Apple nutrition maintenance is a year-round project but we are in the peak part of the year for accurately monitoring and also applying the correct amount of nutrients for our apples, especially to prevent bitter pit and other post-harvest disorders. Most growers in the state will have already sent in leaf samples from the mid-point of 1-2’ new shoots for tissue analysis between July 1-15. However, it is not too late, especially for more northern growers to do the same.

When you receive your report back from the plant tissue analysis lab, it should have a list of the concentrations of N (nitrogen), P (phosphorus), K (potassium), Ca (calcium), Mg (magnesium), S (sulfur), Zn (zinc), Mn (manganese), B (boron), Cu (copper), & Fe (iron) in your sample. If a soil sample was concurrently submitted with the tissue sample, the soil test results for pH, organic matter, P, and K and any special soil test results will also be reported. Analytical levels of nutrients in the tissue and soil will be interpreted to reflect nutrient deficiencies, toxicities or imbalances by typically the sufficiency range approach, or if calibration data are available, the nutrient ratio method.

Usually, fertilizer recommendations will be made base on the analytical results. These reports will be your primary guide for your orchard over the next year and are only as good as the quality of the samples and sampling technique used by you, the grower! Remember, mixing leaves from multiple cultivars in 1 sample will render that sample inaccurate and of questionable use. The following is a typical range and should help in the interpretation of your sample results:

The ranges listed will allow apples to grow, flower and fruit sufficiently for high-quality commercial crops. The nitrogen range is broad due to variables like tree age, fruit use and cultivar differences. Nitrogen should be higher for young, nonbearing apples trees.

So, how do we develop a fertilization program? A few key variables to keep in mind:

1. Don’t ignore your common sense or past records

2. Maintain uniform and proper soil moisture

3. Regularly use tissue tests

4. Do NOT use a fertilizer (balanced or otherwise) unless you can prove you need it!

5. Know how to interpret results

6. Know the nutrients, what they do for the plant, how mobile they are and what form is best (and most cost- effective)

7. Know when to apply the fertilizer.

One of the most important aspects of good apple nutrition is to maintain the available soil moisture for most dwarf trees between 50-70% of field capacity to about a 36” depth, using trickle irrigation . Monitoring can be done with tensiometers, soil moisture meters or the use the Evapotranspiration (pan method) + the crop canopy density (K_c coefficient). Remember, nutrients cannot get into the tree without first dissolving into soil water in ionic form, unless you are using some supplemental foliar applications for nutrients like Ca. Remember, if you use supplemental trickle irrigation, you can not only use foliar and granular soil fertilizer applications; you can also inject fertilizer into the drip lines. Irrigation by itself will contribute to larger fruit size, better quality and bigger crops!

Nutrient application timing for foliar applications depends on the individual nutrient. If your tissue analysis indicates a need, here are some general guidelines for forms and rates of macro- and micronutrients: Mn [ manganese sulfate (24%, @ 5 lb./A)], Cu [ copper sulfate (22% Cu, 4-6 lb/A)]and Zn [zinc sulfate (89%, 5.5-11 lb/A)] are typically applied during dormancy or petal fall stages; whereas K [potassium sulfate, (27%, @ 6-10 lb/A] is at either dormancy or post-harvest;  Mg [ magnesium sulfate (11%, 10 lb./A)]at petal fall; and boron [Solubor (20.5% B @ 4 lb/A] @ either petal fall/first cover or post-harvest. Nitrogen is usually applied at pink or petal fall with a low biuret nitrogen form like urea, (45% and 10 lb/A). However, adjustments should be made based on estimated N efficiency of your orchard conditions:

Another aspect to monitor is the ratios of nutrient levels to one another. Excessive fertilization, and over-liming or too much sulfur application to certain soil types create conditions that are much more prone to cause what we call antagonism among nutrients which affects their absorption. Some of the more common antagonisms are between K & Ca and Mg & Ca. In general, a ratio of 58:12:2 of Ca>Mg>K should be maintained, although some literature indicates this base saturation ratio should be closer to 70:10:3.

For those growers looking for a better way to fertilize more precisely, fertilizing based on tree nutrient demand (TND) can provide superior results. This includes the nutrient amounts the tree requires for growth and fruiting; along with the allocation to buds, fruit, roots & permanent wood. The TND assumes a ratio of each nutrient in the tree and fruit for the whole growing season that remains proportionately constant across the fruit yield range. Red apple cultivars need about 1.9-2.2, 0.4, and 3.0-3.9 lbs of N, P, & K, respectively, per US ton of fruit produced, whereas green apple cultivars require 3.1, 0,4 and 3.4, respectively, per US ton of fruit produced. Standard-sized apple trees have a large framework that uses more N than smaller trees and may require up to 100 lb/A/yr. Dwarf trees use proportionately less N and require as little as 30lb N/A/yr. Here are some very specific tables based on Gala on M9 rootstock:

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The one nutrient that must be applied on a semi-regular basis throughout the summer is Ca, usually in the form of CaCl₂ (77-80%), at the rate of 15-50 lb./A. Ca is part of lime in soils and deficiencies can happen in especially sandy & acidic soils due to leaching. Soil testing is a useful tool to determine soil Ca levels but Ca soil deficiencies are uncommon in WI if proper pH is maintained (>5.0). However, orchardists typically still add supplemental Ca due to partitioning problems in the trees as Ca is immobile in the apple tree; & apple fruit are not part of the tree’s transpiration stream that would help to more evenly distribute Ca. Ca fertilizer is usually applied regularly in summer, as Ca is a necessary component in cell walls, involved in sugar & starch movement in the tree and helps prevent disorders like bitter pit (BP). Honeycrisp and Cortland are particularly prone to bitter pit, along with Delicious, Empire and Spartan.

