Agriculture Reference
In-Depth Information
bylimitationofNfertilizerapplicationandirrigationcanreducefireblightrisk.
These treatments may be difficult to reconcile with the need for rapid growth
of young trees to attain high cropping levels soon after planting. Control of
shoot vigour by the use of spur-type cultivars and the application of some plant
growth regulators can reduce susceptibility, but the use of dwarfing rootstocks
does not achieve this. Pruning and training techniques that minimize vigorous
regrowth may be useful.
The first step to reduce inoculum level is by plant quarantine to prevent
the importation and movement of infected material. The explosive spread
of the disease indicates that quarantine is less effective than previously but
at present some major apple and pear producing countries are still free of
fire blight and prevention of spread by close supervision of nurseries is still
very important. Removal of potential hold-over cankers in winter has a major
effectinreducingsubsequentdiseasedevelopment.Eradicationofearly-season
infections by pruning out and burning reduces further spread of inoculum as
well as checking damage to the trees. Disinfection of pruning tools to prevent
spread by these is important also. Over-tree sprinkling may lead to increased
inoculum production and spread so should be discontinued if fire blight is
present. Early-season copper sprays may reduce inoculum production.
Interference with the infection process is achieved by treatments to inhibit
bacterial multiplication on blossoms and to reduce populations of insects likely
to spread shoot blight. Fire blight is a sporadic disease and control of infection
needs to be based on risk assessment. This is because routine, rather than
targeted, monitoring of other potential host plants as well as apples and pears
is laborious and expensive and, especially, because the most effective control
treatment at present is with streptomycin, where this is permitted. As well as
being expensive, use of this when not needed can contribute to development of
antibiotic resistance. Mills (
◦
C, together with precipitation
or high relative humidity, as the threshold temperature during or shortly after
bloom for fire blight development in New York State. In California excellent
control of fire blight was achieved when streptomycin was applied only after
the mean of maximum and minimum temperatures exceeded a 'prediction
line' drawn from
) identified
◦
Con
◦
Con
.
March to
.
May (Thomson
et al.
,
). This gave appreciable cost savings compared with routine sprays
(Aldwinkle and Beer,
,
) found that the inci-
dence of epiphytic bacteria on pear flowers was related to the accumulated
number of degree-hours above
). Zoller and Sisevich (
◦
C, and a total of about
accumulated
degree-hours indicates the need for bactericide application just prior to the
next rainfall (van der Zwet
et al.
,
.
). This dependence on temperature is
largely explained by Billing's (
) finding that
E. amylovora
increases rapidly
◦
C, which is the approximate threshold for rapid growth.
The minimum conditions for blossom infection defined by the Maryblyt
TM
◦
Cto
from
.
