Agriculture Reference
In-Depth Information
recent research has shown that the widespread disorder of 'Conference' pear
on quince rootstocks known as Parry's Disease is associated with pear decline
MLO (Davies et al. ,
). The symptoms are
similar to the mild or slow forms of pear decline. The leaves of affected trees
develop a premature red colour followed by early leaf fall. In the following
spring leaves remain pale and small, there is little or no shoot growth and no
fruit production. A necrotic line is often evident in the bark at the stock/scion
union. The majority of trees are affected for only one growing season but
the loss of early yield can be economically damaging. In some cases death or
severe stunting may occur. The cvs. 'Comice' and 'Concorde' are relatively
unaffected. Pear decline is spread by the pear psyllid Cacopsylla pyricola Foerster,
the most common insect pest in pear orchards.
,
; Davies and Eyre,
Bacterial disease
Fire blight
Fire blight, caused by Erwinia amylovora (Burrill) Winslow et al. , is the most
important bacterial disease of apples and pears. It has been reported from
) and is so
lethal and fast-spreading under suitable conditions that it has become a ma-
jor factor in determining where different cultivars of apple and pear can be
grown.
During the growing season blighted blossoms and shoots wilt and turn
dark brown or black. The scorched appearance of affected leaves, twigs and
branchesgivesthediseaseitsname.Youngshoottipsbendtoforma'shepherd's
crook'duetolossofturgorandcelldeath.Inhumidweatherinspringandsum-
mer bacterial slime may ooze from affected shoots, blossom trusses, branches
and fruits. The bacterium moves in the intercellular spaces of parenchymatous
tissues, spreading into spurs and along branches at up to
countries around the world (Bonn and van der Zwet,
mm per day in
susceptible hosts, forming cankers and sometimes affecting and rotting fruits.
The cortexof the outer bark of the affected branches is typically dark green
and water-soaked at first but later shrinks and turns brown. The inner bark
is typically moist and red where the disease is active and red-brown streaks
are often found in the sapwood (van der Zwet and Beer,
-
). Leaves and
fruits on affected branches characteristically remain attached after normal
leaf fall in the autumn. In autumn or after prolonged drought the cankers
cease extending and cracks may appear at the margins. In mild wet autumns
cankers may remain unsealed and these are the ones most likely to be active in
the following spring. In less vigorous pear trees and in apple trees the spread
of bacteria within the tissues is slower and the damage proportionately less
(Lelliot,
).
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