This week I participated in Apple Talk organized by the IPM Institute with John Aue and Peter Werts. Here’s the summary of the questions I addressed regarding tree stress and nutrition.

How critical are water relations early in the season for trees in their second to fourth leaf and how quickly should we consider supplying water to these established but immature trees? Is it possible the evapotranspiration estimates are a poor indicator of water stress early when these trees have little leaf surface area?

In trees of any age, the canopy is proportional to the size of the tree and roots underground. In these young trees, the root systems are small not able to explore and search for water compared to a fully established root system. If you have irrigation or the capability to irrigate, trees should be watered right now. If you are handing watering, focus on the youngest trees first as these will be more stressed than younger trees.

The Apple Irrigation Model in NEWA considers the phenological stage, tree age, trees per acre and amount of transpiration ongoing right now. All of these factors are used to determine water surpluses or deficits in the tree. If you know the output of your irrigation system, it should not be hard to determine how much irrigation your orchard needs to address this deficit.

How significant is a warm and cloudy day or night on the trees right now?

The conditions described in this question are more relevant at fruitset and for informing thinning decisions, as it relates to carbohydrate surpluses and deficits. Right now those conditions are not likely generating a lot of tree stress. Climate conditions that induce stress include hot temperatures and wind that speed up evapotranspiration rates, which results in more water usage. If there is not enough rainfall or if trees are not irrigated, then trees can begin to accumulate water stress. As trees develop more leaves, evapotranspiration rates increase and when the trees are not fully leafed out, evapotranspiration rates are lower.

Prior to last week’s freezes we experienced 10-14 days of cool weather. Might this have acclimated the trees/flower buds ahead of the nights where temperatures dropped below 32F, particularly in orchards where the lowest temperatures experienced were relatively moderate, e.g., 26F or above?

Unfortunately, these conditions are not able to “acclimate” buds and blossoms prior to a freeze event. The tree’s ability to acclimate occurs when buds are dormant, (i.e., cold hardiness), and trees can gain and loose this cold hardiness as temperatures fluctuate during dormancy. Once trees begin to grow there is green tissue a significant amount of water circulating through the vascular system of the tree, the trees no longer able to dehydrate and move around water resources. All the last two weeks of cool weather did was slow us down a bit.

Some growers applied Promalin immediately after the first freeze night, even though most of their cultivars were at tight cluster or very early pink. If this product supplements the gibberellic acid (GA) normally provided by the newly fertilized seeds after pollination, and these applications were applied two weeks prior to bloom, will the Promalin supplied GA still be active at bloom?

Promalin applied any time between green tip and tight cluster will not be effective at helping the trees set fruit. The ovaries and ovules must be able to accept the GA and since the phenology is not far enough along, the blossoms are not expanding and developed enough.

Promalin applied to early varieties at pink last week may have done something. The perfect stage to apply Promalin would be at a plump pink and at petal fall and would only work if the ovaries are still viable. The earliest time Promalin can be applied to have an affect is at the beginning of pink and if applied after petal fall, is also not likely to have an impact.

John has observed trees with red/orangish leaves that are also pushing out lots of blossoms. If a foliar nitrogen were to be applied, would that be a benefit the tree, blossoms or both?

Nitrogen (N) applications right now would help the trees put on additional vegetative growth. Applications of N, boron (B), and zinc (Zn) will also help the tree develop more chlorophyl, which will help the leaves improve their ability to photosynthesize. What will really help trees overcome existing stress are warmer temperatures and more water. These foliar nutrient applications, including some Urea, could help the trees overcome the stress.

Labels for liquid formulations of B and Zn typically differ in their recommended application rate. Liquid B labels recommend a pint per acre, while the chelated Zn products list a rate of a pint per 100 gallons of water. If growers are typically spraying their orchard at 60-80 gallons per acre of water, will they still be supplying sufficient Zn? How critical is the timing of the first of two applications of B and Zn at pink and petal-fall? Is tight cluster too early, or early bloom too late? John recent spoke with a private nutrition advisor who reported that Zn chelates have very low absorption rates and was promoting a Zn-nitrate formulation that is claimed to have much better absorption rates and Zn-chelate, and can be applied at far lower rates, at a marginal savings. Is there any benefit to using a Zn-nitrate over a Zn-chelate?

