Organic Broadcaster

Integrated systems approach needed to control fire blight without antibiotics

By Harold Ostenson and David Granatstein

Editor’s Note: The National Organic Program’s sunset date for the use of antibiotics for fire blight control in organic orchards passed last month. U.S. organic apple and pear growers with fire blight-prone cultivars need to find new management tools. The Organic Center has produced a publication with emerging research and experiential knowledge from growers who have implemented non-antibiotic fire blight control over the past decade. This article has been compiled from that publication, which is on the MOSES website orchard section.

There is no cure for fire blight (FB), and there is no single “silver bullet” (including antibiotics) that will prevent FB infection. Successful non-antibiotic FB control combines an integrated systems approach with fungal control, insect control, bloom thinning, spray coverage, tree training, soil and foliar nutrients, and cultivar and root stock selection.

Growers will need to evaluate ideas presented here in light of their particular orchard situation and align this information with new research and new products as they become available. Not all organic materials discussed are approved for use in every state. Always check with your organic certifier before using a new organic material.

General Management Considerations

Integrated Systems Approach
For the most part, FB suppression sprays are the same materials already being applied against other organic orchard horticulture problems such as scab, mites, mildew, fruitlet thinning, and insect pest control (e.g., delayed dormant sprays of copper, oil, lime sulfur; fall sprays of oil, lime sulfur). Integrating your overall orchard spray program with effective FB suppression sprays will be paramount to your non-antibiotic FB control success.

Tree Training Systems
FB-prone cultivars grow best in high density, eight-foot “pedestrian” (no ladders), two-dimensional type plantings. Growers should consider this approach when planning orchard renewal. This type of planting reduces FB infection potential, improves the full foliage coverage required of FB control sprays (the same for scab control, etc.), and can transition tree pruning from winter to summer and early fall, when warm, dry conditions make pruning “wounds” less prone to shoot blight infection.

Crop Load Management
Consistent crop loads from year to year reduces FB infection potential by controlling over-vigorous shoot growth in the “off” production, biennial year. Crop load management of apples and some pear cultivars for organic growers occurs during bloom when the potential for FB infection is usually the highest. Organic thinning to control apple fruit set usually involves a combination of materials including lime sulfur which provides FB suppression during bloom. (Note: lime sulfur applications will be toxic to biocontrol materials such as BloomTime™, Blight Ban™, or Blossom Protect™ and should not be tank mixed).

Bloom thinning, especially for apples, is a key component of a successful integrated systems approach, specifically because: (1) it compacts the bloom window and reduces bloom exposure to FB infection; (2) the same organic bloom thinning materials are effective against scab, mildew, and other fungal threats; (3) these materials suppress over-wintering pests such as aphids, leaf rollers, scale, and others that can spread the FB bacteria; (4) a good bloom thinning program reduces excessive tree shoot growth in light crop years. The end of the bloom-thinning window marks a pivotal point in the annual fruit production cycle because many of the issues facing the organic grower for the rest of the season will be determined at this time: crop load, tree vigor management, FB infection potential, scab-mildew-fungus infection potential, pest level potential, and return bloom and next year’s crop potential.

Symptoms of fire blight include dead branches, water-soaked blossoms, light brown to blackened leaves, discolored bark, black “shepherd’s crook” twigs, and dried fruits. Photo by Dr. Jay Norelli, USDA-ARS

Two items that could reduce FB potential as much as other actions are a chain saw to lower the tree canopy, and a tower sprayer to apply the chemical sprays downward onto the tree canopy and flowers. A tower sprayer can provide economic benefit in high-density orchards by reducing spray materials by up to 50%. It also provides consistent spray coverage throughout the tree canopy on older trees with large canopies. If a ground power air blast sprayer is the only option, consider 300 gallon per acre (GPA) nozzles on the top one-third of the sprayer application pattern, 200 GPA nozzles in the middle, and 100 GPA on the bottom third nozzles. This will “throw” more spray farther to the tops of the trees and reduce the fruit russet potential from material blast in the tree bottoms. Invest in a good pH meter to ensure that your tank mix is at a pH level that maximizes the survival of any antagonistic FB organisms you are applying.

