Symptoms and Signs
Poor shoot and spur growth, dieback of shoots, premature reddening and upper rolling of leaves, reduced leaf and fruit size, and premature leaf drop are symptoms associated with pear decline.
Sudden tree collapse can result from hypersensitive tissue damage at the graft union on highly susceptible Asian rootstocks such as Pyrus serotina or P. ussuriensis. Because of this rapid tree collapse, commercial pear varieties are no longer grafted on Asian rootstocks where pear psylla, which vectors the disease, is present.
The more typical symptoms of pear decline on trees grafted to tolerant rootstocks is a very slow decline when trees are not receiving adequate water and nutrition. In addition, trees on tolerant rootstocks may show mild to moderate symptoms if very high psylla numbers occur, especially during the early growing season. This is because the pathogen in the upper canopy is crushed in the phloem during dormancy and then slowly recolonizes the upper canopy in spring as new phloem is generated. If pear psylla numbers are high in late spring or early summer, they can reintroduce the pear decline phytoplasma into the upper canopy and speed up the progression of pear decline symptoms.
Comments on the Disease
The phytoplasma organism that causes pear decline is transmitted by pear psylla when feeding on pear foliage. An infected pear tree is the only known host from which uninfected pear psylla can acquire the pear decline phytoplasma. The expression of the disease depends on rootstock susceptibility, tree vigor, and psylla numbers. The organism multiplies in both pear trees and in pear psylla. Good horticultural practices and pear psylla control can help decrease the impact of pear decline when pear trees are grafted onto tolerant rootstocks.
Commercial pear rootstocks currently available, with the exception of Pyrus calleryana, are essentially tolerant to pear decline and produce excellent crops in spite of recurring pear psylla populations and exposure to pear decline. Tolerant rootstocks include Bartlett seedling, Winter Nelis, Old Home x Farmingdale, and Pyrus betulaefolia. To keep the disease in remission on susceptible rootstocks:
- Control pear psylla.
- Maintain trees in good vigor by reducing stress caused by inadequate irrigation, nutrient deficiency, weed competition, lack of pruning, and pest damage.
There is no known biological control of the pear decline phytoplasma organism. Indirectly, biological control of pear psylla can reduce disease severity.