How do I keep my fruit trees free of pests and disease?
For home fruit trees, the recommended approach to pest management starts with growing otherwise healthy, vigorous trees capable of resisting pest and disease pressures. Deciduous fruit tree health and vigor depend on good cultural practices with respect to variety and rootstock selection, planting, irrigation, fertilization, pruning, weed control and orchard sanitation. Strong-growing trees are generally much more tolerant of pests than weak ones.
Chemical pesticides are used only when absolutely necessary, based on a correct diagnosis and assessment of damage, and only in the safest, most limited way possible. Note that fruit trees do not have to be completely pest-free to be strong and healthy.
Trees growing in fertile, aerated (well-drained), consistently moist soil tend to have greater resistance to pests than trees stressed and weakened by too much or too little water. Do not over-fertilize. Too much nitrogen leads to excessive vegetative growth and increased disease susceptibility (as well as lower fruit production). Control weeds; they compete with trees for soil moisture and nutrients, in addition to providing habitat for pests. If a tree is not growing well and neither too much nor too little soil moisture is the problem, get a soil analysis.
Keep up with pruning for air circulation and sunlight penetration in the canopy. An open, airy canopy will dry much more quickly after rain or morning dew, minimizing the spread of disease. If possible when planting, choose sites that receive morning sun and are exposed to good air circulation.
Refer to your local retail fruit tree nursery and Cooperative Extension office for spray guidelines and advice about what to look for. If pest damage is observed and you are not familiar with the pest or symptoms, take leaf, twig or fruit samples (sealed in a plastic bag) to your local retail fruit tree nursery for diagnosis and recommended course of action. Insecticidal soaps, applied correctly, can be effective against aphids, mealybugs, spider mites, psyllids, scale crawlers, thrips, and whiteflies. Chemical pesticides are used only when necessary; the wrong treatment or timing does more harm than good if it harms a pest's enemies more than the pest.
In fall, as soon as the leaves are off the trees, remove all leaves, twigs and fruit that might harbor pests, including any mummified fruit left in the trees. Hot composting of the debris (large pile, kept moist and well aerated) will kill most pathogens, but the usual recommendation is to burn or discard.
Dormant oil sprays can be essential for the control of many overwintering pests, such as scale, mites and aphids. Dormant sprays with copper or other pesticides also may be necessary to control such diseases as peach leaf curl and shot hole fungus.
See also the questions "When is a winter dormant spray necessary for home fruit trees?" and "Do I need to spray for peach leaf curl every year?"