MSU Extension Apples

In a series of four virtual meetups held every other week this summer, experts on Honeycrisp production offered their insights and answered questions posed by growers. The program was offered in a unique format that centered around a discussion Q&A session and connected producers from across the country. This nationwide extension effort was conducted in close collaboration with IFTA Education Director Greg Lang of Michigan State University and scientists from USDA-SCRI Root2Fruit (R2F) project led by Lailiang Cheng at Cornell University. Each webinar included short presentations or a video clip from the 2021 IFTA virtual Honeycrisp tour recordings, followed by a panel of growers and R2F researchers to answer grower questions.

Over 300 people registered for the meetups, and nearly 150 attended each session. Recordings of the webinars are available online at the MSU Kaltura Mediaspace website.

Below are a few of the key questions that were answered by presenters and panelists.

Meetup #1 – Crop Load Management

Terence Robinson of Cornell University began this session by explaining why crop load management is so important in Honeycrisp, and how to successfully accomplish it. Stefano Musacchi, Washington State University, then offered insights about how crop load management influences fruit quality.

Q1. Why do we practice crop load management (in Honeycrisp)?

• Control biennial bearing
• Improve fruit size
• Achieve desire fruit quality

Q2. How do we achieve successful crop load management and control biennial bearing?

Crop load management begins with understanding floral bud initiation. There are two competing forces in apple trees that control whether buds will become floral or vegetative. Gibberellins (GAs) inhibit floral bud initiation, while Cytokinins, ethylene, and other hormones promote it. GAs are produced in large quantities in the seeds, so the question becomes: how to we control seed load?

To achieve the desired crop load, Robinson recommended the following program:

• Dormant pruning. Prune to a bud count 1.8 times the desired crop load.
• Blossom thinning and early thinning. It is critical to remove blossoms and fruitlets as early as possible to reduce the seeds and gibberellins that are produced before floral initiation during the period following bloom.
• Return bloom sprays. Use four sprays of ethephon beginning as early as 16 millimeters and repeating every 10 days. Be cautious when beginning early, as this will also lead to thinning.

Q3. How is fruit quality influenced by crop load?

Overcropped Honeycrisp trees will produce smaller fruit with lower quality, including:

• Poor color. Trees with the target crop load will have redder fruit than overcropped trees.
• Delayed maturity. Overcropped trees will mature more slowly and may not catch up to the quality of well cropped trees. This was measured by DA index, quantifying chlorophyll content.
• Decreased dry matter. Trees with target crop load will have higher dry matter content. This is one of the most important qualities correlated with consumer perception.
• Poor return bloom. Overcropped trees will have poor return bloom and associated poor quality, including higher probability of bitter pit.

There can be variability among rootstocks with respect to the effects of crop load on fruit quality. For instance, B.9 loses dry matter as crop load increases above desired levels. But this is not necessarily true for other rootstocks such as G.41 and G.935. Mineral content of fruit (e.g., K, Ca, Zn) is also affected differently by rootstock.

Q4. When does floral initiation occur in Honeycrisp?

Floral bud initiation takes place when bourse shoots slow down growth and set a terminal bud. Work by Poliana Francescatto showed that this happens early in Honeycrisp, typically within 30 days of full bloom. In other varieties, floral initiation can be much later. For Fuji, this is approximately 75-80 days after full bloom, Gala as much as 100-plus days after full bloom. This difference is a key reason to thin early in Honeycrisp. According to Musacchi, another indicator of floral initiation is when the distance between nodes in the bourse shoot starts to shorten.

Q5. What caused the poor return bloom observed across large parts of the eastern US?

The best hypothesis for this right now is that there were stressful conditions during floral initiation period of 2020, preventing the formation of flowers in 2021. During the period 30 days after full bloom in 2020, conditions were very hot and dry in much of the eastern U.S. Digging deeper into floral bud physiology: bourse shoots must form 10 nodes (within the bud) in order to create a flower bud. If growth was inadequate in the bud due to drought or heat, a flower bud would not have been produced.

Q6. What strategies can we use to control return bloom in 2022?

