OM; composted wood products, chitin/chitosan and biochar, were selected for investigation of their potential to suppress Rhizoctonia diseases of potato.
5.2.2.1 Composted bark products
Based on the recommendations of Bonanomi et al. (2007; 2010) summarised above,
composted OMs were targeted. When Scheuerell et al. (2005) tested 36 different composts for
control of soilborne phytopathogens in a containerised (peat/perlite) system, six were found to suppress Rhizoctonia damping-off of cabbage. Of these, the most consistent were hemlock bark, dairy fir-bark compost, mushroom compost and nursery regrind compost. However, suppression was not related to any single physical, chemical or biological factor, making it difficult to predict which composts will suppress Rhizoctonia diseases. Of the variety of OMs
reported to suppress R. solani reviewed by Litterick et al. (2004), the wood products (wood chips and tree barks) stand out as often inducing disease suppression. In a review by Noble and Coventry (2005), one of the largest reductions of Rhizoctonia disease (81%) was with a hardwood bark. Composted barks, therefore, appear to be strong candidates for suppressing
Rhizoctonia diseases. Supporting this is the study by Krause et al. (2001), which
demonstrated that composted pine bark/peat potting medium was more frequently suppressive to Rhizoctonia diseases of radish and poinsettia, compared with sphagnum peat media of different decomposition stages. An explanation of why composted wood products may offer more consistent disease suppression than fresh material was summarised in a review by Hoitink and Boehm (1999). They suggested that low cellulose levels in composted bark
encourage antagonism towards R. solani, as lignocellulosic substances are colonised by
Trichoderma spp., and while fresh hardwood bark stimulates populations both of R. solani and Trichoderma spp. parasitic to them, damping-off severity is still increased. However,
composted bark controls the pathogen even though the populations of parasitic Trichoderma
spp. are far fewer than in fresh bark. If cellulose is combined with composted bark it returns
to being conducive to Rhizoctonia diseases, even though the Trichoderma spp. populations
rise considerably. Therefore, low cellulose levels promote fungal interactions, where the R.
solani is suppressed. These authors also suggested that it is likely that production of chitin
degrading enzymes decreases when the substrate cellulose, preferred by Trichoderma spp., is
available. Van Beneden et al. (2010) demonstrated that the addition of lignin (1% w/w)
reduced R. solani sclerotial viability in one soil type but not another, which was linked to
differences in the response of the soil microbial communities to the amendment. In New
Zealand, Monterey Pine (Pinus radiata D.Don) is grown extensively for the building and
paper industries, so by-products such as bark are widely available, and likely to be economic sources of OM amendments.
5.2.2.2 Chitin and chitosan
Chitin is another OM with potential as a commercial soil amendment. Chitin is the second most abundant bio-polymer in the world, and a massive quantity of chitin-rich waste is regularly discarded by the seafood industry. Because it is a major component of fungal cell walls, it also has potential for increasing the activity of chitin-degrading communities. A
study by Sneh and Henis (1971) found that while R. solani is able to colonise chitin particles,
it was displaced from them within 30 days in non-sterile soil, suggesting that it is not a strong competitor for chitin. There is evidence from a variety of sources that chitin soil amendments reduced Rhizoctonia diseases. Huber and Sumner (1996) summarised that increasing the incubation time after chitin amendments to soil decreases the saprophytic growth of
Rhizoctonia. This suppression was also greater for the chitin amendment compared with a variety of plant residues, which the authors suggested was due to the accumulation of anti- fungal substances as a result of chitin degradation. There have been several more recent studies in which chitin amendments have reduced Rhizoctonia disease severity. Glasshouse
assays (Rajkumar et al., 2008) have shown that the addition of chitin increased the
populations and disease suppressive capacities of two fluorescent Pseudomonads antagonistic
to an R. solani isolate causing damping-off of pepper. The addition of chitin to the biocontol
treatments of two Bacillus spp. isolates and one Trichoderma harzianum isolate improved the
suppression of R. solani pepper root-rot (Sid Ahmed et al., 2003). Sultana et al. (2000)
demonstrated that amending soil with crustacean chitin reduced R. solani infection on
chickpea and sunflowers. Ellis et al. (1998) reported that a fine dust formulation of crab waste
(270 to 1350 g chitin per m2 equivalent) improved sugarbeet seedling emergence in R. solani
infested soil. On potato plants, Davies et al. (2002) found that the addition of chitin at
‘realistic commercial rates’ reduced potato black scurf severity but not length of stem canker
lesions. Lewis et al. (1996) successfully used chitin as a nutrient base in alginate prills of two
biocontrol strains of Trichoderma spp. and one Gliocladium virens strain, and improved
biocontrol of R. solani (AG 4) damping-off on zinnia. In addition, chitin was one of the best
nutrient bases for Trichoderma spp. to reduce the survival of R. solani in infested beet seed.
