Induced resistance for plant disease control - Disease Control in Crops - page 69

A. alternata , for ASM-treated fruit (Cao & Jiang, 2006). After 17 days incubation at
25 o C, disease incidence and severity on treated fruit were reduced by 2342% compared
with control fruit. Postharvest infi ltration of Yali pears with ASM, 315 days before inoc-
ulation with P. expansum , resulted in a reduction in disease incidence and severity (Cao
et al., 2005). In fi eld trials on 'Kosui' pear, four applications of ASM showed control
effi cacy against Japanese pear scab ( Venturia nashicola ) equal to that of the commercial
fungicide (Ishii et al. , 1999).
The use of activators in combination with antagonistic yeast offers a promising method
for controlling wound-invading pathogens in pears. Yu et al . (2007b) found that posthar-
vest application of SA, together with the biocontrol yeast Cryptococcus laurentii , signifi -
cantly reduced blue mould and grey mould ( B. cinerea ) in wound inoculated 'Shuijing'
pears . The combination treatment was more effective than the individual components
alone and provided greater disease control when applied 2 hours rather than 10 hours
before pathogen inoculation. In related studies, combinations of C. laurentii plus gib-
berellic acid (Yu et al. , 2006) and C. laurentii plus cytokinin (Zheng et al., 2007) have
been shown to control blue mould in 'Shuijing' pears when applied to wound-inoculated
fruit. Enzyme activity studies suggest that the 'enhanced resistance' associated with gib-
berellic acid and cytokinin may, in part, be linked to the delay of senescence in the pear
fruit.

4.2.1.5 Citrus

A range of commercial inducing agents demonstrated effi cacy against citrus scab ( Elsinoe
fawcettii ), melanose ( Diaporthe citri ) and Alternaria brown spot ( Alternaria alternata )
in different citrus fruit under glass (Agostini et al. , 2003). The most consistent prod-
ucts were ASM, Re-Zist (Stoller Enterprises Inc., USA) and Aliette (fosetyl-Al, Aventis
Crop Science, USA). In fi eld studies, 16 applications of ASM failed to control Alternaria
brown spot on 'Murcott' mandarins (Miles et al. , 2005). However, tank-mixing ASM with
azoxystrobin improved the effi cacy of the fungicide by over 50% and the level of disease
control was comparable with an industry standard programme. In contrast, three or four
ASM applications on 'Imperial' mandarin and 'Navel' orange reduced citrus black spot
( Guignardi citricarpa ) by ca 50% when compared with controls, but did not signifi cantly
improve fungicide effi cacy when tank-mixed (Miles et al. , 2004).
In glasshouse studies, foliar application of ASM and harpin, three to seven days before
inoculation, reduced the incidence of citrus bacterial spot ( Xanthomonas axonopodis pv .
citrumelo ) and citrus canker ( X. axonopodis pv. citri ) on citrumelo leaves (Graham &
Leite Jr., 2004). ASM reduced the incidence of both diseases by up to 80% and was
found to be more effective and more consistent than harpin. Based on this activity under
glass, ASM and harpin were then assessed in four orchard trials for their potential to
complement the activity of copper-based fungicides on sweet oranges. In these trials,
the inducing agents did not improve the effi cacy of the copper programmes against cit-
rus canker on fruit and leaves, and had no effect on fruit drop. For sweet oranges used
in juice processing, prevention of fruit drop is more important than reduction of fruit
blemishes. Postbloom fruit drop, caused by Colletotrichum acutatum, was reduced on
sweet orange and grapefruit following application of SA, ReZist and ASM in multiyear
screenhouse trials (Chen et al ., 2006; Liao et al ., 2006). Treatments were applied seven