Organic residue amendmentinduced biological soil suppressiveness

A diversity of soil amendments has been explored for the potential to yield a disease suppressive soil. Composts have been the most commonly used substrate in this context and extensive literature exists concerning development and utilization of plant-based composts for control of soil-borne plant diseases (Hoitink and Boehm, 1999; van der Gaag et al., 2007). These organic substrates have demonstrated significant capacity to induce disease suppression in defined environment or growth media conditions (Mandelbaum and Hadar, 1990; Widmer et al., 1998). However, while it may be arguable, there has been minimal effective use of such materials in field-level production agriculture for this intended purpose. There is no doubt that composts are utilized in a multitude of plant production systems, but consistently and predictably realizing the intended goal, that being the development of soil suppressiveness, has been elusive due to an inability to predict effects on soil biological composition and function. This in part can be attributed to variability in consistency of compost activity, which is a function of multiple factors including substrate composition (Termorshuizen et al., 2006), storage conditions (van Rijn et al., 2007) and curing duration (Danon et al., 2007). Disease control achieved with any given compost may also be a consequence of host-mediated effects (van Rijn, 2007). In addition, there exists the potential that organic amendments including composts will yield not only increases in the microbial consortia responsible for potential disease suppression, but may also enhance disease development due to an increase in populations or activity of non-target or target plant pathogens (Cohen et al., 2005; Termorshuizen et al., 2006; Mazzola et al., 2009).

That being said, there are significant opportunities to utilize more clearly defined bio-based products to enhance specific processes including soilborne disease suppression. By 'clearly defined', reference is being made to the consistency of product composition which will enable reproducibil-ity of function, and a capacity to determine functional mechanism(s) involved in such processes. Certain of these amendments, such as fish emulsion or bone meal, operate primarily, though perhaps not exclusively, through chemical mechanisms (Tenuta and Lazarovits, 2004; Abbasi, et al., 2009). However, there are other examples in which the use of residues from specific sources, such as an individual plant species, act to selectively modulate the resident biology in a manner that yields a suppressive soil. Residues from plants belonging to the family Brassicaceae have been studied extensively for their potential to yield suppression of various plant pests including pathogens, insects and weeds (Brown and Morra, 1997). Active interest in these plant residues as a soil amendment emanated from the fact that members of this plant family produce glucosinolates which, upon hydrolysis, yield several biologically active compounds, including isothiocyanates. Chemical mechanisms have long been viewed as the dominant mode leading to pest suppression as a result of brassicaceous amendments (Matthiessen and Kirkegaard, 2006). However, several studies have revealed that other functional mechanisms operate to yield pest control. In some instances soilborne disease and weed suppression obtained in response to specific brassicaceous amendments is not chemically mediated but rather functions at least in part through the resident soil biology (Mazzola et al., 2001; Hoagland et al., 2008).

Brassicaceous seed meal amendments effectively control a number of soilborne diseases (Smolinska et al., 1997; Mazzola et al., 2001; Chung et al., 2002). The operative mechanism(s) differs according to the target pathogen and seed meal plant source (Mazzola et al., 2007a), and in certain instances disease control requires specific changes in microbial community composition that yield soil suppressiveness (Cohen and Mazzola, 2006; Mazzola et al., 2007a). Another level of complexity is realized when the temporal nature of disease suppression is examined and changes in functional mechanism(s) are revealed. This phenomenon has been most apparent in seed meal-induced control of Rhizoctonia root rot of apple where several lines of support implicated the need for a functional microbial community to attain seed meal-induced disease suppression. Such evidence includes the fact that Brassica napus seed meal (BnSM) provided disease control irrespective of glucosinolate content; the capacity of BnSM amendment to suppress Rhizoctonia root rot was abolished if soil was pasteurized prior to introduction of the pathogen, and only seed meals such as BnSM, but not soybean meal, which significantly elevated densities of resident Streptomyces spp. provided effective disease suppression (Mazzola et al., 2001; Cohen et al., 2005). Introduction of individual Streptomyces sp. isolates from seed meal amended soils provided a level of disease control that was equivalent to BnSM amendment, and the majority of Streptomyces isolates provided control of Rhizoctonia solani through the induction of host defence responses (Cohen and Mazzola, 2006).

