This plant disease is caused by a bacterium (Xanthomonas campestris pv. pruni) that also attacks apricots and plums. This is one of the more destructive diseases of stone fruits east of the Rocky Mountains. In the spring, bacteria ooze from lesions on the twigs to be carried by splashing rain to the young leaves, shoots, and developing fruits. Frequent rainfall favors the infection. Trees that defoliate early in the summer are weakened and produce small crops of poor-quality peaches and nectarines.

This plant disease, caused by either of two closely related fungi (Monilinia laxa or M. fructicola), is very destructive to all of the stone fruits. The fungi spend the winter in twig cankers or in rotted fruit (mummies) in the tree or on the ground. In the spring, spores are blown or splashed from cankers or mummies to healthy flower buds. After penetrating and decaying the flowers, the fungus grows down into the twigs, producing brown, sunken cankers. During moist weather, a thick, gummy sap oozes from the lesions, and tufts of gray spores may form on the infected areas. Spores from cankers and infected blossoms or mummies are splashed and blown to the maturing fruit. Young peaches and nectarines are fairly resistant to infection, but maturing fruit is vulnerable. Brown rot develops most rapidly in mild, moist conditions.

This plant disease is caused by a fungus (Taphrina deformans) that attacks peaches and nectarines wherever they are grown. Infection occurs as soon as the buds begin to swell in the very early spring. Fungal spores are splashed from the bark to the buds by spring rains. Later in the season, the infected leaves develop a grayish white covering of spores that are blown onto the bark. Infected trees are greatly weakened by the premature loss of foliage in early summer. Leaf curl is most severe when spring weather is cool and wet.

Synanthedon species These moth larvae are very damaging to peaches and nectarines. The adults are blue to black clear-winged moths that resemble wasps. In mid- to late summer, females lay eggs near the base of the tree. Emerging larvae bore into bark near the soil surface. Their tunnels interfere with water and nutrient circulation, causing twigs and branches to wilt and die. These borers feed throughout winter and into spring. A gummy sap-often mixed with sawdustlike particles-oozes from the tunnels. The borers pupate in early to midsummer in cocoons located at the base of the tree or just inside their tunnels. The moths emerge several weeks later.

Although many different species of scale insects attack peaches and nectarines, the two most common are San Jose scale (Quadraspidiotus perniciosus) and white peach scale (Pseudaulacaspis pentagona). The scales lay their eggs or bear live young in the spring. In late spring to midsummer the young scales, called crawlers, move about and then settle on leaves and twigs. These small (1/16 inch), soft-bodied young feed by sucking sap from the plant. Their legs atrophy and a hard crusty shell develops over the body. An uncontrolled infestation of scales may kill large branches after 2 or 3 seasons.

These pests, related to spiders, attack many garden and greenhouse plants. They cause damage by sucking sap from the undersides of leaves. As a result of their feeding, the plant''s green leaf pigment disappears, producing the stippled appearance. Spider mite webbing traps cast-off skins and debris, making the plant messy. Mites are active throughout the growing season but are favored by hot, dry weather (70°F and up). By midsummer, they have built up to tremendous numbers. Severely infested trees may produce small, poor-quality fruit.