Above and below are weather details from highlandsweather.net. The graphic below allows you to see the current humidity percentage and temperature.
Last week, I had the opportunity to speak to my friend, Sara Villani, who is the leading scientist at NC State University, studying Boxwood Blight. I enjoy catching up with her at least once a year to see if her research is revealing anything new about the fungal pathogen. As they continue to learn more, it's important to understand two things. Boxwood Blight will not show physical symptoms on the shrub unless the humidity is at 98% and the temperature is around 71*F. It is still important to spray because you could have infected shrubs and the treatments can continue to help knock the fungus back.
Other advice she offered:
Boxwood Blight is more prone to shrubs that have excessive leaf wetness. Leaf wetness is caused not only by rain, but also dew in the morning. This means Boxwood that are growing in the shade compared to those in full sun will have more daily leaf wetness. Overgrown boxwood that are very dense with little air space inside the shrub are also more prone to Blight because they don't dry out as fast as those with good air movement in and around the shrubs. Remember that Boxwood Blight is a sticky fungal spore that's found on the ground. If you can trim the boxwood to allow for a slight gap between the ground and the foliage, you have a better chance at avoiding the fungus. A healthy application of mulch below the shrubs can suppress the fungus as well, by covering it up. It's important to treat boxwood using 2 fungicides with 2 different modes of action; one that is a contact (Daconil) and the other being systemic fungicide, working from inside the leaves. It also goes without saying that cleaning tools with bleach and soap is critical between jobs, especially if a landscaper is bringing tools from another site! But again, the biggest take away is this:
98% humidity + 71*F + excessive leaf wetness = Boxwood Blight
If you are able to reduce just one factor in the equation above, you will not see blight symptoms on Boxwood. As temperature decreases, so does Boxwood Blight incidence. Interestingly, as temperature increases, the same can be said. This means in environments like Atlanta, Boxwood Blight is more of an issue in spring and fall and less of an issue in summer.