2020 Bloom Considerations
As the 2020 almond bloom/crop year begins, there are several considerations to keep in mind. These points may be useful to help understand observations from the field as well as provide an understanding of any direction received.
Nut set.
Flower set percentages will vary from year-to-year. Formature trees, about 20-30% of the flowers will set while younger trees will beslightly higher. Even though this is low, crop loads are still high due to thenumber of flowers that are in each tree. A mature tree will have an estimated 35,000-50,000flowers, depending on tree size. For more information on this, please see thisarticle: http://thealmonddoctor.com/2016/03/07/almond-set-and-nut-drop/which discusses the results of a tagging study which followed the developmentof almonds through the season.
External factors greatly influence the set percentages. Cropload from the previous year impacts the amount of energy available for flowerformation and bloom. High yields from the year before will reduce the number offlowers that will set nuts. Poor weather conditions can reduce bee flighthours, kill flowers in freezing temperatures, or promote disease. The period ofweather risk extends into the fertilization period, which continues for severalweeks. Keep in mind that the crop is susceptible to frost and diseases as longas there is a risk of frost and rainfall.
Several factors can be mediated by good farming practices.These include the presence and strength of pollinators (usually honeybees),post-harvest practices, and nutrient deficiencies. Honeybees are required forvarieties needing cross pollination (most CA orchards – 2.0-2.5 hives/acre) andrecommended for self-compatible varieties (0.5-1.0 hive/acre, although theexact number is unclear). Stronger hives (8+ filled frames) are essentially aninsurance policy for poor weather conditions. Hive numbers and placementsshould take into account the weather.
Post-harvest treatment of the trees is also a stronginfluencer on flower set percentage. Moderately severe to severe water stress duringpostharvest will lead to the reduction of flower buds and more leaves pushingearly. Boron deficiency impacts the rate of fertilization, leading to fewer setnuts. Potassium deficiency will reduce spur survivability from year-to-year,leading to fewer fruiting positions. Preventing these stresses from occurring willhelp maintain a high production orchard.
Disease control.
Diseases can kill the flowers or developing nuts. As such, protectionshould be considered when rainy or wet conditions exist. If there is no rain inthe forecast, sprays can be delayed until a wet period occurs. Generally, ifperiods of dew exist, it is still good practice to make a full bloom/petal fallspray to prevent disease occurrence. Sprays should be continued through thespring during rainy periods to reduce the risk of crop loss from disease. Moreinformation on spray timings and materials here: http://ipm.ucanr.edu/PDF/PMG/fungicideefficacytiming.pdf
Spray rig speeds can be faster during bloom than whenin-season. This is due to the reduced surface area of the tree since leavesmake up about 90-95% of the tree’s surface area. A 33-50% increase in spray rigspeed should be possible with no impact on coverage (e.g. 2 mph to 3 mph).
Bloom spray additives.
With the exception of boron, bloom spray additives have had varied results. Boron should be applied as early as possible in the process, preferably at post-harvest, dormant, or pink bud. Multiple UC studies have failed to show statistical results with various products that do not include boron, but newer products are being developed. These additives should be trialed to determine if they work on the specific operation. Additional surfactants and Insecticides should not be added to protect pollinators. If managing PTB (Anarsia lineatella), consider the use of Bt which has been shown to be safe for honeybees (more information here: http://thealmonddoctor.com/2016/01/18/bloom-bt-for-peach-twig-borer-control/). Since many fungicides already contain surfactants to optimize performance, surfactants should only be used if specified on the fungicide label.
Bacterial blast?
There has been a lot of discussion regarding bacterial blast.This disease, which appears to be a combination of weather events and presenceof the bacteria Pseudomonas syringae pv. syringae, kills flowers and thusreduces crop set. This bacterium is naturally occurring on the tree’s surface,and only becomes a problem when conditions are favorable. Low temperatures(generally below freezing), which are thought to damage the flower or flower bud,followed by rain often lead to greater levels of blast. All varieties appearsusceptible, but Aldrich and Independence seem to be showing a greater occurrenceof disease in previous years. Copper treatments have been used to reduce blast occurrence,but generally are inconsistent. This is most likely due to the timing ofapplication and quick formation of copper resistance within P. syringae.An antibiotic, Kosumin 2L, may have a section 18 for use in almond. Research upuntil now has been minimal since it has been a few decades since we had theblast levels similar to what was observed in 2017 and 2019.