Managing crop disease through cultural practices - Disease Control in Crops - page 17

In some recent work, Messiha et al. (2007) examined the incidence and severity of brown
rot in different soil types. They found that cow manure amendment signifi cantly reduced
disease incidence in organic Dutch sandy soils, although populations of the bacterial
pathogen Ralstonia solanacearum were not affected. In Egyptian sandy soils, how-
ever, population density of the bacterium was reduced, probably as a result of micro-
bial competition (Messiha et al., 2007). This work indicates that the mechanism of
disease suppression of soil-borne plant pathogens may vary greatly depending on the
soil type.

2.4.3.2 High nitrogen amendments

There are numerous reports of the effi cacy of a range of high nitrogen amendments in
controlling a variety of pathogens (see Bailey & Lazarovits, 2003). For example, meat
and bone meal and soy meal signifi cantly reduced the incidence of verticillium wilt, com-
mon scab and populations of plant parasitic nematodes in potato fi eld trials (Conn &
Lazarovits, 1999; Lazarovits et al., 1999). Amendments such as soy meal and blood meal
are degraded in the soil leading to the release of ammonia, which is toxic to many organ-
isms, including the resting structures of plant pathogens (Bailey & Lazarovits, 2003).
A low level of organic carbon in the soil was found to be critical for the accumulation
of ammonia, while high levels of soil organic matter prevents ammonia accumulation
(Tenuta & Lazarovits, 1999).

2.4.4 Irrigation

Although an adequate water supply is vital to crop production, irrigation can play a det-
rimental rather than a benefi cial role in managing plant diseases. For example, irrigation
water can spread pathogen propagules and under dry conditions can prevent desiccation
of such propagules, thereby effectively increasing the level of inoculum in soil. Watering
from overhead prolongs leaf wetness, thereby providing favourable conditions for ger-
mination and infection by fungal spores. Overhead watering also increases the risk of
splash-dispersal of spores, thus increasing pathogen spread. However, irrigation can be
used to reduce the level of pathogen inoculum. Thus, the activity of microbes that destroy
fungal sclerotia can be increased by alternate wetting and drying of the soil. Generally,
drip or trickle irrigation, which delivers water directly to the root zone at a rate insuffi -
cient to lead to pathogen spread, is least likely to encourage disease development.
In a study of the control of the downy mildew pathogen Peronospora sparsa on black-
berry, O'Neill et al. (2002) found that the use of sub-irrigated sand beds resulted in very
low disease incidence, whereas the use of overhead irrigation led to the disease develop-
ing in 97% of plants. In southern Israel, less frequent and reduced irrigation was shown to
lower the incidence of Monosporascus cannonballus on melon and to postpone plant col-
lapse, although yields were reduced (Pivonia et al., 2004). Gitaitis et al. (2004) examined
the effects of a number of treatments, including irrigation, on centre rot of onion caused
by the bacterium Pantoea ananatis. They found no effect of drip or overhead sprinkler
irrigation on the incidence or severity of centre rot (Gitaitis et al., 2004). Phytophthora
capsici is a serious soil-borne pathogen of pepper ( Capsicum annuum L.) and causes
signifi cant crop losses worldwide. The pathogen has been shown to spread under high