In Brassica juncea seed meal (BjSM) amended soil, the temporal dynamics in elevation of resident Streptomyces populations corresponded with the induction of soil suppressiveness, providing further support for this phenomenon as a functional mechanism. Disease control in response to BjSM amendment was attained even in pasteurized soil, but only if the amendment was made at the time of pathogen infestation. When addition of R. solani inoculum was delayed until 24 h post-seed meal amendment, pathogen suppression in native (Fig. 11.1) or pasteurized soil was not observed. The pattern of observed disease suppression corresponded with the pattern of allyl isothiocyanate (AITC) generation, a process that was completed within 24 h of seed meal amendment (Mazzola et al., 2007a). Soil suppressiveness to Rhizoctonia root rot was restored to native soils when incubated for a period of 4 weeks, and the re-establishment of disease suppression was associated with the elevation of resident

Streptomyces spp. populations (Mazzola et al., 2007a). Seed meal-induced soil suppres-siveness towards Rhizoctonia root rot of apple was also obtained in field trials through the use of various brassicaceous seed meals including that of B. napus (Mazzola and Mullinix, 2005; Fig. 11.2).

Accumulating data demonstrate that soil biology may also contribute to seed meal-

Rhizoctonia solani root infection (%) Streptomyces population (log cfu/g soil)

Fig. 11.1. Duration of Brassica juncea seed meal (BjSM) incubation period in soil prior to pathogen infestation affects native Streptomyces densities, level of Rhizoctonia solani AG-5 infection of apple seedling roots and mode of disease suppression induced by the seed meal amendment. Relative to a non-treated control, disease was suppressed when the pathogen was introduced into soil at the time of seed meal amendment (BjSM 0h) or at 4 weeks after amendment (BjSM 4 wk) but not when the pathogen was introduced at 24 h after application of the amendment (Bj SM 24 h) (left panel). Populations (colony forming units, cfu) of resident Streptomyces spp. in the corresponding soils, and their proliferation at 4 weeks post-seed meal amendment are evident (right panel). Disease suppression attained when the pathogen was introduced at the time of seed meal amendment (0 h) was attributed to the generation of allyl isothiocyanate, but this chemical was evacuated from the system by 24 h post-seed meal amendment (Mazzola et al., 2007a). Disease suppression attained when the pathogen was introduced at 4 weeks post-seed meal amendment was attributed to the elevated populations and activity of resident Streptomyces.

Control

BnSM

Fig. 11.2. Effect of soil treatment on Rhizoctonia solani infection of Gala/M26 and Golden Delicious/M7 roots in two apple orchards in Washington state: CV (black bars) and WVC (grey bars), respectively (Mazzola and Mullinix, 2005). Soil fumigation and seed meal amendment significantly reduced root infection relative to the non-treated control at both orchard sites (treatments with different letters differ significantly P < 0.05), and there was no significant difference between seed meal and fumigation treatments. BnSM, Brassica napus seed meal soil amendment; 1,3-D:C17, 1,3-dichloropropene-chloropicrin soil fumigation.

Control

BnSM

Fig. 11.2. Effect of soil treatment on Rhizoctonia solani infection of Gala/M26 and Golden Delicious/M7 roots in two apple orchards in Washington state: CV (black bars) and WVC (grey bars), respectively (Mazzola and Mullinix, 2005). Soil fumigation and seed meal amendment significantly reduced root infection relative to the non-treated control at both orchard sites (treatments with different letters differ significantly P < 0.05), and there was no significant difference between seed meal and fumigation treatments. BnSM, Brassica napus seed meal soil amendment; 1,3-D:C17, 1,3-dichloropropene-chloropicrin soil fumigation.

induced suppression of root disease incited by Pythium spp. BjSM effectively controls Pythium root rot through the release of AITC (Mazzola et al., 2009). However, other mechanisms of disease suppression must function in a time-dependent manner as at least partial disease control was attained in response to seed meal amendment even when inoculum of Pythium irregulare was introduced into soils 16 weeks post-seed meal amendment (Fig. 11.3.). Analysis of fungal communities using a taxonomic macroarray indicated that Trichoderma spp. were preferentially dominant in amended soils suppressive to Pythium whereas the fungal community was more evenly distributed in soils conducive to Pythium spp. (Izzo and Mazzola, 2007). It is plausible that these fungi, which possess a well-known capacity to provide biological control of Pythium spp. (Howell, 1982; Wolffhechel and Jensen, 1992), contributed to the observed disease suppression.

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