I've been receiving a lot of calls lately about trees and diseases in the last few weeks. Calls or emails usually include the statement of what's going on and how do I stop it and fix it. The first thing to remember with foliar diseases – by the time you see a problem – the leaves were infected long before that time. For example, trees that show signs of Anthracnose, were infected in the spring so by the time you see issues the window for prevention has passed. I know that I've mentioned this in past articles, but it's very important to properly identify the problem, and the plant for that matter, before pulling out the sprayer and spraying plants.
Once you are seeing various foliar issues, the first as I said above is to properly identify. There are some cases where leaf troubles can be indicators of other issues such as Oak Wilt or Ash Yellows for example. That's when bringing in a sample to your local Extension office and asking for assistance is often the first step in the right direction. When it comes to determining future chemical control needs, the best way is to determine if they are warranted or necessary or if they will even do any good to address the problem.
I'm a huge proponent of good cultural practices when it comes to plant care to reduce stress on plants to help reduce the impact of foliar diseases that result in lost foliage and/or reducing the susceptibility of the tree to other insects and/or diseases. This is especially critical for plants such as trees.
Ways to help reduce stress on plants is to mulch using an organic based mulch 2-4" deep and providing supplemental irrigation when we are not receiving ½ inch to an inch of rain a week when it's hot and dry. As the days get shorter and the temperatures get cooler you may not need to water as often, but if we still aren't getting much rain, plants still benefit for added soil moisture as long as the ground isn't frozen. This might mean getting down on hands and knees and digging down into the soil a bit to see if it's dry or not.
If you have leaves that have fallen are due to foliar disease, do make sure to clean them up so that they aren't providing inoculum to reinfect plants next year. The recommendation is to not add them to the compost pile just to be safe. There are plenty of disease organisms that can be killed if the compost pile runs hot enough (about 140-160F) and all of the diseased materials is exposed to those temperatures long enough, which requires active turning and proper compost management. So, if you aren't sure if you're compost pile gets hot enough or your nervous, then don't feel bad about NOT adding diseased leaves to the compost pile.
Once you have determined what the problem is see if there is a need for chemical controls the following year. Sometimes an excessively wet spring will increase the occurrence of foliar diseases but if the spring is milder, preventative treatment might not be necessary. If you notice that your plant is having the same problem year after year no matter what the weather seems to be like, then preventative sprays might be warranted. Check with your local Extension office for current recommendations as well as assistance for proper diagnosis of plant health issues.