For many years now, since the mid-1960s, almond growers have been told to defoliate their trees shortly following harvest. In most cases the agent used to defoliate trees was zinc sulfate (36%), applied between 10-25 pounds/acre with a 100 gallon ride. The rationale behind such a recommendation was as follows:
- Remove the tissue upon which inoculum can be carried and which constitutes an overwintering site.
- Induce an earlier stage of quiescence. This would theoretically place the trees physiology into a deep state of dormancy.
- The earlier stages of dormancy are thought of as conditioning the trees to withstand freezing weather during the winter and early spring frosts.
The realities behind these hypotheses are as follows:
- While a large source of pathogen carryover would be reduced, there would still be numerous ways in which inoculum would be readily spread.
- There would still remain a large inoculum to provide a source of infection.
- For example, shot hole conidia are commonly found lodged between the bud scales of both vegetative and flower buds.
- Anthracnose conidia are sticky spores easily picked up by flying insects or birds and deposited on areas of a new host.
- Phytophthora syringae can grow and reproduce even under very cold temperatures. The predominant tissues affected are woody tissues.
Inducing a state of dormancy in the trees by defoliating the canopy is a high risk method, as the additional stress can kill tissues before they become conditioned. Cold and/or heat tolerance are best imparted by balancing the tissue levels of minerals and maximizing density of tissues by maximizing photosynthetic efficiency.
Zinc sulfate sprays are a very radical and damaging method to achieve defoliation.
In studies of ZnSO4 solutions, we consistently observed concave indentations or lesions where the droplet was formed. This miniscule zone of necrosis represented tissue that had lost the ability to control differential permeability.
As a result cells adjacent to the damaged area began to leak much of its contents into the cavity. Pseudomonas syringae, a common epiphyte, living on the surface of many plants would use this sap for primary sustenance and rapid proliferation, including the production of low molecular weight glycosides, syringomycin and syringotoxin ultimately eventuating in the initiation of bacterial canker.
Canopy tissue hardening and gaining tolerance to many types of environmental stress events should be the goal of the farmer. Trees will receive the most damage if the tissue has low integrity and is grown under carelessly imbalanced fertility regimes.
Balanced nutrition is best achieved through the use of field specific blended fertilizers day to day nutrient deliveries to the field. Through all of this, it will be imperative to balance numerous minerals and nutrients tailored to best address thorough and balanced plant nutrition requirements of a unique field.
Alternate bearing is a common physiological setback occurring when energy and carbon compounds are not harvested in an efficient and timely matter.
The harsh and phytotoxic effect of ZnSO4 defoliation sprays does more damage than good to the orchards. Considering the many negative effects of the ZnSO4 defoliation sprays, it is extremely important that you allow Sunburst PDC to design a field specific program. The other major drawback of ZnSO4 is that it robs the grower of an additional 2 months of photosynthetic harvest. Our clients have been able to overcome the low bud counts in their orchards following a heavy crop year. We have been able to take these blocks that have been showing a very low bud count and with specific nutritional balancing, have gained sufficient flower bud numbers to avoid alternate bearing. By the end of November these fields will manifest impressive flower bud counts.
Through the use of these and other techniques, we were able to help several growers do away with alternate bearing. All of these benefits have been able to materialize by keeping viable foliage active through the end of November.
It is helpful to understand these concepts by taking a moment to estimate the growth and development of the crop lost when premature crop loss is incited by premature defoliation.
- Growers’ on-year yields are 4,000 lbs/acre.
- In the off-year, yields are 2,000 lbs/acre.
- 2,000 lbs/acre when expressed through the metric system is approximately 908,000 grams.
- The trees’ expenditure of energy for producing an additional 908,000 grams of almonds requires approximately 40,860,000 calories of energy per acre.
- The majority of almond planting run between 75-109 trees per acre or an average of approximately 92 trees per acre. Each tree must harvest roughly 45,000 calories to produce an additional 2,000 lbs/acre.
- An average, mature almond tree will harvest about 1,350 cal./day. Thus, if the trees are allowed to continue to remain physiologically effective, the 60 days remaining between mid-September and mid-November would allow for an average energy harvest of ~81,000 cal./tree over that period.
- Superior trees with balanced nutrition have been observed to operate between 200%-500% of normal with 300% of normal being common when we are able to institute superior nutrition and irrigation. Thus instead of 81,000 cal./tree, production is about 243,000 cal./tree over that 60 day period.
- The additional photosynthetic efficiency, then allows us to harvest 162,000 additional calories. This surplus energy, if available to the trees, allows for the necessary support of tissues including both the need of the canopy and root system.