2022 Disease Update: Fire Blight Risk for May 4 and 5

When it is apple bloom, we live day-to-day when it comes to monitoring weather data to determine if fire blight will be an issue. This time last week, fire blight was not on the radar; today is a different story. We will be meeting the requirements for fire blight infection risk this week. According to MaryBlyt, there is an infection risk forecasted for May 4–5 with an EIP over 100 for both days. When the EIP (epiphytic infection potential) is between 100 and 150, conditions are sufficient to support blossom blight. With our protracted bloom, it is critical growers protect open blossoms during this period to prevent blossom blight. In addition, scab infection risk is forecast for May 3–7. For growers in the northern part of the state or at higher elevations whose trees may be a bit further behind phenology-wise, fire blight prevention needs to start now. Fire blight risk increases as May progresses. There is a forecasted high risk for fire blight infection May 12–15.

For folks using NEWA, the above announcement may come as a surprise since this is not what NEWA is forecasting this week. Through some sleuthing, it appears that the National Weather Service data NEWA pulls for the forecast is not being updated as timely as other outlets, such as Weather Underground. 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.

Fire blight management

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 even 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 100–150 is low but is sufficient to support a blossom blight epidemic, and an EIP of more than 200–250 indicates that large numbers of infections are likely should a wetting event occur.

Consider applying products that activate plant defenses to your fungicide 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).

Tight cluster

(especially if warm weather is moving trees along quickly)

• LifeGard 4.5 oz/100 gal
• Vacciplant 16 fl oz/A
• Apogee/Kudos 6 oz/A (only apply to high vigor apple trees)

Pink

• 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 hr of an infection event for the defenses to be activated
• Apogee/Kudos 3 oz/A (only apply to high vigor apple trees)

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.
• 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. LifeGard and Vacciplant are additional options.
• 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.

Apple scab management

As a reminder, apple scab disease pressure is the greatest from bloom through petal fall. Considering May 1–7 is almost one long continuous scab infection period, do not take chances.

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 this time, 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)

When necessary, 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 if you use a single mix or a pre-mix). Save two FRAC Group 7 and 11 fungicide sprays for the summer and late-season to manage fruit rots.

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.