Agriculture Reference
In-Depth Information
In the following spring active overwintered cankers extend and ooze is
produced externally. This inoculum can be spread by rain or a variety of
insects, not including bees, to open flowers. Pollinating insects, bees and
flies, then spread inoculum rapidly between flowers on warm sunny days.
The pathogen multiplies rapidly on stigma surfaces at temperatures above
◦
C and is transported in water from rain, fog or dew to the region of
the nectaries where infection occurs. Blossom infection is particularly likely
on warm (
◦
C) sunny days when insect activity is high. In north-
western Europe the coincidence of these conditions with open blossom is
greatest with secondary blossoms, to which some pear cultivars are partic-
ularly prone, and with late-opening flowers on one-year-old wood on apple
trees.
Storms greatly increase the chances of infection, especially when flowers
or young shoots are damaged or leaves detached by hail or strong winds.
Exposure of young vascular tissue aids infection but the pathogen must invade
cortical tissues to produce typical symptoms.
The pathogen has only a very short life on the leaf surface. Persistence
within host tissues, overwintering in inconspicuous cankers, and the existence
of symptomless carriers are, however, important factors in its epidemiology.
Spread can result from the movement of infected plants from nurseries and,
at a more localized level, by transfer of bacteria on pruning tools.
Tissue susceptibility is generally much greater in young tissues and trees
than in older ones. It is generally greater in pears than in apples but there
are also large differences between cultivars. Tables
-
show relative
tissue susceptibility, as measured by response to artificial inoculation of shoots
in the field (Lespinasse and Aldwinkle,
.
and
.
).
The 'Delicious' group of apples are relatively resistant, 'Golden Deli-
cious' and its progeny and 'Fuji' moderately susceptible, 'Granny Smith' and
'Braeburn' susceptible, and 'Rome Beauty', 'Cox' and 'Idared' very suscep-
tible. Spur-types tend to be less susceptible than the corresponding non-spur
cultivars. The dwarfing apple rootstocks in Table
.
are all susceptible or
highly susceptible. 'MM.
' which appeared highly susceptible in inoculation
studies (Table
.
) has shown little natural infection in the USA (Lespinasse
and Aldwinkle,
). A rootstock breeding programme at Geneva, New York
has produced a number of dwarfing and semi-dwarfing apple rootstocks that
are resistant to fire blight. This resistance is derived from 'Beauty Crab',
M
). Some are resistant to
some other diseases and pests and have given promising initial results in terms
of effects on cropping (Robinson
et al.
,
.
×
floribunda
or 'Robusta
' (Robinson
et al.
,
).
Someofthemostimportant
P. communis
cultivars,'Bartlett','PasseCrassane',
'Packham's Triumph' and 'Doyenne du Comice' are very susceptible. Most
P. pyrifolia
(Nashi) cultivars are susceptible,
P. ussuriensis
and
P
.
×
bretschneideri
