2022 Disease Update: Beware of Fire Blight Conditions May 13–20

Much of Pennsylvania is still experiencing some state of apple bloom. As temperatures are feeling more like spring, we are creeping higher into the risky territory for fire blight infection, especially if rain is in the forecast. Many folks in the state have been hit hard by the freeze/frosts this spring. If you have blossoms that have black pistils, they are still susceptible to fire blight. Blossoms are susceptible to infection as long as the nectaries are open, regardless of blossom viability. It is important to still protect these trees.

As I have been reporting at the spring meetings this season, I'm still having fits with the fire blight model in NEWA functioning correctly. At the Fruit Research and Extension Center, we are determining fire blight risk by using the MaryBlyt software and inputting our weather data from our on-site weather station, as well as the forecasted data from Weather Underground. The NEWA output and my MaryBlyt output are not aligning, and we're still trying to figure out the discrepancy.

In the meantime, I have been alerting growers to pay attention to the daily max and min temperatures, including any wetting events. The higher the average temperature, the risk for fire blight increases. We focus on the epiphytic infection potential (EIP) calculated from the MaryBlyt software to determine fire blight risk. To assess fire blight risk, the EIP is calculated based on four criteria. The minimum requirements for blossom infection and the order in which they must occur are:

1. Flowers must be open with petals intact (flowers in petal fall are resistant)
2. An accumulation of at least 198-degree hours above 65ºF
3. A wetting event such as dew or rain
4. An average daily temperature of 60ºF

The EIP is based on the assumption that abundant inoculum is available in and around the orchard. If the EIP is less than 100, few if any, infections are likely to occur; an EIP of 100 to 150 is low but is sufficient to support a blossom blight epidemic, and an EIP of more than 200 to 250 indicates that large numbers of infections are likely should a wetting event occur. During May 13–20, the EIP will be ranging from approximately 120 to 190.

Rain is in the forecast and this will trigger a potential apple scab infection event on May 13–16. If growers are in full bloom, this will be an important apple scab infection since the disease pressure will be high due to the maximum number of mature overwintering spores being available during this time. For growers in post-bloom or petal fall, the disease pressure is much less since we are on the downside of available spores.

Fire Blight Management: Risk May 13–20

The majority of Pennsylvania is experiencing different shades of bloom: early bloom, full bloom, and petal fall. We are finally entering typical spring conditions: warm days, mild nights, and threats of rainfall. These are perfect conditions for fire blight. Please remain alert.

For those in bloom, consider applying products that activate plant defenses to your fungicide sprays and/or early streptomycin sprays to help get a jump start on management, especially since it takes approximately 48 hours to observe the benefit of the product. We have observed an additive effect of repeated applications of plant defense activators (applying every 3–5 days).

• Actigard 1 – 2 oz/A
• LifeGard 4.5 oz/100 gal
• Vacciplant 16 fl oz/A
• Reminder for Actigard, LifeGard, Vacciplant: Needs to be applied within 48 hours of an infection event for the defenses to be activated

Streptomycin remains the best option for management. Do not use ARM sprays; complete sprays will avoid an unintentional infection trigger, especially under the current conditions.

• A typical streptomycin application gives you 48 hours of protection.
• When applying streptomycin, you are killing the bacteria within the flower at that moment and resetting the clock.
• Adding Apogee/Kudos to streptomycin spray for blossom protection will help manage the shoot blight phase of fire blight that may occur after bloom.
• Organic management options include Blossom Protect or Regalia 1 qt/A plus Magna-Bon CS2005 1 pt/A. Blossoms must be open and these products need to have been applied prior to the anticipated infection event to be effective.

Shoot Blight Management Options

With the precarious nature of this protracted bloom, we are running the risk of possible infection events slipping through the cracks. Consequently, we need to be proactive in limiting any fire blight spread, particularly through shoot blight.

• For added protection, include a plant defense elicitor (example: Actigard 1–2 oz/A) or Apogee/Kudos (2–6 oz/A) in your streptomycin application to help limit potential shoot blight post petal fall.
• Spraying Cueva at 2 qt/A starting at petal fall has shown to limit shoot blight. This needs to be done on a weekly basis through approximately mid-June.

Apple Scab Management: Risk May 13–16

Tank mix with a rainfast mancozeb. In addition to fungicide resistance management for apple scab, this will manage rust infections. Growers are highly encouraged to use complete sprays instead of ARM from bloom through petal fall, especially if frequent rain events favor extended wetness periods.

During bloom through petal fall, it is best to use FRAC Group 7, 7 + 9, 7 + 11 fungicides:

• Aprovia (FRAC Group 7; 5.5 fl oz/A)
• Excalia (FRAC Group 7; 3 – 4 fl oz/A)
• Fontelis (FRAC Group 7; 16 fl oz/A)
• Miravis (FRAC Group 7; 3.4 fl oz/A)
• Sercadis (FRAC Group 7; 4.5 fl oz/A)
• Luna Tranquility (FRAC Groups 7 + 9; 11.2 fl oz/A)
• Luna Sensation (FRAC Groups 7 + 11; 5 fl oz/A)
• Merivon (FRAC Groups 7 + 11; 5 fl oz/A)
• Pristine (FRAC Groups 7 + 11; 14.5 – 18.5 oz/A)

Post petal fall and for fungicide resistance management, rotate FRAC Group 7 fungicides with the following:

• Cevya (FRAC Group 3; 5 fl oz/A)
• Indar (FRAC Group 3; 8 fl oz/A)
• Inspire Super (FRAC Groups 3 + 9; 12 fl oz/A)
• Scala (FRAC Group 9; 5 – 10 fl oz/A)
• Vangard (FRAC Group 9; 5 oz/A)

Limit FRAC Group 7 and 11 fungicides to two complete applications during this period of high disease pressure. A maximum of four complete applications are allowed per year for FRAC Groups 7 and 11 fungicides for fungicide resistance management (regardless of if you use a single mix or a pre-mix). Save two FRAC Group 7 and 11 fungicide spray for the summer and late-season to manage fruit rots.

A note for bitter rot control: aggressive bitter rot management does not begin until the latter half of June and will continue through harvest. Any bitter rot that may be lurking in the orchard right now will be suppressed by your apple scab management.

For growers who may not have an apple or peach crop this year due to freeze or frost events: Foliar diseases still need to be managed to prevent inoculum build-up for the coming year. Apple diseases that still need to be managed are fire blight, apple scab, rust, powdery mildew, apple (Marssonina) blotch; bacterial spot, peach scab, and cherry leaf spot need to be managed on stone fruit. Growers can be conservative when managing these diseases. For fungal diseases, sulfur or other broad-spectrum fungicides will do a great job keeping inoculum suppressed. Apogee/Kudos will help suppress shoot blight. A rotation program of copper and biologicals (Serenade or Double Nickel) will be adequate to control bacterial spot.

Commercial fruit growers, please note

When controlling for disease, weather and tree growth conditions need to be monitored at a local level within one's own orchard. To assist with management decisions (especially infection events), growers can use the NEWA website, which has weather stations all over Pennsylvania. Before chemical products are applied, be sure to comply by obtaining the current usage regulations and examining the product label. Product information can be easily obtained from CDMS.