The light bloom this year may result in a very heavy bloom next season. Several options were discussed to mitigate this. For some of these the time has passed for the 2021 season, but the strategies can be kept in mind for future years:

• The most important thing you can do is practice excellent dormant pruning and bloom thinning in 2022. This will reduce heavy crop expected next year and improve fruit size and quality for 2022. It will also remove excess flowers before seeds and GAs are produced that will inhibit floral initiation for 2023.
• Exogenous GAs. These may be applied in the off year to reduce return bloom. Tory Schmidt’s research (Washington Tree Fruit Research Commission) has shown that applying GAs such as the new product Arrange (Fine Americas) beginning at 10-millimeter fruit size can inhibit floral bud initiation in the following year.
• Applications for return bloom may be helpful, but research is still improving our understanding of this. NAA applications may need to be applied earlier to correspond with the floral initiation period. It also can lead to seed abortion, which would contribute to reducing GAs.
• Late spring or summer applications can promote return bloom but have mixed results. Robinson recommended a program of 0.5 pints per 100 gallons TRV dilute beginning at 15 millimeters. At 25 millimeters, the rate could be increased to 1 pint.
• ACC applications. This new product may help promote return bloom when applied in the off year. The product is a precursor of ethylene, but results have shown it is likely safer than ethephon applications (which are somewhat unpredictable and very sensitive to heat).
• Summer hedging. Summer hedging often results in the formation of a flower bud at the end of the new growth. Length of the new shoots depend on timing, with June hedging leading to 10 centimeters of growth and July hedging only 3-5 centimeters in the Eastern U.S. Summer hedging in Washington state when done too early (June) can lead to an undesired second bloom in fall.
• Precision pollination? This idea was included in the brainstorm as a way to control how many fruit are successfully set. In the East, it may be a challenge because there is an abundance of wild pollinators that assist with pollination. In contrast, Musacchi indicated that in Italy, bee exclusion has been utilized successfully to control pollination. In western U.S., precision pollination approach has greater potential due to the relative lack of native pollinators. Recent trial by M. Whiting in apples, sweet cherry, pears, almonds and other annual crops have reveal potential to improve fruit set with artificial pollination technologies.

On Honeycrisp, trees that are excessively thinned or thinned early can become too big, a condition not desired by growers, packers and consumers, and higher risk for bitter pit. Thus, in off years or in on years with early thinning, growers have applied two main strategies to reduce the risk of over sizing.

• Deficit irrigation. Bruce Allen (Washington State Grower) has started practicing deficit irrigation to control growth to compensate the effect of oversize fruit.
• Delay hand thinning.

Meetup #2 – Rootstocks

This meetup began with an introduction from Robinson, followed by perspectives from Tom Auvil on rootstocks appropriate for Honeycrisp in Washington State.

Q1. What is the best rootstock for Honeycrisp?

The short answer is it depends. When selecting a rootstock for any site, it is important to take into account the scion, planting site characteristics (especially soils), orchard system, and climate. Over the past 20 years, a number of new rootstocks have become available, after rigorous testing through trials such as NC140 evaluations. Designer rootstocks, so called by Robinson and Gennaro Fazio that growers may choose to grow the tree well enough for their variety, their soil, their climate, so that it can fill the space by the end of the second year.

For Honeycrisp in particular, Robinson outlined two primary goals: Select a rootstock that will 1) provide adequate vigor to balance the low-vigor scion and fill the canopy space in 2-3 years, and 2) produce high yields of bitter pit-free fruit.

Q2. Why are some rootstocks more susceptible to producing fruit with bitter pit?

Bitter pit is a complex disorder influenced by several factors, many of those factors are influenced by the rootstocks. One hypothesis is that some rootstocks are inherently more efficient at taking up potassium and nitrogen. This results in higher K/Ca ratios, resulting in higher bitter pit incidence. Essie Fallahi, University of Idaho, indicated that larger rootstocks take up more water through the growing season leading to more K and N uptake. This increases the nutrient imbalance in fruit and shoot/fruit ratio and is also known to affect fruit Ca partitioning. Vigorous rootstocks have higher shoot/fruit ratio and lower crop loads, leading to oversize fruit, shoot water demand and hormonal imbalance, all of which are predisposing factors for bitter pit susceptibility. (More on vigor and bitter pit in “nutrient management” section below.)

Q3. Which rootstocks are best for preventing bitter pit?