In addition to evidence of effect of chitin as an amendment, Sadeghi et al. (2006)
demonstrated that the biocontrol potential of some R. solani antagonists (Streptomyces spp.)
was linked, in part, to their chitinase producing capacity. Incubation of chitin in soil has been
shown to disrupt both pathogenic and saprophytic activity of R. solani and was linked to
increases in the population of actinomycetes (Henis et al., 1967). Also, transgenic expression
of fungal chitinolytic genes can confer resistance of plants to fungal pathogens, including R.
solani (Kumar et al., 2009). Introduction of a novel chitinase gene into a fluorescent
pseudomonad enhanced its levels of control against rice sheath blight and cotton damping-off
caused by R. solani (Xu et al., 2004).
Chitosan, the de-acetylated form of chitin, is produced as a bio-fertiliser in Asia (El Hadrami et al., 2010), and is more water soluble than chitin. Chitosan, like chitin, demonstrates antimicrobial (including suppression of several phytopathogenic fungi) and plant defence
promoting capabilities. Mazaro et al. (2009), investigating chitosan as an elicitor of plant
defence genes, found it reduced Rhizoctonia damping-off of beet seedlings. Chitosan can also chelate minerals and metals, reducing their availability to pathogenic fungi, but has also been
growth agar reduced R. solani growth by about 30%, but the fungus would not grow on Water
Agar amended with chitosan. However, the growth of two mycoparasitic Trichoderma spp.
was strongly inhibited by chitosan amendments in agar media. Applications of chitosan to potato tubers have also been reported to decrease black scurf incidence (Kurzawinska and Mazur, 2008). If chitosan is able to reduce Rhizoctonia diseases of potato, it could potentially be through direct mycotoxic activity, stimulation of acquired resistance in host plants or possibly through mediating other physical processes in the soil, perhaps including changes in the microbial community. If use of chitosan becomes more popular globally, investigating the mechanisms of action of this compound would be of interest.
5.2.2.3 Biochar
Soil amendments with biochar (charcoal produced from biomass pyrolysis) have been the focus of recent research, as this material can act as a carbon sink as well as having positive effects on soil quality, fertility and leachates (McHenry, 2011). One biochar has been shown to reduce the leaching of at least one pesticide from soils, which can protect the surrounding environment, but it also slowed pesticide biodegradation and could reduce pesticide efficacy
(Jones et al., 2011). Biochar has also been shown to induce plant defences and protect against
foliar pathogens when used as a soil amendment (Elad et al., 2010). Graber et al. (2010)
found that wood-derived biochar amendments enhanced pepper plant development, possibly due to promotion of plant health promoting rhizosphere communities. There have been several reports that biochars/charcoals, including wood charcoal, can increase beneficial plant-microbe interactions such as mycorrhizal partnerships, although these materials often had to be ‘charged’ with fertilizers, and could have a substantial impact on soil microflora (Ogawa and Okimori, 2010). Because biochars are porous and can retain soluble nutrients, ‘charging’ them with fertilisers means that factors such as the C:N ratio can be easily adjusted for studies. For example, Huber and Sumner (1996) summarised that the addition of N
fertilisers to amendments with high C:N ratios and suppressive to Rhizoctonia spp.
counteracted the suppressive effect. They also noted that nitrate sources of N tended to reduce disease severity, while ammonium sources increased severity. This could offer an interesting area of study should biochars demonstrate any suppression of Rhizoctonia diseases of potato. While there is little published work on the effects of biochar on suppression of soilborne phytopathogens, the influence they can have on soil structure, plant health and soil
communities means they have potential to yield positive results in this field. If their use in agriculture grows, then understanding their influence on plant-pathogen interactions will be important.