Induced resistance for plant disease control - Disease Control in Crops - page 67

hydroxide, disease control was equal to that obtained with weekly applications of copper
hydroxide plus maneb and there was no loss in yield. The effect of ASM on yield was
cultivar specifi c and correlated with application frequency.

4.2.1.3 Apple

Field application of ASM has been shown to protect the apple cultivars 'Golden Delicious'
(Brisset et al., 2000), 'Jonathan' and 'Fuji' (Maxson-Stein et al. , 2002) from infection
with Erwinia amylovora, the causal agent of fi re blight. ASM application in each of
these studies was initiated approximately one week pre-bloom with successive sprays at
7- or 14-day intervals. On 'Golden Delicious', blossom blight incidence was reduced
by approximately 50% when ASM was applied twice before inoculation or twice before
and twice after inoculation. On 'Jonathan', ASM signifi cantly reduced the incidence of
natural fi re blight strikes associated with severe weather conditions at the end of petal
fall. ASM was more effective when applied at weekly intervals than when applied fort-
nightly. Furthermore, there was a positive correlation between ASM concentration and the
reduced spread of fi re blight cankers on inoculated shoots of 'Fuji' apple. The combined
use of ASM and streptomycin was more effective at limiting canker development than
either treatment alone (Maxson-Stein et al. , 2002). It was suggested that the integrated
use of ASM with streptomycin could reduce the need for streptomycin in the post bloom
period and so reduce the risk of the development of streptomycin-resistant strains in the
fi eld.
Prohexadione-calcium (Pro-Ca, Regalis ® ; BASF, Limburgerhof, Germany), a growth
regulator that is used to reduce shoot growth in pome fruit, has also been shown to activate
host defences including PR-proteins (Bini et al., 2008) and phenylpropanoid metabolism
(Roemmelt et al., 2003). Application of Pro-Ca to container-grown apple trees 'Idared'
and 'Freedom', two weeks before shoot inoculation with E. amylovora, resulted in a sig-
nifi cant reduction in the development of fi re blight lesions (Buban et al ., 2004). Blossoms
represent the primary site of infection for the fi re blight pathogen. In fi eld experiments,
Pro-Ca and the growth regulatory compound, acylcyclohexanedione (TrixE, Moddus ® ,
Syngenta), reduced the severity of blossom blight infections on 'Golden Delicious' and
'Pink Lady' when applied to open fl owers 14 days before pathogen inoculation (Spinelli
et al., 2007). In the same study, experiments on 'Golden Delicious' and 'Abbe Fetel' pear
plantlets demonstrated that the growth regulators treatments the migration of the patho-
gen in host tissues in a similar manner, and to a similar extent, to that observed following
ASM application. Whilst the above results are encouraging, the use of different activators
have not exhibited suffi cient effi cacy against fi re blight to merit consideration as complete
replacements for bactericides. However, they do provide viable alternatives to enable a
reduction in the frequency and/or application concentrations of bactericides and fungi-
cides that are currently used for pome fruit disease management.
Pro-Ca has also demonstrated effi cacy against apple scab caused by Venturia inaequalis.
Treatment of 'Golden Delicious' seedlings with Pro-Ca, 10 days before inoculation with
V. inaequalis, afforded a signifi cant reduction in apple scab symptoms on leaves when
compared with untreated seedlings (Bini et al., 2008). Furthermore, the expression of
PR genes, in response to pathogen inoculation, was greater in seedlings treated with
Pro-Ca than in non-treated seedlings. The authors offered a cautionary note regarding the