Crop Load Management of Dwarf Apple Trees for Best Economic Results Terence Robinson, Alan Lakso and Steve Hoying Dept. of Horticulture NYSAES, Cornell University Geneva, NY 14456

The Counter Balancing Responses to Crop Load Adjustment As crop load is reduced by thinning (or by pruning) fruit size of the remaining fruits increases. As crop load is reduced by thinning (or by pruning) yield is also reduced. The best way to evaluate the benefits or costs of thinning is to convert yield/acre to crop value/acre taking into consideration fruit size Yield (bu/acre) Fruit Size (g) Crop load (fruit no/cm 2 TCA)

The Optimum Crop Load is When Crop Value is Maximized Yield (bu/acre) Fruit Size (g) Crop load (fruit no/cm 2 TCA)

The Optimum Yield Varies with Variety For small fruited varieties (Gala) optimum yield is ~75 t/ha (with a Tall Spindle) For large fruited varieties (Jonagold) optimum yield is ~100 t/ha For biennial bearing varieties (Honeycrisp) optimum yield is ~55 t/ha

The Optimum Yield Varies Between Years In 2004 the optimum yield for a Gala orchard was 75 t/ha In 2003 the optimum yield for the same orchard was 45 t/ha was a very rainy year, 2003 was a dry year

In most years there is a strong relationship between fruit size and crop load Fruit Size (g) Crop load (fruit no/cm 2 TCA) Relationship between Fruit Size and Cropload in 5 Gala Orchards. Geneva Geneva Geneva Geneva Hudson 2003

Fruit size of Gala in 2007 was 70g less than in 2010 (2 inch vs. 3 inch Gala Fruit Size in the Severe Drought of 2007

What is the Optimum Crop Load? What is the Optimum Thinning Level? In four Gala orchard of the same age but with different tree sizes had optimum crop loads of fruits/cm 2 TCA. In 2005 which was a dry year the optimum was significantly lower Crop Value ($/ha) Cropload (fruit no/cm2 TCA) Optimum Cropload to Maximize Crop Value of 5 Gala Orchards

What Fruit Size will Give the Greatest Returns? The optimum fruit size for Gala appears to be 100 count fruit (~170g) except in dry years. 0 2,500 5,000 7,500 10,000 12,500 15,000 17,500 20,000 Crop Value ($/ha) Fruit Size (g) Hudson 2003 Geneva Geneva Geneva Geneva 's

There is a Huge Financial Cost of Over Thinning

Short History of Chemical Thinning in Apple 1.Apple Growers have had a historical problem with biennial bearing. In 1920's and 30's researchers learned that hand thinning of flowers or fruits could control biennial bearing. 2. Chemical Thinning. In the 1940's DNOC was noted to burn flowers of apple which led to chemical thinning of apples NAA was identified as a chemical thinner in the 1950’s. 3. Types of Chemical Thinners. Caustic bloom thinners: DNOC (Elgetol), Ammonium Thiosulfate (ATS), Pellargonic Acid (Thinnex), Urea sulfonic Acid (Wilthin), Endothalic Acid (Endothall), Tergitol, Entry Auxins: (NAA – Sodium Salt=Fruitone N, Potassium Salt=K-Salt, Fruit Fix, Ammonium Salt=Tree Hold, Acetetamide conjugate=NAD) Cytokinnins: Benzlyadenine (Maxcel), TDZ, CPPU Ethylene Releasing chemicals: (Ethrel) Carbamate Insecticides: Carbaryl (Sevin), Vydate

Can we do chemical thinning more precisely? Although we have 50 years of experience with chemical thinning it remains variable and difficult to predict The carbohydrate model allows more confidence in predicting the results of chemical thinning Precision Thinning is a strategy to manage the chemical thinning process better - Use multiple applications of thinner - Apply thinner then assess results and then re- apply

Variability in Thinning Efficacy Within a Season

Variability in Thinning Efficacy is Caused by: 1. The chemical thinner concentration 2. Fruit Size 3. The application process -sprayer setup- water volume, air speed, droplet size 4. The chemical uptake process -cuticle thickness -environment during and after applying the chemical (temperature, humidity, application coverage, drying conditions) 5. The sensitivity of the tree -bloom density -initial set -temperature -sunlight -tree vigor

Summary of Observations on Weather Effects: Dark, cloudy weather of more than 1-2 day duration reduces carbohydrate supply and results in greater natural drop and greater chemical thinning. High night temperatures (>60°F, 15.5°C) increase carbohydrate demand and increase natural drop and chemical thinning response. Very high day-time temperatures (>85°F, 35°C) increase carbohydrate demand and causes excessive thinning. Very cool temperatures (<65°F, 17°C) reduce fruit demand and results in poor thinning response.