This question may be answered in more than one way. First, you can evaluate percent of fruit with bitter pit by rootstock. Budagovsky 9 (B.9) has earned a strong reputation for the lowest percent of bitter pit, According to Robinson’s data, B.9, G.30, G.65, and G.214 had the lowest observed bitter pit incidence compared with the highest in B.118, G.41, G.814, G.6210, and M.7. Intermediate levels of bitter pit have been observed in G.11, G.16, G.935, M.26, M.9 Pajam2, and M.9-337.

Alternatively, it may be more important to look at the highest cumulative yield of bitter pit-free fruit over time. This means evaluating percent bitter pit incidence, taking into account rootstock productivity. Robinson did this productivity adjustment by taking the recorded productivity per tree and multiplying it by “ideal” planting density, then subtracting the bitter pit percentage for each rootstock. Pit-free yield was reported as the calculated tonnes per hectare over a 14-year period. The “ideal” tree spacing were: 2 by 11 feet for B.9 and G.65, 3 by 11 feet for G.11, G.16, G.41, G.214, G.222, G.4202, G.5046, G.935, M.9 and M.26, 4 by 12 feet for G.30, G.202, G.210, G.814 and M.7, and 6 by 11 spacing for B.118.

Based on this adjustment, rootstocks producing the highest bitter pit-free fruit were G.11, G.30, G.214, and G.935, which translated to a higher crop value. Data on bitter pit-free yield that included additional rootstocks also highlighted B.10, G.11, G.214 and M9-337 as better performers compared to B.9. Much of the work presented in this session was conducted in New York. More information on rootstock performance in Washington was shared by S. Musacchi (session 1), E. Fallahi and L. Kalcsits (session 3). Given the dependance of rootstock on environmental conditions (soil, moisture, risk factors), more work is needed to determine the best rootstocks for other climates such as Michigan and Washington.

Meetup #3 – Nutrient Management

In this meetup, presentations providing an overview of nutrient management for Honeycrisp were given by Lailiang Cheng, Cornell University; Essie Fallahi, University of Idaho; and Lee Kalcsits, Washington State University.

Q1. Nitrogen management – How much should be applied and at what time?

Adequate N is necessary for Honeycrisp, particularly because it is a weak growing variety. Most N is supplied by mineral content of the soil, provided mostly by organic matter. In highly demanding crops, N applications are necessary. The amount needed can range from 0-25 pounds actual N per acre on Honeycrisp, although this is highly dependent on yield per acre. Leaf analyses should be done annually to determine the amount. Take samples near the end of June or early July, before zonal chlorosis begins to develop.

Nitrogen applications should be made early in the season when shoots are actively growing, and N demand is highest. This is prior to petal fall; after petal fall, there is higher risk of increasing N levels in fruit.

• Fertigation: apply from pink to petal fall.
• Ground application: should be made in a split application, at bud break and again at petal fall.

Q2. Are post-harvest applications of N beneficial?

Post-harvest (early October) can be beneficial for Honeycrisp when vigor is low. At this time in the season, we do not have to worry about affecting fruit quality. Post-harvest applications will promote root growth and help replenish N reserves for early growth in the following season. Apply 3% foliar urea (25 pounds per 100 gallons).

Q3. What is bitter pit and why does it occur in Honeycrisp so frequently?

Bitter pit is a physiological disorder, caused by an imbalance in calcium (Ca) in the fruit. Cell membrane integrity (stability) relies on Ca. When Ca is insufficient, cells become leaky and break down leading to cell death. Pits, typically seen in the calyx end of the fruit, are pockets of these dead cells.

Inadequate Ca in the calyx end of the fruit is the result of internal deficiency. Ca comes from the soil and then is transported and partitioned throughout the plant, via xylem with the transpiration flow:

• It is taken up by the roots from the soil, along with other nutrients (K, Mg, N) and transported plant organs,
• Then partitioned between transpiring organs: shoots and fruits,
• Next it is transported through fruit from stem to calyx end, and
• Finally, it is partitioned within the cell to the membrane and other cellular components.

In Honeycrisp, xylem functionality is reduced compared with low bitter pit cultivars like Gala, and less effective at transporting Ca. Other susceptible cultivars include Cox Orange Pippin, Braeburn, Golden Delicious, Juici, among others.

Q4. How do rootstocks affect the risk of bitter pit?

