Fire blight risk increasing: treatment options in counties affected by Section 18 for Kasumin

The risk for blossom blight infection is extreme this week as of May 14, 2013. Kasumin is the best material available for control in orchards in regions affected by streptomycin resistance in the fire blight pathogen.

With predicted temperatures in the high 70s to low 80s for most of Michigan for the next seven days (as of May 14, 2013), this will be a critical period for potential fire blight infection. We have not experienced these types of temperatures for this duration during bloom in many years. 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 to 45 minutes on stigmas. Pollinators are also quite active as well, and will also increase the movement of the pathogen around in orchards and facilitate colonization of new flowers.

Last week, conditions were present for a fire blight infection in most regions of the state; however, these fire blight-conducive conditions were quickly followed by the freeze event in the morning of Monday, May 13. After a freeze, the MaryBlyt fire blight model resets the EIP value to zero because cold temperatures are inhibitory to growth of the pathogen. Also, since this possible infection was relatively early in the bloom period for most, there was likely not a strong buildup of pathogen cells in most orchards.

What typically happens when temperatures get cold is that the pathogen cells remain on flowers, but do not grow. As the open flower ages to five to six days after opening, the flower reaches a state where the pathogen cannot infect it. Thus, we can escape infection of that flower. However, those pathogen cells do not disappear; they remain on the flower and can serve as inoculum for subsequent infection if they are disseminated to newer recently-opened flowers.

Therefore, I think our risk in this coming week (May 15-22) may actually be higher than we think because of the potential that more inoculum is present in orchards from the buildup of last week.

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, ranging up to 200. These numbers change every day as predicted high temperatures change. They have increased for most stations from yesterday. EIP values around 200 are higher than Michigan State University Extension has seen during bloom for a number of years. 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 inch 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 significant fire blight year we had in 2005, EIP values peaked at 255 for Bainbridge, Mich., and 182 in Sparta, Mich. Of course, weather will play an important role in the potential for fire blight epidemics and subsequent shoot blight infection, but we do know that the occurrence of blossom blight puts trees at significant risk for further infection events of shoots during the summer.

What is the relationship between EIP values and choices for blossom blight control? The main choices we have are streptomycin (Agri-Mycin, FireWall), oxytetracycline (Mycoshield, FireLine), Kasumin and the biological control Serenade MAX. Remember, streptomycin resistance is widespread in southwest Michigan, the Fruit Ridge, and Oceana County. Streptomycin-resistant strains of the fire blight pathogen are beginning to be spread more widely in northwest Michigan. Thus, streptomycin should not be used in these areas, leaving our choices as oxytetracycline, Kasumin, and Serenade MAX.

In general, if EIP values are less than 50, any of the three would be effective as we would be under low fire blight pressure. For EIP values ranging from 50 to 90, oxytetraycline would be a good choice because the disease pressure is moderate and due to the high costs of Kasumin. Remember that this material inhibits growth of the pathogen and must be applied in advance of rains. For EIP values greater than 90, Kasumin is the best choice as our field data indicates Kasumin has better efficacy than oxytetracycline under high disease pressure.

For this upcoming week, the warm-up is slated to begin Wednesday, May 15. The best strategy would be to protect trees with Kasumin prior to any rain event. The interval between the first Kasumin application and the next one should be tight (two to three days) as this will be a period of rapid pathogen growth and new flowers will continue to open that were not protected with the first Kasumin application. Thus, Kasumin should be applied again prior to predicted rains during the weekend. The Kasumin rate is 2 quarts per acre in a maximum of 100 gallons. With such a high likelihood of infection, tank-mixing Kasumin with oxytetracycline would have limited value because of the reduced efficacy of oxytetraycline and also because oxytetracycline is degraded rapidly by sunlight and thus would not last too long on flowers anyway.

Finally, as we approach king bloom petal fall, I strongly suggest that you incorporate Apogee into your spray to initiate your shoot blight protection program. Please see two previous articles on Apogee from 2012: “Use of Apogee for shoot blight control in 2012” discussing fire blight control, and “Applying Apogee in 2012” discussing timing and rates. Treat 2013 as a severe risk year and consider increasing the rate for your first application to 150 percent of a split rate.

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