A Carbon Based Hypothesis of Fruit Growth and Abscission Fruitlet sensitivity to chemical thinners is primarily a function of carbon supply available for fruit growth from current production. –Temperature and sunlight influence the trees carbon production. –Temperature affects demand from competing sinks and demand from fruits. –When demand for fruit growth exceeds supply from current production the least competitive fruits abscise. –Trees are more susceptible to chemical thinners when carbon supply is limiting and less susceptible when carbon is ample.

During the chemical thinning window there is competition for resources between sinks : 1. Between fruits in the cluster 2. Between adjacent clusters 3. Between fruits and shoots 4. Between shoots and roots

Competition between shoot growth and fruit growth in Empire apple trees at the time of thinning. Empire Apple % Available Sunlight Shoot or Fruit Growth Rates as % Control Shoots Fruit Shoot growth was not affected by the reduction in carbohydrates caused by shade, but fruit growth was severely reduced at lower light, defruiting the trees at the lowest light.

In a normal thinning year we can detect fruits that will abscise at 6 days after treatment.

In cool years we can detect fruits that will abscise at 9 days after treatment.

Rate of Chemical NAA – rates from 2.5ppm-15ppm –good rate response in some years, but in many years there is no rate response between 5-10ppm. –above 15ppm suppresses fruit growth rate too much Maxcel – rates from ppm –good rate response up to 150 ppm –Above 200ppm causes branching Carbaryl – rates from ppm –not rate responsive above 600ppm Caustic Blossom Thinners - small rate effect but at high rates can cause phytotoxicity (burning of leaves).

Important Cautions about Chemicals: NAA should be avoided on small fruited varieties and high rates should be avoided (>10ppm). NAA should be avoided on Spur Delicious because of pygmies. The use of Regulaid with NAA will increase response The use of Carbaryl with NAA will increase response Maxcel should always be combined with carbaryl (XLR formulation) when thinning is desired. The use of Ultrafine oil (1qt/100) with Maxcel will increase response.

Fruit Size at Time of Application The greatest sensitivity to NAA is when fruit size is 10mm. The greatest sensitivity to BA is when fruit size is 12mm Recent Results: Combining 15 years of thinning trials in VA there was no significant effect of fruit size at time of application on thinning results. Conclusion: Weather (carbohydrate balance) is probably more important than fruit size in determining response.

Thinning Efficacy During the Thinning Window

Initial Cropload Thinners remove a similar percentage of the crop whether initial crop load is high or low. Initial crop load must be an important factor in determining rate of chemical.

Temperature at Time of Application Research has shown greater uptake of NAA at higher temperatures than at lower temps. This led to recommendations to apply sprays only when temperatures were between 70-80° F. Recent Results: Combining 15 years of thinning trials in VA there was no significant effect of temperature at time of application on thinning results. Conclusion: Cool temperatures result in slower drying and similar chemical uptake as warmer temperatures which cause faster drying and shorter uptake times.

Weather Before Application Cloudy weather for the 3-5 days before thinning can result in: –more succulent leaves, –reduced carbohydrate supply for fruit growth, –increased natural fruit drop, –greater chemical thinning response. Frost can damage leaves and fruit greatly increasing natural drop and chemical thinning response.

Weather After Application (Carbohydrate Balance) Cloudy weather 2-3 days in duration during the week after thinning can result in much greater thinning than if sunny weather prevails: High temperatures during the 3-5 days after thinners are applied have a significant effect on thinning response. High night temps cause excessive thinning (fruits use up carbohydrates faster resulting in a deficit of resources for fruit growth).

