The Rosaceae plant family is a large one with over 4,800 species, and its members are commonly planted in our Central Valley gardens. Roses (as you might guess from the family name), ornamental pears including Bradford pear trees, apple and pear fruit trees, pyracantha, hawthorn, spirea, contoneaster and many types of flowering fruit trees such as flowering plum, flowering cherry and Carolina laurel cherry are members of the Rosaceae. All are susceptible to fire blight, a bacterial disease that spreads in warm wet weather. We had a warm wet spring this year and are seeing more fire blight damage than in the previous drier, hotter drought years.
Fire blight damage shows in all parts of infected trees and bushes. Shoots, flowers and fruit turn black and die. Twig tips develop a “shepherd crook” shape and look as though they have been burned. The bacteria overwinter in sunken cankers or lesions that form on the branches and trunk; the cankers ooze a sticky amber substance. The bacteria from the cankers are spread to neighboring trees and bushes in spring by bees, other insects and splashing water. In fall, search out cankers and diseased limbs on plants that have been infected by fire blight and cut them out. Make the cuts at least 8 to 12 inches below the canker or diseased branch and disinfect pruning tools between cuts with a 10 percent bleach solution. Antibiotic sprays were used in the past to control for fire blight, but the bacteria have become resistant to the applied antibiotics (streptomycin, terramycin). Some control is provided by spraying copper solutions in spring, but the cultural control of pruning out diseased wood and cankers in fall and winter is an effective method of fire blight control.
Peach leaf curl is a fungus that infects peach and nectarine trees. In spring, the first set of leaves is thicker than normal; the leaves turn a reddish color, pucker and curl.
If left untreated, the fungus can eventually affect the fruit and then the entire tree. In fall as the weather cools and the rains begin, the fungal spores that were dormant during the hot summer begin to proliferate on the bark surface where they overwinter. Peach leaf curl fungus is then spread in warm wet spring weather by splashing rain or overhead irrigation.
There are varieties of peach and nectarine that are resistant to peach leaf curl. Treatment for the disease is made after leaf fall in late November in our climate and consists of spraying a fungicide to drench all surfaces of the tree. Unfortunately, most of the commonly used copper-based and lime-sulfur products are no longer available to the home gardener. Bordeaux mixtures (copper sulfate and hydrated lime) work well in controlling peach leaf curl but the separate ingredients must be mixed together just before application and are now difficult to find. Copper products build up over the years in the soil, and care should also be taken to prevent runoff and keep copper products out of the water supply.
Chlorothalonil is a synthetic non-copper fungicide used to control peach leaf curl in the home garden.
Send Elinor Teague plant questions at firstname.lastname@example.org.