Cheng indicated that some rootstocks (e.g., G.11 & G.41) are more efficient at taking up K compared to others (G.214 & B.9). If you are using a rootstock that is more efficient at K uptake you need to compensate for this by lowering the K supply to allow more Ca to be taken up. Honeycrisp requires lower K levels for optimum growth and fruit size compared to Gala. Cheng recommended leaf K levels of 1.0 to 1.3% and fruit K levels of 0.5 to 0.7%.

Fallahi, University of Idaho, pointed out that fruit yield and fruit size cannot be separated. However, in extensive research on rootstock differences, he found that G.202 had low yield and also smaller fruit, while G.30 and M.26 EMLA had greater yield and fruit size, compared with V.1, G.202, and G.969. Regarding bitter pit levels, G.30 had higher levels of BP incidence. Fallahi pointed out the differences observed between bitter pit incidence at harvest vs. post-harvest between rootstocks. At harvest, V.1 and G.202 consistently had the lowest BP levels and G.30 had higher bitter pit levels (5-15%), while at post-harvest G.30 and M26 EMLA had the lowest bitter pit incidence (approx. 20-25%).

Lee Kalcsits also indicated that the effect of rootstocks relates to crop load and K/Ca ratios. He showed that in WA, B.9 CG.4003 and G.969 were tolerant to BP, while G.41, CG.2034, G.30, V.6 were more susceptible to bitter pit.

Q5. How do crop load and size affect the risk of bitter pit?

Fallahi showed that time and amount of thinning can also affect bitter pit. The control (no thinning) had the lowest levels of bitter pit, but also the smallest fruit (153 grams). Early thinning at bloom (to one fruit per spur) increased bitter pit levels compared with the control or the same level of thinning conducted later. Light thinning (two fruit per spur) when done early as well as late thinning (one or two fruit per spur) did not affect fruit size (approximately 256 grams) and reduced bitter pit levels to half of that reported in the heavy thinning early. Results presented by Fallahi agree with the results presented by Kalcsits regarding a close relationship between crop load and bitter pit development.

Kalcsits indicated that, after evaluating over 3,000 samples in several U.S. apple growing regions, crop load was one of the key factors and positively correlated with bitter pit incidence.

Q6. When and how we manage water for reduced bitter pit?

According to Sallato, excessive vigor is the key factor that leads to bitter pit in apples. All management strategies that are conducive to reduce excessive vigor will help reduce bitter pit incidence. For example: dwarfing rootstocks (as opposed to vigorous rootstocks), summer pruning, PGRs, water management to control excessive growth, reduced N and K supply, etc.

Regarding timing, is important to make sure that all conditions are secured for early nutrient uptake during cell division, including adequate moisture and nutrient supply. Any restriction to uptake, such as low temperatures in the soil, excessive water, or dry conditions can affect early Ca uptake, when it is most important. If the orchard has excessive vigor, is in its off year, or has low crop load, deficit irrigation can reduce BP incidence when done during fruit elongation.

Q7. What is the efficiency of Ca foliar sprays?

Calcium sprays have shown low effectiveness, ranging from 1 to 2% uptake. This can increase Ca levels in fruit by approximately 10%, which is insufficient to overcome Ca deficiencies at a cellular level in bitter pit susceptible cultivars. Kalcsits recommended focusing on crop load and rootstocks and using Ca sprays as a complement to other practices.

Q8. What harvest and post-harvest practices should be used to manage bitter pit?

Fruit that is harvested at greater maturity is more susceptible, so fruit should be harvested at optimal timing and not delayed. Application of the PGRs ReTain and Harvista increase the risk of developing bitter pit. Controlled atmosphere storage, application of 1-MCP, and storage temperatures of 33 degrees Fahrenheit reduce the risk of bitter pit.

Q9. How can I test my fruit for risk of bitter pit?

Lailiang Cheng and Mario Miranda Sazo conducted significant research to identify the best tissue and method for testing fruit for the risk of developing bitter pit. Peel sap (the water soluble nutrients in the peel) of fruitlets of 55 grams size was found to be the best indicator. It is the site of water transpiration, and higher concentration of nutrients including Ca, Mg and K end up in the peel.

The peel sap analysis method is currently being used by New York State to help growers predict bitter pit risk. Research this season (2021) is being conducted in Washington and Michigan to evaluate this method for other regions.

