Most pruning is done in winter or early spring when deciduous trees and bushes are dormant. However, because rain can carry bacterial pathogens into fresh pruning cuts, there are several susceptible plants that should be pruned during the summer dry season.
Apricot trees are pruned after harvest, at least six weeks before the rainy season begins (in late October in our area). If pruned during wet weather, the bacteria that causes Eutypa dieback can infect the tree leading to sudden branch death in summer. Eutypa dieback infection first appears as oozing cankers on the bark surface. If the branches on your apricot tree have died so quickly that the leaves are still on or if you see a gummy substance and sunken spots on the bark, remove dead branches and prune out diseased portions at least 10 inches below the obvious infection. Disinfect pruning tools between cuts with a 10% bleach solution. Summer pruning is the most effective means of preventing Eutypa dieback in apricots.
Olive trees are susceptible to waterborne infection by the bacteria that causes olive knot, a warty, woody growth that appears predominantly at leaf nodes, but also on branches and sometimes on the olives. The knots or galls usually do not affect the fruit quality or overall health of most olive varieties (except for the Manzanita olive).
Prune olive trees in the dry summer and, because the knots or galls contain some bacterial pathogen that can be spread by rain, remove them as well in summer. Break off or cut off the galls and follow the same disinfection procedures given above.
Oleanders that are pruned during wet weather are also susceptible to infection by the bacteria that causes oleander knot or gall. The bacterial pathogens that cause olive knot and oleander gall are closely related. In fact, olive trees can become infected with the same bacteria that causes oleander knot. (So be careful of cross-contamination when pruning olives and oleanders). All parts of oleanders are toxic. Follow disinfection procedures when pruning and then dispose of all oleander cuttings, leaves and flowers in the green waste bin.
There are a few other plants that are best pruned or sheared in summer. Euryops should be sheared lightly in summer.