Precision Thinning 1.Calculate the desired fruit number per tree (This defines the target). 2.Use the carbohydrate model to assess tree sensitivity to a chemical thinning spray before application. 3.Apply a chemical thinner spray. 4.Use the fruit growth rate model to assess the effect of the chemical thinning spray after application. 5.Use the carbohydrate model to assess tree sensitivity before re-application of a second chemical thinning spray. 6.Use the fruit growth rate model to re- assess the effect of the second thinner.

Calculation of Desired Fruit Number (Tall Spindle) 1.Determine desired yield/acre (75 t/ha) and desired fruit size (100 count)=375,000 fruits/ha (375,000 fruits per acre / 3,333 trees/ha = 112 fruits/tree 2.Count flowering spurs on 5 representative trees at pink. (Count = 200 flowering spurs X 5 flowers per spur = 1,000 potential fruits/tree) 3.Calculate percent of fruits needed = thinning task (112 desired fruits per tree/1000 potential fruits per tree = 11.2%)

Chemical Thinning Windows Bloom Ammonium Thiosulfate (ATS) Lime Sulfur and Fish Oil Promalin Maxcel NAA Petal Fall (fruits at 5-6mm) Sevin Maxcel + Sevin NAA + Sevin Maxcel + NAA Fruits at mm NAA + Sevin Maxcel + Sevin Maxcel + NAA Fruits at mm NAA + Sevin Maxcel + Sevin + Oil Ethrel + Oil

Example of Precision Thinning of Gala Bloom ATS (2.%) Petal Fall (5-6mm) NAA (7.5ppm) + Sevin (1pt/100gal) mm fruit size Maxcel (100ppm) + Sevin (600ppm) (directed to the upper part of the tree) mm fruit size (desperation spray) Maxcel (125ppm) + Sevin (600ppm) + Oil (0.125%) (directed to the upper part of the tree)

Best Thinning Strategies for Improving Gala Fruit Size % Full Bloom + 600ppm PF + 100ppm BA plus 600ppm 10mm fruit size. (av. size=185g) % Full Bloom + 600ppm PF + 100ppm BA plus 600ppm 10mm fruit size. (av. size=191g) 0.25% Full Bloom + 50ppm BAplus 600ppm PF + 50ppm BA plus 600ppm 7DAPF + 50ppm BA plus 600ppm 14DAPF. (av. size=194g) % Full Bloom + 600ppm PF ppm NAA plus 600ppm 10mm fruit size. (av. size=166g) 2 % Full Bloom + 600ppm PF + 100ppm BA plus 600ppm 10mm fruit size. (av. size=164g)

Multiple Sprays to Thin Gala in a Difficult Year

Precision Crop Load Management and Follow-up Hand Thinning Use the Target Fruit Number to precisely hand thin 1.Count representative trees before and after hand thinning 2.Review the results with hand thinning crew 3. Simple trees are easier to manage precisely. 4. Tall Spindle/Fruiting Wall with 4 wires= ~30 fruits between each wire

Return Bloom Flower initiation is inhibited in the bourse shoot by gibberellins produced in the seeds and shoots and transported down the fruit stem and the shoot stem Total Seed number per tree is related to return bloom. More seeds=less return bloom. NAA and/Ethrel applied in summer counter act the effect of seed produced GA’s and shoot produced GA and stimulate more flower bud initiation The mechanism is thought to be a temporary reduction in shoot growth rate Spray 4 weekly sprays of a low dose of NAA (5-7ppm) Spray 4 weekly sprays of a low dose of Ethrel 150ppm Begin sprays on June 21

Recommendations for Managing Biennial Bearing of Honeycrisp 1)Manage bud load by pruning (precision pruning) 2)Use 2 % ATS at 80% Bloom or 3)Use Aggressive Petal Fall Thinning (10ppm NAA + Carbaryl). 4)Use 7.5ppm NAA+ Carbaryl at 10mm fruit size as a second spray). 5)Hand thin early to 5 fruits/cm 2 TCA on young trees and 8 fruits/cm 2 TCA on mature trees. 6)Apply 4 sprays of summer NAA (10ppm) at 10 day intervals beginning in mid-late June (in some years NAA helps).

Is it Worth the Effort to Precisely Manage Crop Load? Days After Petal Fall Crop Value ($/acre) BA/Sevin NAA/Sevin Untreated Control

Thank You for Your Attention Questions?