Fire blight risk is very high for May 7-10, 2015 in Michigan

The risk for blossom blight infection is very high this week. Kasumin is best for control in orchards affected by streptomycin resistance in the fire blight pathogen. Streptomycin and Kasumin are the best materials in other areas.

With predicted temperatures in the low 80s for most of Michigan May 7-8 and in the 70s May 9-10, this will be a critical period for potential fire blight infection. In addition, potential rain events are forecasted for any of those days and in particular Friday and Saturday, May 8-9.

The MaryBlyt model on the Michigan State University Enviro-weather website is predicting Epiphytic Infection Potential (EIP) values for fire blight as high as:

Southwest Michigan
222 for Bainbridge for Friday

Fruit Ridge
222 for Sparta for Saturday
222 for Belding for Saturday

West central Michigan
98 for Hart for Friday

Northwest Michigan
175 for Suttons Bay for Friday
176 for Elk rapids for Friday

Eastern Michigan
283 for Petersburg for Saturday and 234 for Sunday
185 for Romeo on Saturday

The EIP value is telling us the potential is there for significant fire blight infection. Temperatures in the high 70s to low 80s are optimal for growth of the fire blight pathogen on apple flower stigmas. At these temperatures, populations of the pathogen can double in size every 30-45 minutes on stigmas. Rain events will trigger fire blight infection events on flowers that harbor very large populations of the fire blight pathogen. Free water from rain enables the fire blight bacteria to “swim” down the style from stigmas to the nectaries at the base of the flower where these bacteria cause an infection.

Although your orchard may be in a location where few flowers are open currently, trees have been developing and kind of “holding” during the last few cooler days. I would expect consecutive days of 80 degree weather to result in a significant percentage of flowers opening. If these flowers are colonized by fire blight bacteria soon after they open, populations will be large enough to cause an infection within two days and likely in less than two days.

This current situation will require a minimum of two antibiotic applications for successful blossom blight management. The first spray should be applied prior to the first rain event, whether that will occur Thursday or Friday. With warm temperatures and further rainfall expected, the second spray should be applied within two days of the first. After Sunday, temperatures are expected to cool into the 50s or low 60s and the threat of new infection will subside. However, infections that occur this weekend will eventually show up as blossom blight symptoms. This is why the second application is necessary. Ensure optimal coverage during these sprays; remember that every flower is a potential site for infection.

The antibiotics of choice for this week are Kasumin in areas where we have streptomycin resistance and streptomycin in areas with no resistance. Please see my accompanying article, “Three antibiotics available for fire blight management during bloom,” posted at the Michigan State University Extension website with details on antibiotic use for blossom blight management.

Finally, even though temperatures will cool down, do not forget to start an Apogee program for shoot blight control. The timing for the first Apogee application is at king bloom petal fall when new shoots are about 1-2 inches long. If you miss that timing, you can still get some shoot growth inhibition and shoot blight control, but the effect will not be as strong. With the tremendous risk of blossom blight infection, it is imperative that we do the best job possible for shoot blight control.

Be sure to use the fire blight model on the MSU Enviro-weather website to see what the EIP values are as predicted from the weather station closest to your orchard. The current predicted MaryBlyt EIP values for later this week are very high in most regions, and over 200. These numbers change every day, as predicted high temperatures change. They have increased for most stations from yesterday. This indicates the risk of blossom blight infection will be at a critical point later this week. Any rain that occurs during this period (as little as 0.01 inches of rain) will trigger a fire blight infection event.

The higher the EIP value gets, the higher the risk of infection. When the number reaches 100, this indicates the potential for significant infection. At an EIP of 200, the risk is extreme. For historical reference, during the last really regionally-significant fire blight year we had in 2005, EIP values peaked at 255 in Bainbridge, Michigan, and 182 in Sparta, Michigan. Of course, weather will play an important role in the potential for fire blight epidemics and subsequent shoot blight infection, but we do know the occurrence of blossom blight puts trees at significant risk for further infection events of shoots during the summer.

Dr. Sundin’s work is funded in part by MSU’s AgBioResearch.

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