Understanding the fire blight model on the Enviro-weather web site

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

One of the useful models on the new Enviro-weather web site (www.enviroweather.msu.edu) of MSU is the fire blight model for apple. This model is based on the blossom blight model in Maryblyt™ developed by Paul Steiner (deceased) and Gary Lightner, both formerly of the University of Maryland. Maryblyt™, developed for the older DOS operating system, is no longer commercially available.

I am providing here a brief explanation of the blossom blight model on Enviro-weather for those wanting to understand how to use it more efficiently. Enviro-weather uses the EIP (epiphytic infection potential) value of Steiner. Basically, when the EIP value reaches 100% or above, it is assumed that a rain or heavy dew can initiate a fire blight infection. See the accompanying section for a more in depth explanation of the EIP term.

So, as shown in Figure 1, if a group of blossom opened on May 29, by June 1 these flowers could be infected if they were wet from dew or rain. This EIP chart gives several starting dates (May 29 to June 4) for use with orchards with different bloom starting dates.

Why do the EIP numbers decrease?

Those of you who have studied the EIP values on the Enviro-weather charts may wonder why the numbers can decrease. For example the EIP value for bloom opening on May 29 is 201 as of June 2 but is 168 one day later. This is because in an orchard, flowers age and become less receptive to bacterial growth. Enviro-weather, mimicking Maryblyt, eliminates heat units from the accumulation that these older, now non-receptive blossoms, would have been subjected to.

Cool weather can reduce the risk for blossom blight. Accordingly, the blossom blight model will reduce the EIP values by a third, then by a half, or then to zero for one, two, or three consecutive days with no temperature above 64°F.

What column do I select later in the bloom period?

Some of you may notice that as the bloom time progresses the blossom blight chart no longer shows a column labeled with the first bloom date for your orchard. If that is the case, simply pick the oldest column as this will show the EIP values for the oldest flowers in that orchard still receptive to bacterial population buildup.

If my orchard was unprotected for an infection, when will symptoms show up?

One of the nice features of the original Maryblyt™ program is that it predicted when symptoms due to an infection would show up. This feature has not been implemented in Enviro-weather, but can be calculated with a little work. Maryblyt™ predicts fire blight symptoms will show up 103 GG55 following an unprotected infection. MAWN, the Michigan Agricultural Weather Network (www.agweather.geo.msu.edu/mawn/) has a degree day calculator that can be used for this task. For example, several southwest Michigan weather stations showed a blossom blight risk (EIP > 100 plus rainfall) on May 10, 2006. Using the MAWN calculator with a starting point of May 10, the GDD55 accumulation as of May 28 was 89. So, symptoms expression from this May 10 point is expected around May 29 or 30, when the total reaches 103. This feature will be added in the future to Enviro-weather when time permits.

The Epiphytic Infection Potential

According to Paul Steiner, once a blossom opens, fire blight bacteria multiply on the tip of the flower pistil whenever the air temperature is above 65°F. The higher the temperature, the faster the bacterial population will grow on the pistil tip also called the stigmata. This population was modeled by Steiner by calculating the heat unit accumulation above the base of 65°F. Because bacteria grow so rapidly, Steiner used degree hour base 65°F, rather than the more typical degree day used for insects. For example, a day that had a constant temperature of 70°F would accumulate 70 – 65 = 5 degree hours every hour for a total of 24 hr X 5 = 120 degree hours. According to Steiner, an open blossom exposed to 198 degree hours base 65°F could potentially have enough fire blight bacteria on the pistil sufficient to cause an infection if a rain or heavy dew washed the bacteria into the inner flower. To make the numbers easier to work with, Steiner expressed the degree hour accumulation as a percentage of the target 198 degree hours. So an accumulation of say 150 degree hours expressed as a percentage is 100 x 150 / 198 = 75.5%. Notice that the EIP is roughly half of the degree hour accumulation. Steiner called this percentage the “epiphytic infection potential” (EIP). Epiphytic means “on the plant surface.”

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