Moderate tree vigor should be the goal as significant Ca amounts may be used by the tree for vegetative portions if vigor is too high.  Prohexadione-calcium (Apogee)has been used to suppress BP in Honeycrisp when applied at the pink stage @ 6 oz./A. Tree vigor, crop load and irrigation should be optimized to avoid Ca deficiency leading to bitter pit. Ca apps. are even more important if fruit is to be stored as fruit may show no issues at harvest but will develop problems post-harvest. Younger trees are more likely to produce fruit that develops BP. Some other strategies to minimize bitter pit is to avoid excessive levels of N, K & Mg and deficient levels of Ca, B & Zn. These relative levels can also be expressed as nutrient ratios and are indicators of potential corking and BP problems that will appear in storage and include high N:Ca, K:Ca, Mg:Ca & N+K+Mg:Ca ratios in fruit peel and/or leaf samples. POMA (foliar Ca, 6% calcium chelate)when applied in 5 weekly apps @ 2 quarts/acre starting at petal fall (mature tall-spindle trees) suppressed BP in New York 54% in 2017 and 42% in 2018. [Concentrating on cell division stage of fruit (PF to 45 days post full bloom)].

In general, when applying Ca, it typically takes > 300 gal of dilute spray/A for thorough coverage. Here are some more specific guidelines for foliar rates:

For bitter pit/senescent breakdown developing post-harvest, dipping/flooding of fruit with a solution containing 20 lb of CaCl₂ (7.2 lb Ca)/100gal water works very well.

Pre-harvest fruit drop and control

One of the worst things that can happen is when you have gotten through most of the growing season with a good fruit load and then you experience pre-harvest drop due to ethylene production in the tree and fruit. This is a left-over wild trait that I see in almost all 600 wild Kazakhstan (center of evolutionary origin for apple) trees that I have growing in my research plots on the UW-River Falls campus. The tendency helps wild trees distribute seed as early and thoroughly as possible.

 It may seem a little early to be thinking about harvest but when I was growing up in SD, early cultivars were much more popular. We had cultivars like Oriole and Mantet coming in at the latter part of July and also the notorious dropper, Yellow Transparent in early August. One of the most easily recognizable relationships with premature drop, then, is genetic predisposition, as the cultivars Liberty, Lodi, Zestar, Honeycrisp, Jerseymac, Golden Delicious, Red Delicious and Spartan are very prone to drop. McIntosh will drop if you even look at the tree the wrong way!

However, there are other factors besides the genetics that I have already mentioned in this article such as water availability in dry years and tree nutrition. Trees low in Mg and high in K & B are more likely to drop fruit. Throw in hotter temperatures than usual (this year thus far) and other stressors like heavy crop load, clustered fruit/short-stemmed cultivars, heavy summer pruning, insect & mite activity, disease pressure and sandy areas in the orchard and you have yourself a much greater than usual risk.

Sometimes we can also get too concerned waiting for that all-important market-driven color development and find the apples are dropping before that happens. The cultivars Cortland and Jonagold are less likely to drop than the highest risk category and cultivars Gingergold, Goldrush, Fuji, Paulared, Empire, Jonamac, Gala and Jonathon are the least likely to drop. Given the presence of multiple aforementioned factors, however, and all cultivars should all be monitored. Being mindful of some of the risk factors may help prevent premature drop in the future but if high-risk drop is staring you down this season, then it will probably be necessary to apply a stop-drop spray. NAA (Fruit Fix 800, Fruitone N, PoMaxa) should be applied 7-14 days prior to anticipated harvest and before any fruit drop is noticed. Summer cultivars are generally sprayed with 5 ppm and later cultivars at around 10-15 ppm. A follow-up spray within 7-10 days of the first can be applied if fruit were not harvested. As is usually the case with growth regulators, use a dilute spray and consider your tree/foliage density. One of the unfortunate consequences of NAA use is that it can decrease shelf life and storage potential.

AVG (ReTain) blocks ethylene synthesis but in so-doing, also delays ripening and color development. That can be helpful, though, if you spray some of your orchard, you could more efficiently spread out harvest. ReTain also greatly improves fruit size, storage quality and shelf life and reduces cracking and watercore. ReTain sprays should have a silicone-based spreader sticker additive (like Silwet L-77 or Sylgard 309 @ 12 oz/100 gal.) and applied approximately 1 month prior to the anticipated harvest date. Again, just like NAA, the best results will be obtained when a dilute spray is applied to achieve thorough coverage. Gala and Jonagold have high sensitivity to ReTain and Honeycrisp has intermediate sensitivity. All 3 respond as well to a ½ rate of ReTain (165 g/A) as a full dose (333 g/A).

 NAA + ReTain sprays can give you the best of both worlds but more research needs to be done on specific rates tied to growing conditions and cultivars. Philip Schwallier and Amy Irish-Brown at MSU have found for example, that Honeycrisp responds well to between a ¼-1/2 rate of ReTain + NAA 14-21 DBH (days before harvest). Jonagold and Gala work well with ½ rates of ReTain and adding NAA as needed. Most other cultivars respond well to a range of ½-2/3 rate ReTain + NAA 14-21 DBH and on up to full rates.

This article was posted in Apples, Disease and tagged Apples, Bitter pit, disease, fertilizer, post-harvest disorders, pre-harvest, pre-harvest drop, pre-harvest stop-drop sprays, soil moisture, Soil pH, summer apple nutrition.