Spray Volumes
Growers have two approaches: (1) lowest spray volume with high active material concentration, which still gives full canopy coverage while also keeping spray deposited on fruit from forming droplets on the calyxes which dry and turn black; and (2) dilute spray volume in the 300 GPA range, with active material rates very low so that the droplet residue concentration forming on fruit calyxes is too low to cause residue marking. Weather, spray equipment, pear cultivar mix and tree canopy size all contribute to the spray volume decision.

Pre-bloom Foliar Nutrient Sprays
Pre-bloom nutrient sprays accelerate tree leaf expansion and the startup of photosynthesis, resulting in a shortened bloom period, faster fruit set, and a reduction in FB infection exposure. Growers have found sprays of cytokines, fish products, and carbohydrates to be very effec-tive in apples and pears. A condensed bloom period results in less bi-annual bearing, consistent tree shoot vigor, larger fruit, and less shoot FB during the 30 days following bloom.

To mitigate a spring bloom time environment that favors FB infection conditions, avoid irrigating during bloom. If irrigation is required, a drip (trickle) irrigation system would help keep orchard humidity lower. If you have wind machines for frost control, think about using them to increase air movement in your orchard to shorten the dry-out period during potential FB infection windows. Reduce tree canopy wetness by striving to increase air flow and sunlight in your tree training systems, including two-dimensional canopies conducive to fast drying.

In addition to standard orchard sanitation practices to control FB infection levels, consider a fall application of dilute lime sulfur and oil and/or copper just prior to leaf drop. This action may reduce overwintering pests, scab, and FB inoculum levels. The spray should be timed late enough that predator insect populations have ceased to be active. Sprays help dry out cankers and reduce insect transfer on the surface. A mix of horticulture oil with copper painted on trunks in late winter has been used by growers for 20 years and is very effective in curing overwintering FB cankers and drying them up prior to spring. The copper treats the cambium layer. In many orchards, the copper/oil mix is still visible in subsequent years with no new canker activity.

Vigor Control
It has been said that going organic results in less production, less tree vigor, and smaller fruit. For many organic growers, this is not the case. Tree vigor can be controlled in organic orchards while producing prime sizes and volumes of fruit through these techniques:

• Reduce dormant pruning and move to controlling growth in summer and early fall. This will help reduce excessive spring shoot growth.
• Use a steady diet of compost and build up the soil biology and nutrient reserve which will keep tree canopy and fruit production levels in balance.
• Use pre-bloom foliar sprays to produce strong flowers, shorten the bloom window, and reduce the excess vigor potential that leads to increased FB damage.
• Aggressively bloom thin to increase fruit size, maintain consistent crop loads, improve fruit color, and avoid biennial bearing.

Most orchards need to apply the same amount of calcium as nitrogen each year. Sulfur applied in low concentration with full canopy coverage in early June will help to set tree shoot terminal buds, reduce shoot blight, and counter aphids and mildew. Both of these actions improve the tree nutritional balance, reduce vigorous green shoot growth, reduce FB opportunities and increase fruit size and quality parameters.

Specifics for Apples

Cankers on the bark ooze liquid that contains the pathogen, which is picked up and spread by insects. Photo by Dr. Jay Norelli, USDA-ARS

Dormant Stage to Tight Cluster Stage [silver tip, green tip, half-inch green, tight cluster]
Spray applications during these early apple bud development stages for arthropod pest and disease control, as well as crop load success for the entire growing season. This is also the time when sprays will have a minimal impact on beneficial insects. The more susceptible your cultivar is to russetting, the more intense the spray program must be during these early spring bud stages. Use active organic coppers, combined with or separate from lime sulfur and oil in dilute sprays of 200-500 gallons/acre with full coverage on every row.

