Agriculture Reference
In-Depth Information
computermodel,whichhasprovedreliableforapplesineasternUSA(Lightner
et al.
,
), are: open intact flowers, a wetting event comprising
.
mm of
◦
C
rain or heavy fog or dew, an accumulation of
degree-hours above
.
◦
C or above. Blossom blight
prior to this and a mean daily temperature of
.
◦
C have accumulated
is estimated to appear when
day-degrees above
.
after infection (Billing,
) found
similar but not identical temperature dependence of the infection process,
and a likely time between direct blossom infection and visible disease of
). In southeast England, Billing (
,
day-
◦
C for apple and
degrees above a mean temperature of
such day-degrees
forpearandhawthorn.Shealsoemphasizedtheimportanceoforchardfactors.
Fireblightriskassessmentsbasedsolelyonregionalweatheranditsexpectedef-
fects on the population growth of the pathogen can result in underestimation of
riskor,morecommonly,overestimationandunnecessaryspraying.Thishasled
to the development of approaches using orchard weather data and categorized
orchard-specific factors such as the local history of fire blight, cultivar suscep-
tibility, orchard sanitation, etc. (Shtienberg
et al.
,
). Summer
infections can occur on secondary blossom of pear, on storm-damaged young
shoots and hail-damaged young fruits. Infection-control measures can be tar-
geted accordingly. Infection can be reduced by controlling insects which con-
tact and feed on the ooze and carry the pathogen so acquired to natural
infection sites and which also, through creating wounds by feeding, initiate
infection sites and infect vegetative shoots (van der Zwet and Beer,
; Smith,
).
Streptomycinandothermedicalantibioticshavebeenwidelyusedtocontrol
bacterial multiplication at the time of, and just after, inoculation, but are
not permitted to be used in many countries because of the potential risk
to human health of developing antibiotic-resistant bacteria. They are only
locally systemic and are not effective when applied to unopened blossoms. A
streptomycinsprayonedaybeforeoronedayafterinoculationwith
E. amylovora
prevents infection (Gouk
et al.
,
). This short time-span of effectiveness may
resultintheneedformultipleapplications.However,
E. amylovora
hasdeveloped
resistance to streptomycin in many fruit-growing areas in the USA (Breth
et al.
,
). Copper compounds can be effective
but can cause excessive russeting (Momol
et al.
,
) and in Israel (Manulis
et al.
,
). Resistance-inducers,
natural plant extracts, bacterial powders and mineral powders are also being
evaluated for fire blight control (Momol and Saygili,
).
Fungal diseases
Fungal attack in the orchard requires frequent and expensive chemical spray-
ing for its control, and many apple and pear breeding programmes give high
priority to resistance to fungal pathogens.