First, it is important to check foliar nutrition results and make application decisions based on these. Concentrating nutrient applications will result in reduced rates, but if this delivers acceptable results, the reduced rates may be fine. It depends on what the tree actually needs, and trees should be on a maintenance program where this year’s test results inform next year’s fertilization program. If trees are healthy and foliar-nutrition results are not showing deficits, then the reduction in the overall amount of nutrients applied in 60-80 gallons/ac versus that in 100 gallons will not make a difference. Growers can also apply the same pint of Zinc chelate product in 60-80 gallons/ac if they want to maintain the total amount of zinc recommended.

Regarding the timing of these applications, B and Zn need to be applied pre-bloom where B supports growth of the pollen tube and Zn fruit and shoot growth. Post bloom Zn is also important for fruit growth, which is why these are often applied twice, once pre-bloom and once post-bloom. Sometimes the trees start the growing season with Zn or B deficiency, and this is why these applications are made in the spring. If applied post bloom, these applications help that year’s growth and help mitigate against deficiencies for the following season. The pre-bloom spray is the most important for B and the post-bloom spray is the most important for Zn, since it helps with fruit growth.

There are several Zn formulations available and organic growers are often sourcing Zn-sulfates. Studies have shown that other forms of Zn are more readily absorbed by the plants, and an analysis of Zn absorption in California peach and pistachio found significant differences in absorption of different Zn formulations. However, these other formulations have an increased risk of russeting injury to fruit as well as phytotoxicity to leaves. Zn chelate, while having lower rates of absorption, has a much lower risk of fruit russeting.

Thinking ahead to planning calcium sprays, is there any advantage to beginning these sprays early at petal fall or fruit set?

A study from Washington State University looked at calcium (Ca) uptake using isotopes and found that fruit can uptake Ca during its entire development. The study began the first application at June drop and continued into August. At fruit set, when the little apples are still fuzzy, they can take up more Ca than later in the season. However, the surface area of the fruit is so small, a lot of calcium will be wasted trying to achieve adequate spray coverage. As the apples grow and develop a waxy surface, the fruit will not take up as much calcium, but there is more surface area to absorb the product because the fruits are bigger. The best strategy in terms of dollars spent on calcium is to make many low-rate applications beginning at June drop and continuing to just before harvest.

Do you have any recommendations on how to overcome biennial bearing in Honeycrisp? Are there any differences in rootstocks or chemical strategies?

There may be some rootstock differences, but Amaya is not aware of any hard data on this. Biennial bearing of Honeycrisp is partly a trait of the cultivar and early thinning is one of the most important ways to reduce biennial bearing. The induction of bud development for the following year is already happening by petal fall. The production of GA by new fruits will inhibit bud production of fruiting buds for the following year. Organic growers should be very focused on early bloom thinning to control the crop load. There will be more discussion on bloom thinning using a variety of materials next week.

What practices can you recommend for establishment of high-density orchards that help to promote growth, fill space in the row and mitigate against fire blight risk for one and two-year old trees?

Good irrigation is critical during establishment, if the trees are stressed, they have already lost growth potential. The trees also need proper support, e.g., tying leaders off to the wire, using a bamboo stick, etc. The more the leader grows up, there should also be greater overall growth through the canopy. Allowing the trees to sway in the wind may stunt their growth and it is critical to not delay on trellis installation.

Regarding nutrient management, applying a quarter pound of calcium nitrate around the base of tree between bud break and July 1st will also help growth.

This article was posted in Apples and tagged Amaya Atucha, Apples, freeze, freeze injury, tree nutrition.