Question
I've been using a systemic made by Bayer for two years with great results.The flowering crab apple tree is old and was diseased. It is now flourishing. My question is; should I continue to use the systemic or can I miss a year or two?

Answer
It depends on which product you are using. Bayer Advanced has a product for diseases and one for insects. If it is the one for insect control if you are not seeing any signs of insect problems, leaves chewed, sticky sap dropping insect presence, I would not use the product. Although the product will prevent insect attack If no   insects are there I would wait until you see any signs.

If you are using the Disease product I might go ahead and use it. Once a foliage disease starts it is difficult to reverse the damage. The prevent for the disease problems can be beneficial.

Of course it is up to you--if you want to prevent any insect or disease problems use the product but as you are aware this product is not cheap. IF it were my tree I would wait until I saw a problem and then use the Bayer  product.