Pear decline—phytoplasma
Pear decline is caused by a phytoplasma, a microscopic bacterialike organism. Phytoplasmas require a living host to survive and reproduce. To survive, the pear decline phytoplasma requires either a pear tree or the pear psylla (Cacopsylla pyricola), an aphidlike insect (psyllid) that vectors the phytoplasma when it feeds on pear.
Identification
Symptoms of pear decline generally appear the year after the growing season when the tree became infected. In spring, infected trees leaf out more slowly than healthy trees and the leaves are typically pale green to yellowish. In late summer or early fall, leaves of infected trees thicken, curl downward, and change color prematurely. Leaves of Bartlett and Bosc pears turn red to purple and show the most pronounced leaf curl. Leaves of the Comice pear variety turn bronze and develop less leaf curl. Infected Asian pear varieties (e.g., Chinese pear) rarely develop curled leaves.
During hot weather, infected trees on Asian rootstock may develop quick decline; foliage wilts rapidly and affected trees die within a few days. A brown line caused by death of phloem cells develops on the outside of the wood at the graft union. This dead phloem can be seen by carefully peeling away bark. The dead phloem cells block nutrients from entering roots, so root growth is reduced and fibrous roots die of starvation.
Trees on other rootstocks rarely show brown, dead phloem cells at the graft union. Non-Asian rootstocks may instead develop ridges on the bark and wood at the graft union.
Life cycle
Pear trees become infected with decline when a pear psylla infected with the phytoplasma feeds on their foliage. Trees planted on tolerant (non-Asian) rootstocks do not develop quick decline (rapid death) but may have an overall loss of tree vigor and decline slowly over several years.
The presence of the decline phytoplasma in pear leaves causes leaves to curl and turn color sooner than they normally would in the fall. This premature color change is due to plugging of phloem cells in the leaf veins and petioles, which prevents movement of photosynthates out of the leaves. The first symptoms of leaf curling usually appear in early July and intensify up to leaf fall. Leaves begin to turn bronze, red, or purple as early as mid-August. Leaves that drop prematurely carry with them an abnormally high amount of sugars and nitrogen, which normally would have been moved to limbs and roots and stored there during the dormant season. The premature leaf drop reduces the tree's vigor the following growing season.
The pear decline phytoplasma is slowly eliminated by the host tree during the dormant season. Infected trees recover if not reinfected by psylla feeding. Some phytoplasma overwinters in roots of infected trees growing on tolerant rootstocks, but the main source of a continued infection is feeding by infected pear psylla.
Damage
Pear decline can rapidly kill pear planted on Asian rootstocks: Pyrus pyrifolia (=P. serotina) and P. ussuriensis. When infected pear grow on other rootstocks, if leaf symptoms are severe and persist year after year, tree vigor is reduced. The decline symptoms include leaves that are pale and undersized, reduced shoot growth, and fruit that is smaller and less abundant than normal.
In the past, pear decline widely killed pear trees that were planted on Asian rootstocks. Since the mid-1980s the incidence and severity of pear decline has been greatly reduced by planting trees on other rootstocks more tolerant of the disease, more effectively controlling pear psylla, and using good cultural practices.
Solutions
Consider planting pear grown on rootstock tolerant of pear decline. If trees are performing poorly, consider replacing them with pear on decline-resistant rootstocks. Tolerant rootstocks include Bartlett seedling, Old Home X Farmingdale rooted cutting, Pyrus betulaefolia, and Winter Nelis seedling.
Reducing the abundance of pear psylla will reduce transmission of the phytoplasma. For example, trees can be sprayed with horticultural oil during the dormant season. If the psyllid was abundant the previous growing season, a second oil application just before bloom can be made.
Trees infected with pear decline may benefit from increased nitrogen applications and frequent, light irrigations that maintain optimal moisture in the root zone. Trees may also recover more quickly if the fruit load is thinned. Heavy winter pruning will help to restore tree vigor. Also control weeds near pear trees to reduce competition for soil moisture and nutrients.
Adapted from Integrated Pest Management for Apples and Pears and Pest Management Guidelines: Pears, University of California Statewide Integrated Pest Management Program (UC IPM). |