This video and protocol provide instructions to prepare samples for peel sap analysis:

• How to Prepare and Submit Peel Samples for Peel SAP Analysis
• Protocol for Honeycrisp Sampling for the Passive Bitter Pit Prediction Model

Fallahi indicated that both the calyx peel Ca levels, and calyx flesh plus peel Ca levels were the best indicators for bitter pit incidence analyzed in pre-harvest apples (correlation of 0.86 and 0.90).

Meetup #4 – Harvest and Post-harvest Management

This webinar began with an introduction from Randy Beaudry, MSU, who summarized the primary disorders afflicting Honeycrisp and the research that has been done to understand and mitigate them. Chris Watkins, Cornell University, followed this with what is known about how PGRs such as ReTain and Harvista affect storage of Honeycrisp.

Q1. What are the recommended harvest maturity indicators for Honeycrisp?

Ripening is a continuous process that differs from season to season and block to block. Post-harvest disorders and consumer perception are dependent on maturity at harvest, so it is important to harvest at the appropriate time. These are the rough recommendations:

• Starch Index: 4-6. This is approximately 40-60% starch cleared. You can use this chart developed by Washington State to assess Honeycrisp starch patterns.
• Firmness: about 15-16 pounds of pressure. However, Honeycrisp is never a particularly firm apple and this is not the most important indicator.
• Background color: 2-3 on the green background color scale used for McIntosh. Background color should be just beginning to turn yellowish. This is a very important indicator.
• DA meter index: 0.8-1.2. This metric measures the chlorophyll content in the fruit peel.
• Internal ethylene: 0.5-1.0 ppm. This is a difficult metric and does not behave consistently year to year.

Q2. What are the primary storage disorders that affect Honeycrisp?

Honeycrisp, pomology’s problem child, is susceptible to many storage disorders. The following table summarizes each disorder, the symptoms associated, and conditions that improve or exacerbate each. Storage Control Systems and Jennifer DeEll also partnered to make a very effective visual summary of “Storage Disorders of Apples.”

Q3. What are the recommended storage conditions for Honeycrisp?

In most situations, Honeycrisp requires a conditioning treatment due to chilling sensitivity. Fruit should be held at 50-70 F for five to seven days prior to CA storage. To mitigate CA injury, the following options are available:

• Do not use CA, store in air with 1-MCP treatment
• Use delayed CA, keep CO₂ low (<1%)
• High temperature conditioning prior to CA
• DPA treatment prior to CA

Q4. What is DCA and should I use it for Honeycrisp?

Dynamic controlled atmosphere (DCA) storage is a relatively new method of storage in which temperatures are kept very low (33-34 F) and O_2­ level is also minimal (0.4-0.6%) in order to reduce respiration as much as possible. Fruit must be monitored to make sure conditions do not become anaerobic and fruit transition to fermentation. This is also monitored by measuring ethanol production (the product of fermentation) but is complicated because Honeycrisp does not have a normal fermentation or ethanol production response compared to other cultivars. Research on DCA is ongoing several labs across the country, and recommendations will be forthcoming.

Q5. Will pre-harvest treatments of Harvista and ReTain negatively impact my fruit?

Harvista and ReTain are two PGRs that act by inhibiting ethylene biosynthesis or ethylene action, respectively. Generally, the effect of these products on post-harvest are nearly the same. Fruit treated approximately one week before anticipated harvest date in research trials had increased bitter pit, leather blotch, core browning, and CO­₂ injury. However, leather blotch was controlled with treatments of 1-MCP with or without PGR treatment. On the other hand, PGR applications reduce senescent breakdown and wrinkly skin, both of which are associated with advanced maturity. PGR treatment can reduce soft scald, even if stored at 33 F.

Q6. What causes and enhances lenticel disorders in Honeycrisp?

Research is still ongoing to identify factors that contribute to lenticel breakdown. During the growing season, conditions that impact the cuticle development, including drought or excessive rain, can lead to lenticel breakdown. A major contributing factor post-harvest is the quality of wash water. Both organic materials and inorganic materials (dirt, minerals, and detergents) have been hugely implicated in lenticel disorders. Aluminum is particularly problematic and is present in detergents as well as in fungicides used with overhead irrigation. The take home message is to carefully monitor and change out your packing line wash water regularly.