While there are many different brands and chemicals out there for fruit trees, they're typically meant to do one of two things: control fungus or control insects. Some include both a fungicide and an insecticide for convenience, but there are many things to keep in mind when using chemicals on your fruit trees.
Most important, make sure the fungicide or insecticide you choose is meant for fruit trees. If the product label doesn't list them among the plants it's designed for, do not use it. Also, when time comes to harvest the fruit, make sure it's washed thoroughly, a good practice regardless of whether you've treated a tree with chemicals.
If you had a problem with fungus in a previous season, the best time to start treatment is before it returns. Spray fungicide in the spring at the first sign of vegetation. This preventative measure is often more effective than starting after you see symptoms because by then the disease has already matured and become established. Continue spraying every three to four weeks throughout much of the growing season.
If you are using an insecticide or combination product, stop spraying during the blooming phase. You may switch to a straight fungicide during this time, but never an insecticide. This could kill bees, which are responsible for pollination. Without that, your crop will be substandard in the fall.
Proper identification of the fungus, or any other disease, is important in determining what anti-fungal application is necessary. For example, anti-fungal applications with the active ingredient chlorothalonil is good for controlling brown rot and black knot in a variety of fruit trees, including apricots plums and cherries. Using a product with ferbam, mancozeb, sulfur or neem oil will control rust and scab in both pears and apples.
While controlling these common conditions with chemicals can yield good results, also consider removing all infected material from and around the tree, especially in the case of fungal or other disease infections. Fungal spores can overwinter on dead leaves and potentially reinfect the tree the next year. Remove any dead branches in order to prevent the spread of disease.
Fruit flies, apple maggots, coddling moths, mites and many other species can wreak havoc on fruit trees. Insecticides come as powders or sprays, though sprays generally provide better coverage and are easier to thoroughly apply. Some are ready to use but others are concentrated, and you need to follow label directions for mixing. As with all insecticides, always wash your hands after using and keep pets and children away from the treated tree for at least a few days.