Pink Bud Stage through 50% Bloom
Warmer temperatures and conditions favor FB infection. Since exposing the flower and developing fruitlets to the strongest FB control materials leads to fruit russet, use “softer” materials that only address a single orchard problem category: pest control, fungal control, or crop load management. Spray water volumes need to move from dilute (200-400 GPA) to semi-dilute volumes (100-200 GPA) to reduce the potential for leaf/flower burn and fruit russet. Use fatty acid soluble copper materials (e.g. Cueva™) rather than active copper products during this period to control scab, and FB bacteria growth, and to minimize fruit marking. An upcoming lime sulfur thinning spray can provide mildew control at that time.

Full Bloom Stage
One of the main purposes of the integrated FB control program is to initiate a multiple spray program early in the spring growth cycle to minimize the potential for FB bacteria to infect during bloom. In full bloom stage, FB bacteria have a direct route into the plant via the flower nectary. This period also is critical for controlling scab, mildew, insect pest emergence, and organically managing fruit set, crop load, and return bloom, which means it’s not possible to concentrate solely on FB control.

Lime sulfur or lime sulfur + oil with 200 GPA spray volume suppresses scab, overwintering insect pest emergence, mildew, scale, and FB, while compacting the bloom window. Experienced growers use 2-3 applications of bloom thinning spray in the 200 GPA spray volume range every 3-4 days to cover an 8-12 day bloom window. While a lime sulfur application suppresses beneficial organisms in FB biocontrol products, in most cases, it is worth using for its multiple simultaneous actions on various diseases, insect pests, bloom window compaction, and crop load reduction.

Soluble coppers might be the option of choice after lime sulfur bloom applications to reduce the russet potential from high accumulations of bacteria/yeast on russet-prone apple cultivars under extended wet and humid conditions. Probably any spray applied during this time carries some risk of fruit russet. In most cases, spraying apples post bloom more often with higher rates of water and lower active spray material concentrations will help to minimize russet.

If it is particularly wet (rain or high humidity) during the later portion of bloom, a spray combination of Serenade Optimum™, or similar biological material, or an organic SAR (Sytemic Aquired Resistence) product such as Regalia™, with or without wettable sulfur may be a good option against FB, scab, mildew, and other fungi.
Under heavy scab pressure, it may be necessary to go from the last lime sulfur treatments at bloom directly to a soluble copper program because the bio-fungicide option against FB, Blossom Protect™, will not prevent scab infection.

Petal Fall – Post Petal Fall [+30 days]
This is the stage where spray mix compatibility, coupled with timely spray responses targeting a broad range of fruit and tree quality threats, becomes a critical integrated control challenge for the organic tree fruit grower.

If the integrated FB control program after lime sulfur consists of Blossom Protect™ followed by a soluble copper application (a successful FB control based on preliminary trials), additional soluble copper sprays at petal fall can proceed if required under a forecast of relative dry weather. This is especially true if scab and shoot blight are concurrently the major concern. Consider adding a biological, or SAR product, or Kaligreen™ with the soluble copper if scab or mildew infection potential is high. In the event that petal fall and post petal fall phases are met with heavy rain or extended high humidity conditions, a Serenade™ application should be initiated prior to forecasted rain.

With russet prone apple cultivars, non-lime sulfur and non-copper program alternatives may be a better approach to reduce russet potential.

If shoot blight after petal fall is a major threat to the orchard, multiple applications of soluble copper are probably the best approach even at the expense of fruit russet. Controlling overly vigorous new growth via horticultural practices should also be a high priority. Control of chewing and sucking insect pests during this stage is important as they can be a factor in the spread of the FB bacteria that will cause shoot blight.

Harold Ostenson is a tree fruit consultant. David Granatstein is a Sustainable Agriculture Specialist at Washington State University.

From the November | December Issue

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