Managing crop disease through cultural practices - Disease Control in Crops - page 15

Colletotrichum gloeosporiodes or C. acutatum ) in apples (Biggs, 1999) and to decrease
post-harvest disease development on strawberry (Cheour et al., 1990), while treatment of
tomato with calcium carbonate reduced fusarium crown rot disease (Woltz et al., 1992).
In contrast, Nam et al. (2006) could fi nd no effect of calcium on anthracnose on strawberry.
Because calcium increases resistance of plant cell membranes and cell walls to microbial
enzymes, increasing calcium concentrations in storage organs could lead to enhanced
resistance to pathogens (Conway & Sams, 1984; Biggs & Peterson, 1990). However,
the form in which the calcium is applied can infl uence the mechanism by which cal-
cium affects disease. For example, the addition of lime can affect disease by altering pH,
while calcium salts (e.g. propionate) can be directly inhibitory to pathogens (Rahman &
Punja, 2007). Making general recommendations for the use of calcium in plant disease
control would be unwise due to the range of crops and pathogens affected by calcium
application. Instead, the appropriate amount and form of calcium to be applied needs to
be determined for individual crop-pathogen interactions. The dwindling availability of
fungicides, together with increasing public concern for the environment means that the
use of calcium to control plant disease, especially post-harvest, is attracting increased
attention.

2.4.2.5 Silicon

Although the effects of silicon in reducing disease severity have been known since 1940
(Wagner, 1940), it was not until the 1980s that more detailed work was carried out in
this area. In this work, cucumbers grown in nutrient solutions supplemented with silicon
were found to have signifi cantly less powdery mildew infection than plants not receiving
silicon supplementation (Miyake & Takahashi, 1983; Adatia & Besford, 1986). Indeed,
silicon has been shown to suppress both foliar and soil-borne pathogens in curcubits
(Belanger et al., 1995) and to reduce susceptibility of rice to various pathogens (Datnoff
et al., 2007b). Wheat grown in soil amended with silicon showed reduced infection by
several pathogens, including B. graminis f. sp. tritici, S. tritici and Oculimacula yallundae
(Rodgers-Gray & Shaw, 2000, 2004).
It has been suggested that the effects of silicon in providing disease control are due
to the creation of a mechanical barrier to penetration (Kim et al., 2002). However, this
has been disputed by studies which could fi nd no evidence for the creation of a physical
barrier following silicon treatment in wheat inoculated with powdery mildew and bitter
gourd and tomato inoculated with Pythium aphanidermatum (Samuels et al., 1991; Heine
et al., 2007). Rather, several studies have suggested that silicon activates defences in
plants. For example, in wheat inoculated with B. graminis f. sp. tritici, epidermal cells of
silicon-treated plants were shown to react to attempted infection with specifi c defences,
including papilla formation and callose production (Belanger et al., 2003). In the rice-
M. grisea pathosystem, silicon-mediated resistance was found to be associated with accu-
mulation of antimicrobial compounds at infection sites, including diterpenoid phytoalex-
ins (Rodrigues et al., 2004). In fact, phytoalexin accumulation occurs in silicon-mediated
resistance in both dicots and monocots and since phytoalexins are highly specifi c to plant
species, it has been suggested that silicon might be acting on mechanisms shared by all
plant species, for example, those resulting in activation of plant stress genes (Fauteux
et al., 2005).