Question
This year is starting out pretty weird weather wise. Here in Michigan, summer came a couple days before March 21st. I have a lot of trees with apple scab this year, but temps are already in the 70s and nearing 80s - in march. I've noticed that after some fungicide applications in the past in June and September, the leaves looked like they were drooping more, though the tree seemed better after a few more weeks. I have a lot more requests from new customers for treatment. I'm worried I won't be able to get to everyone in time. I know the NORMAL schedule for applications, but what temperature shifts should I watch out for? So far we have had 10 degree days over 75f, 10 degree days over 60f before that. Next week temps are set to go back to more normal 40s and 50s with chances of frost.

Answer
Hi Sue,

You are lucky to live in Michigan.  The Extension Service in Michigan has an online forecasting system that will help you know when conditions are right for apple scab.  This will help you time fungicide applications and know the risk.  The model is based on years of research where scientists have determined the temperature and moisture requirements that are important in the disease cycle.

You should go to the following web address for Enviro-weather:
http://www.enviroweather.msu.edu/homeMap.php

Once you are at the main page, you can select the weather station that is closest to your location.  Remember that there is always going to be some temperature variation, so you need to use a little of your own knowledge of the site.  For instance, if the orchard is in a low spot that is prone to fogs, you are more likely to have a frost if temps are near freezing as opposed to a higher spot.

After you pick the weather station, you can select "Fruit" from the menu in the green bar at the top of the page.  This will open up a new menu down the left side of the screen.  On this menu you can choose apple and then apple scab.  It opens up a new page that is the specific weather data for your location.

I tried two locations and in the first one, it said no risk.  In the second one, the font changed from black to red.  It said "light" and told me to expect symptoms on 4/8.  So, this would be the time to spray at site 2, but you could hold off at site one.  Hopefully, this will help you select which of your sites have the greatest need for a fungicide application.

Here is one other page that you may find helpful.
http://apples.msu.edu/applescab.htm
As you scroll down the page, there is a table that tells you the temperatures and leaf wetness periods that are needed for apple scab.  This will hopefully answer your general question about temperatures.

Good luck with the trees!