide per 100 square centimeters of leaf surface area, during 1 hour of exposure to noontime sunlight. Not only is Carmel on the lower end of this statistic, but this variety appears to be especially sensitive to shifts in adverse environmental conditions. Like all almond trees, Carmel is what we call in plant physiology a “carbon-3” plant.
Complete characterization of a carbon-3 (C-3) plant is complicated and involved. However, for our purposes what is most pertinent about a C-3 plant is its sensitivity to high summer temperatures and high light intensity. C-3 plants respond to high temperatures and high light intensity by undergoing ‘photorespiration’. During photorespiration, the plant converts about 50% to 60% of its normal volume of carbon dioxide. That is, photosynthesis is reduced to about 50% capacity. Furthermore, when hot summer temperatures arrive, another physiological process, ‘respiration,’ increases proportionately with the rise in temperature. Respiration represents various biochemical processes which, in short, utilize energy. That the rate of respiration is directly proportional to the rise in temperature is well known in physiology, and is oftentimes expressed by scientists as ‘Q-10,’ or the quotient of the rate of respiration at a higher temperature in relation to respiration occurring at 10 degrees Celsius lower.
For example, respiration increases approximately 2.5 to 3.0 times between 45 degrees and 75 degrees Fahrenheit. Between 75 degrees and 105 degrees, the Q-10 reduces to 2.0 (i.e. plant respiration occurs 2.0 times faster at 105 degrees than at 75 degrees). Photorespiration in most C-3 plants is initiated above 90 degrees. The Carmel variety appears to be one of the more sensitive cultivars to hot weather and the induction of photorespiration. In most instances, we have observed Carmel shifting into photorespiration at 94 degrees, whereas Nonpareil, NePlus and most other varieties are a bit less sensitive and can withstand slightly higher temperatures before succumbing to severe photorespiration. The process of shifting into photorespiration takes more of a toll on the tree than is realized as the role of Q-10 factors means that respiration is increasing on the other side of the spectrum. In other words, we have two opposing forces set in motion:
- Food-making is reduced on end due to photorespiration
- Food use is increased on the other end due to Q-10 phenomena
The result is that the tree’s physiology is stressed to the limit. This is especially so if the tree is carrying a crop load that it needs to mature and fill out with carbohydrates, proteins and fats. In a year with hot summer temperatures, this stress can be quite formidable.
For example, in 1996 we experienced over 45 days of 100+ degree temperatures. Most of these temperature extremes came in July and August, with many successive hot days. There were more than 60 days with temperatures of 94 degrees or higher.
The normal season for active photosynthesis of almond trees begins with at least ½+ expansion of the leaves, or about mid-March. The period for active photosynthesis, then, generally spans the months of mid-March to the end of September, or about 6 ½ months. On most warm days, 94 degree temperatures are reached by 11 am to noon, and continue until 5 pm. Thus, a variety that is sensitive to hot weather, such as the Carmel, will have undergone active photorespiration for a minimum span of 300 hours (60 days x 5 hours per day). While in active photorespiration, most trees will be operating at about 50 % of their food-making capacity. Thus, in 1996 the Carmel variety lost approximately 150 hours (300 hours x 50%) of active photosynthesis, or about 15 full days of food-making (150 hours / 10 hours per day). Most of this deficiency in food-making came at a period of high demand during nut fill and maturity, all of which added up to a period of extreme physiological stress. If other health-debilitating pressures are at play, the physiological stress can reach a point of severe economic damage.
For example, consider when there have been several consecutive years of wet weather, which often produces a variety of soil-borne disease pressures. This not only includes water mold fungi, such as Pythium and Phytophthora, but other groups such as Rhizoctonia and Fusarium, and increases in plant-parasitic nematodes. All work in concert, in many cases the physical and physiological damages to the root system by nematodes predispose roots to colonization by water mold and other fungal pathogens.
In many of our examinations of trees afflicted with severe bud drop, the root system is affected with either distinct nematode damage, root tip necrosis, or both. If the tree is grown in a sandy soil with low calcium, boron and other minerals, damages are intensified even more. We have, for example, conducted nutrition studies with sterile, hydroponic tomato cultures varying the levels of calcium from 0 to 600 ppm (with ½ strength Hoagland Solution as a base). After a period of time, all tomato seedlings planted in cultures with 200 ppm or less calcium begin to degenerate. The lack of tissue integrity normally imparted by calcium becomes diluted, and membranes begin leaking out cell contents before undergoing total collapse and necrosis of tissues.
Thus, in a soil environment where important integrity-contributing minerals, such as calcium and boron are limiting, nematode or soil-borne fungal pathogen pressures can incite relatively higher degrees of damage. If the tree has undergone physiological stress during the summer, root tissue integrity is further minimized.
Additionally, in some years we observed that younger trees with minimal canopy and maximum ground floor exposure to direct sunlight experienced a literal cooking of the soil. This was especially true of sandy soils with non-tillage management, which characteristically have many roots within the top 6” of soil. In hot years, soil temperatures exceeding 130 degrees have been recorded. It is difficult for delicate root tips to withstand this form of heat stress. With all the above factors set in motion, the stage is set for considerable setbacks to almond trees.
The tips of the roots are the manufacturing sites for key plant hormones, known as cytokinins. These important compounds are translocated to various growing points where they play a major role in cell division, cell elongation, and creation of a sink for photosynthates. Flower buds are a major repository and site for translocated cytokinins. When in place, the cytokinins not only help to break dormancy, but also help to induce cell division. In sufficient quantities, the delicate balances of various hormones (e.g. abscisin, gibberellin, auxin, and cytokinin) become instrumental in the flower push and overall growth and development of the tree.
A droplet of cytokinin, if placed on detached leaves, will create in the exposed tissue a collection site or sink for plant food. This process also occurs when cytokinins are translocated to flower buds. The buds are healthy and capable of drawing important plant foods to sustain demands on rapid growth and development. If both stored photosynthates and the necessary balances of hormones are suboptimal, the flower bud’s operative metabolism is minimized. As mentioned above in ‘Point 3,’ plant tissues can be made more frost-tolerant with increased photosynthates, resulting in higher solute concentrations within plant tissues.