Biofumigation for plant disease control – from the fundamentals to the farming system - Disease Control in Crops - page 186

appears to result in further spikes in ITC release. The required plant-back time can be
longer where large amounts of seed meals are applied, in some cases effects on apple
replants have still been observed 12 weeks after incorporation (Mazzola et al ., 2001).

(g) Target lighter-textured soils with low organic matter
In common with the synthetic ITC-liberating pesticide metham sodium, the characteristics
of ITCs mean that disease control is likely to be more effective on lighter-textured soils
with lower organic matter. These soils will have lower ITC sorption due to reduced organic
matter, less microbial degradation due to lower microbial activity, greater mobility of ITC
through the soil matrix in both the liquid and vapor phases due to larger less tortuous
pores, and less protection of microorganisms occluded within soil micro-aggregates.
Suppression which is related to non-GSL mechanisms may be more important in soils
with higher organic matter levels by stimulation of the resident microbial populations.

(h) Adjust strategies to incorporate appropriate IPM approaches
Whether disease suppression using Brassica amendments is principally related to ITCs or
to other mechanisms, it is unlikely as a single alternative strategy to match the levels of
disease control achieved by the synthetic fumigants and pesticides which we are seeking
to replace (Lazarovits, 2001; Matthiessen & Kirkegaard, 2006). No examples of this level
of control have been reported. As a consequence, opportunities to integrate Brassica
amendments with other components of an IPM approach should be sought. These include
wider rotations, crop hygiene, resistant/tolerant varieties, solarization, bio-control agents
and augmented pesticide applications. Many of these approaches have been applied in
combination with biofumigation to improve disease control (e.g. solarization Chapter 10).
Some examples of this integration are discussed further in the following case studies.

9.8.2 Case studies

In order to highlight some of the practical issues related to developing biofumiga-
tion as a component of disease management two case studies of specifi c applications
are considered. A more detailed compilation and review of 18 fi eld studies directed at
Brassica -based biofumigation for pest and disease control is provided in Matthiessen &
Kirkegaard (2006).

9.8.2.1 Brassica green manures in potato production systems

A wide range of potato pests and diseases have been targets for control using
Brassicaceous amendments with varying success (Table 9.1). Studies on root-knot nema-
todes in the Pacifi c Northwest of USA demonstrated signifi cant suppression by rapeseed
( B. napus ) compared with wheat green manures and provided evidence for the role of
GSLs (Mojtahedi et al ., 1993). Despite the relatively high levels of suppression achieved
compared to non- Brassica controls (Table 9.1), infection and damage to tubers remained
at levels which required augmenting with the chemical nematicide ethoprop to achieve
levels of control similar to that using 1,3-dichloropropene. Rapeseed green manures in
the same area were also effective in reducing weed populations from 59% to 90% com-
pared to sudan grass or fallow and increased the yield of subsequent potatoes by 17-25%
(Boydston & Hang, 1995). The authors concede that even under the relatively low levels