The NEWA Fire Blight Tool: too much information, too little explanation?

This time of year -- apple and pear bloom -- I/we get lots of grower questions (and I get precious little sleep) about how to interpret the fire blight risk values displayed in the NEWA Fire Blight Tool. I contend it is a case of too much information, too little explanation. Can I boil it down to a simpler take on the risk values? Sure, let's try.

First, you need to understand that on the left, you have Cougar Blight, and on the right you have "NEWA Blight" aka Maryblyt. (Figure 1.) Two different tools. Cougar Blight was developed in arid Washington, while Maryblyt was developed in the more humid east/northeast. Which do you think might be a better tool here? Yea, the one on the right.

Figure 1. Cougar Blight TRV left column, 'NEWAblyt' (aka Maryblyt) EIP right column.

Cougar Blight is much simpler, only accumulating heat units based on -- you got it -- temperature. YOU have to decide whether you had a sufficient wetting event -- be it rain, heavy dew, or even a dilute spray application (fungicide or PGR's only, no insecticides durin bloom) -- to get a fire blight infection. Here's all you need to know about Cougar Blight:

The Cougar Blight V8 (Version 8, not the motor in your 60's Camaro) Daily TRV column header for Cougar Blight means Total Risk Value. No, not Tree Row Volume here either. Arghhh.

1. Use  fire blight "occured in the area last year"
2. MARGINAL = 0-149 TRV (keep checking back); HIGH (pay close attention, have the strep and sprayer handy) = 150-349 TRV; and EXTREME (you will get a fire blight infection if it gets wet and you don't spray streptomycin) = above 350 TRV.
3. Oh yea, you have to have open blossoms too. D'oh!

Simple, you have to go from there, but EXTREME should make you too getting any sleep iffy.

NEWA Blight (aka Maryblyt) is a bit more complex. Study the table below (Figure 2.), and know this: when all FOUR of these requirements are met you WILL have fire blight infection unless you get out there and spray strep (unless you already did just prior the infection event).

BLOOM: flowers must be open (D'oh!) = BLOOM;

DH: (Degree Hours) must accumulate sufficiently >65 degrees F. 

TEMPERATURE: average daily >60 degrees F.;

WETTING: there must be a wetting event, rain or dew, but don't discount a dilute spray application is equivalent to dew (but NEWA does not know you did that, did it?)  Got it?

Figure 2. Simple graphic of how Maryblyt assesses risk of infection.

Now, there are some nuances and significant calculations to NEWAblight/Maryblyt, you don't need to worry about that, NEWA takes care of it. The important thing, any time Infection Potential EIP value (right column) goes above 100, you really need to stay on top of when the color goes ORANGE because then three conditions above have been met, and all it's going to take is some wetting to have an infection which cold turn into a fire blight epidemic. (Yikes!) As NEWA says "risk is 'HIGH' and forecasted wetting events should be carefully considered and a bactericide applied just before or after a rain"when it is ORANGE. If it turns RED and says "Extreme or Infection and antibiotic should (must?) be applied just before or after a rain." Must. Must. Must.

I am going to stop for fear of making it more complicated. It's not really. You just needed that explanation. Excellent references on Cougar Blight and Maryblyt from which I deduced all this for you are below. Read them. Please don't call me anymore... :-)

Cougar Blight Model

Maryblyt 7 Manual -- A Predictive Program for Forecasting Fire Blight Disease in Apples and Pears