NEWA is predicting a high risk for fire blight infection this week as a result of favorable conditions for apple orchards in bloom. Favorable conditions include 1) a certain number of heat units accumulated during bloom for a threshold level of inoculum to be reached; 2) a wetting event to wash the bacteria into infection sites; and 3) the average temperature is above 60°F. If at least two conditions are met during bloom, then the risk for infection is ‘High’ and antibiotics should be applied. A wetting event is not necessary to elevate the risk. Additionally, forecasted wetting events should be carefully considered and a bactericide applied just before or after a rain event.

When using the fire blight model on NEWA, it is important to enter your own “First Blossom Open Date” for your orchard as this can drastically change predictions. You can also enter your orchard’s history with fireblight in the first drop down menu. Lastly, you can enter the date you last applied an antibiotic this season and the model will reset to provide the most accurate predictions. When using the model, it is important to look at the column farthest to the right in the chart titled “Infection Potential EIP Value” since this is the most accurate prediction for our region. Any time the model shows an orange (High) or red (Extreme) output and the value for that date is above 100, an antibiotic application is warranted.

To manage fire blight in both apples and pears, consider using Kasumin at bloom. Kasumin is an antibiotic like Streptomycin and Oxytetratracycline. It contains Kasugamycin which is a different chemistry. Kasumin is only labeled for bloom and petal fall applications while strep and oxytet are labeled for post bloom use (note that Fireline 45 only allows for 1 postbloom application on apple). Using 3 different chemistries in rotation allows for resistance management while extending the protection into the post bloom period when late blossoms typically appear which ever material you use, the addition of Actigard or other plant defense elicitors may improve control. Applications of Apogee or Kudos during bloom can work to prevent shoot blight after petal fall. The use of Actigard and Apogee are especially recommended in new plantings where instances of shoot blight can kill young trees. For further recommendations on these products, Michigan State University published a guide including information on application timing and product rates.

Additionally, for peaches at petal fall, copper formulations should be used to suppress bacterial spot. Generally we recommend starting at 0.5 oz of metallic copper and gradually lowering the rate as the season progresses. The rate applied will depend on the formulation. Dr. Norm Lalancette published a chart listing common copper formulations and rates for peach and nectarine applications. Avoid combining copper with captan especially if it has been overcast for several days. Also avoid acidic spray solutions when applying copper. Dr. Lalancette has published a handy guide for copper applications in early covers. Antibiotics such as Mycoshield or Fireline may also be used and may offer slightly more residual activity during long wetting periods.