How to Manage Pests

Pests in Gardens and Landscapes

Seasonal development and life cycle—Pear decline

Pear trees are infected with decline when mycoplasma-infected psylla feed on their foliage. Trees planted on tolerant rootstocks do not decline quickly, but may experience leaf damage, resulting in overall loss of tree vigor. The mycoplasma in pear leaves causes them to curl and turn red prematurely by plugging the food conducting cells in the leaf veins and petioles and preventing movement of food products out of the leaves. First symptoms of leaf curling usually appear in early July and intensify up to leaf fall. Leaves begin to turn red or purple as early as mid-August. Leaves that fall prematurely carry with them an abnormally high amount of sugars and nitrogen, which normally would have been stored in limbs and roots over the dormant season. This loss reduces the tree's vigor the following season.

Mycoplasmas are slowly eliminated by the host tree during the dormant season, and trees recover if not reinfected by psylla. Some mycoplasmas overwinter within the roots of infected trees growing on tolerant rootstocks, but the main source of a continued infection is renewed feeding the following season by infected pear psylla.

Red foliage is characteristic of pear decline
Red foliage is characteristic of pear decline

Statewide IPM Program, Agriculture and Natural Resources, University of California
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