2026 Disease Update: High Risk for Fire Blight This Week

In a matter of a week, many areas will go from tight cluster to pink to bloom and possibly petal fall. Mother Nature is in rare form this 2026 season. In Adams County, pear blossoms are open, and apples are starting to bloom, especially on early-blooming varieties such as Pink Lady. According to the fire blight models, there is a high risk for infection conditions from April 14 - 18 for orchards where apples and pears are in bloom. If there is a wetting event, including heavy dew, the "high risk" could trigger a severe "infection risk," since EIP values are predicted to be in the 200– 300 range. Currently, there is a low chance of rain throughout the week; however, hot temperatures have historically triggered thunderstorms. Growers need to be on alert. Even if no rain occurs, growers are encouraged to spray their orchards during full bloom as a precaution, as the bacterial load on the stigma will become extremely high this week.

Growers can refer to the NEWA stations in Pennsylvania and Maryland closest to their location and double-check the fire blight risk. We focus on the EIP value calculated by the MaryBlyt model to assess risk. To better understand EIP (or refresh your memory), there is a healthy review of EIP at the end of this article.

We are still experiencing excellent weather for powdery mildew, and management is still necessary for both apples and stone fruit. Please refer to the Disease Update from April 7, 2026, for management recommendations.

Spray options to consider when apples are in full bloom

Apply streptomycin plus an adjuvant, such as Regulaid. Be sure to follow the label for rates. Streptomycin is bactericidal, killing bacteria in the flower. The addition of an adjuvant will push streptomycin further into the nectaries, killing any bacteria that may have slipped by.

• For added protection, combine streptomycin with a plant defense elicitor, such as Actigard (1-2 oz/A).
• Including prohexadione calcium (e.g., Apogee, Kudos) with streptomycin is highly recommended to prevent potential shoot blight post-petal fall. Prohexadione calcium increases the plant cell walls, preventing Erwinia amylovora from gaining entry.
• You can safely combine streptomycin, a plant defense elicitor, and prohexadione calcium. This option will provide you with excellent protection in the short term and long term.
• Organic management options include Blossom Protect or Regalia at 1 qt/A plus CS 2005 1 pt/A. Blossoms must be open, and these products must have been applied before the anticipated infection event to be effective. Organic plant defense elicitors include LifeGard (4.5 oz/100 gal) and Vacciplant (16 fl oz/A).
• Using copper products with copper sulfate pentahydrate (e.g. CS 2005) is probably the best copper option for fire blight management because this is a soluble copper. Soluble copper products have all the copper ions available to kill bacteria at the moment they are applied. In contrast to fixed coppers, where the copper ions are bound to particles and are released gradually. Please note: all copper products may cause fruit finish issues.

As a reminder

When applying streptomycin, you are killing the bacteria within the flower at that moment, thereby resetting "the clock." It is important to apply treatments before any wetting event to prevent bacteria from entering the flower nectaries. Even if it rains within 24 hours, you have achieved your protection with that application for blossoms that are opened. Streptomycin does not need to persist, as fungicides do, to prevent disease. A typical streptomycin application gives you 48 hours of protection.

When conditions are forecast to be favorable for fire blight, consider adding 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 product's benefit. We have observed an additive effect from repeated applications of plant defense activators (every 3-5 days).

For growers experiencing tight cluster through pink bud in the more northern parts of Pennsylvania

Warm weather may move along trees at tight cluster through pink bud quickly in the next few days. Also, the warm weather will be sticking around through next week, so bloom may be around the corner for folks in this region. Consider a pre-emptive strike by priming the tree's immune system or increasing the plant cell walls, as referenced above. If you are applying a fungicide spray, there are options to add to your tank:

• Actigard 1 – 2 oz/A
• LifeGard 4.5 oz/100 gal
• Vacciplant 16 fl oz/A
• Apogee/Kudos 6 oz/A (only apply to high vigor apple trees)

A healthy review of Epiphytic Infection Potential (EIP) to understand fire blight risk

In the context of fire blight, "epiphytic" refers to the Erwinia amylovora bacteria living and multiplying on the surface of healthy plant tissues, primarily stigmas and nectaries within open flowers, before they actually cause an infection. They are essentially colonists waiting for an opportunity.

EIP is a calculated value or index that estimates the size or density of the epiphytic population of Erwinia amylovora bacteria present on susceptible flower blossoms. It represents the amount of potential inoculum (bacteria ready to cause disease) available.

Fire blight prediction models (e.g. MaryBlyt, Cougar Blight) calculate EIP primarily based on temperature accumulation over time (using degree days or degree hours above a specific base temperature, often around 60-65°F or 15.5-18.3°C) since the flowers opened. Warmer temperatures allow the bacteria to multiply faster on the flower surfaces, increasing the EIP.

How EIP Relates to Fire Blight Risk

EIP is a critical prerequisite for blossom blight infection. You need a sufficiently large bacterial population present on the flower before infection can occur.

A high EIP indicates a high potential for infection. If the calculated EIP is high, it means conditions have been favorable for the bacteria to build a large population on the blossoms. If a wetting event (rain, heavy dew, high humidity, or even irrigation) occurs when EIP is high, and flowers are open, the risk of infection is significantly elevated. The moisture washes the bacteria down into the nectaries, allowing infection to start.

A low EIP indicates a lower potential for infection. This suggests the bacterial population on the blossoms is small. Even if a wetting event occurs, there may not be enough bacteria present to initiate widespread infection. Thus, the overall risk is lower.

EIP is not the only factor: High EIP alone does not guarantee infection. You also need:

• Open, susceptible blossoms.
• A moisture event (rain, dew, high humidity >60%) to wash bacteria into the flower's nectaries or facilitate movement into wounds.
• Sufficient warmth during and after the moisture event for the infection process itself to proceed.

The specific numerical scale and thresholds can vary slightly depending on the model version and local calibration. However, a common benchmark uses an EIP value calculated based on degree-hour accumulation:

Low Risk Potential (EIP < 100): An EIP value below 100 generally suggests the epiphytic bacterial population is likely too small to cause significant infection, even if a wetting event occurs. The risk is considered low.

High Risk Potential (EIP ≥ 100): An EIP value of 100 or higher indicates that the bacterial population on the blossoms has likely reached a critical threshold. If a wetting event coincides with this high EIP and open blossoms, the risk of a fire blight infection event is considered HIGH. Some models might use slightly higher thresholds (e.g., >200) for very high-risk warnings, depending on context.

In summary

Epiphytic Infection Potential (EIP) quantifies the estimated population size of Erwinia amylovora bacteria living harmlessly on flower surfaces before infection occurs, serving as a measure of potential inoculum. Fire blight models calculate EIP primarily based on temperature accumulation; higher temperatures increase EIP. A high EIP (e.g., values ≥ 100 in many models) signals a large bacterial presence, creating a high fire blight risk if combined with open blossoms and a wetting event (rain/dew). Conversely, a low EIP (e.g., < 100) indicates a smaller bacterial population and thus a lower potential risk, even if weather conditions become wet. EIP is therefore a crucial component, alongside moisture and host susceptibility, in assessing the imminent risk of fire blight infection.

For additional resources

Dr. Tree Fruit and Don Podcast

Penn State 2026–2027 Tree Fruit Production Guide

For 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 support management decisions (especially during infection events), growers can use the NEWA website, which includes weather stations across Pennsylvania.  Before applying chemical products, be sure to comply with current use regulations and read the product label. Product information can easily be obtained from CDMS, Agrian, or Greenbook.