Fruit trees are susceptible to many pests and diseases, and without more specific information on your tree it is impossible to say with certainty what is affecting it. At first glance, assuming your photo has been taken recently, the raised blotches may be evidence of blister mite, an insect pest, which can be seen in the spring as the feeding of females and their offspring causes tiny blisters on developing leaves. Blister mites affect both apple and pear trees. This may also be a leaf spot disease, of which there are many affecting all kinds of fruit trees. These diseases do not do serious harm to trees, but good sanitary practices – raking and destroying fallen leaves so that the disease-causing pathogen does not overwinter – will be helpful in preventing recurrence.
In general terms, the best way to manage insects and diseases is as mentioned above: good sanitary practices. Remove infested or infected leaves, fruit and pruned branches and dispose of them in the garbage rather than in your compost. Make sure your tree branches are not crowded: this helps to improve air circulation and reduces wetness which promotes the development of disease. If you are replacing a fruit tree or planting a new one, search out disease-resistant varieties. Reduce stress to your tree by making sure it receives sufficient irrigation, and apply a layer of mulch around your trees, making sure that you leave space between the mulch and the trunk of the tree.
Because many of the diseases and pests that afflict fruit trees are specific to the type of tree, more information will help to identify what is affecting your tree. Please do contact us again with further information (type of tree, date of photo, etc.) and we will try our best to give you a more targeted response.