Can Fresh Wood Chip Amendments Suppress Root Pathogens?

by Jim Downer

Plant pathogens, especially root pathogens, survive in container media and can be some of the most destructive nursery pests.  Root pathogens are successful because they are hidden; symptoms don’t show on foliar parts until significant percentages of roots are lost (fig. 1).  Often more than 50% of absorptive roots are dead before wilt, die back, color changes, or leaf drop are apparent. Early symptoms of root disease such as growth cessation or stunting are usually not observed.  Because root-compromised plants use less water due to loss of function, media become and stay wet or saturated.  This increases media breakdown and the development of anaerobic chemistries in the bottom of the container. These conditions prevent root regeneration, dooming plants to early failure. 

Fig. 1.  Incipient root rot does not show on foliage but roots may be discolored. Photo: J. Downer.

In a series of articles in Greenhouse Grower, Brian Jackson and other researchers at North Carolina State University (NC State) detail the benefits of using wood fiber and other wood components in container media, which include increased root growth in floricultural crops (Jackson 2016; Owen and Jackson 2014, Owen and others 2014). Most of the NC State research has focused on small pine wood chips — fresh not composted — engineered as an aggregate for use in peat-based media. While non-composted redwood and fir bark may break down in container media and eventually decrease porosity, Jackson’s research showed that amending peat-based media with the specific fresh pine chips studied at NC State increased porosity, making it a good substitute for perlite.

In addition to vigorous root growth and increased media porosity, fresh wood chip amendments may suppress some soilborne diseases. Jackson’s popular articles do not mention this potential benefit, but there is some evidence that supports this assumption presented in another NC State paper (Kaderabek and others 2013). In this study, disease severity of Pythium and Rhizoctonia was often less in peat-based media amended with fresh wood chips than in peat-based media alone; the media without fresh wood chips also had greater disease earlier in the cropping cycle, which could indicate that the wood chips somehow suppressed disease. The increased porosity in the media with fresh wood chips could explain the observed reduction in disease severity. For example, Filmer and others (1986) showed that increases in porosity of container media lead to control of Phytophthora root rot of toyon.  I like to call managing diseases by modifying media components substrate-mediated disease control (SMDC). Alternatively, since fresh pine bark are known to contain inhibitors that may suppress some soilborne diseases (Hoitink and Fahy 1986), it’s possible that the fresh wood chips used in the study also contained inhibitors. Moreover, fresh wood chip amendments could potentially suppress soilborne diseases by enhancing biological control of root pathogens.   

Soilborne plant pathogens can be parasitized by other non-pathogenic or saprophytic fungi resulting in disease suppression (Hoitink and Fahy 1986). A typical example is the hyperparasitism of Pythium by Trichoderma. The work by Gravel and others (2009), in which T. harzianum (applied as the product Rootshield) successfully limited geranium root disease in peat-based media, is typical.  However biological controls, while effective experimentally, often fail in sustained production and as plants grow to larger sizes. 

Trichoderma is a wood-inhabiting fungus and preferentially grows and sporulates on fresh wood (fig 2.). In my own experience and in observations from experiments that I conducted with other researchers, media lacking fresh wood chips may be less likely to sustain Trichoderma. For example, we observed increases of Trichoderma on fresh wood chips associated with concomitant loss of viability of Armillaria mellea inoculum incubated in piles of fresh yardwaste over time (Downer and others 2008). Targeted or classical biological control seeks to “inoculate” or “inundate” the environment with parasitic or predatory organisms respectively. Curiously SMDC does not require inundative releases of biological control agents.  If the substrate for microbial growth is in place, biocontrol fungi can develop on those particles and impart their disease suppression to the media.  For this reason, media made of stable materials such as pine bark or peat moss may be less likely to support growth of biological control agents than more labile components such as fresh wood chips. 

Fig. 2.  Fresh wood chips become infected with Trichoderma spp. over time. Photo: J. Downer.

If growers decide to use wood chips in container media, one caveat is that nitrogen be supplied in quantities to support both microbial and plant growth. Fresh wood chips added to media increases the carbon to nitrogen ratio of the medium and could lead to nitrogen depletion, depending on the size of the wood chips. See the article published by Owen and others (2014) as part of the Greenhouse Grower series, or their publication in HortScience (2016), for details concerning fertilization with pine wood chips. When the specific pine chips studied at NC State were used in peat-based media at rates of 10% to 30%, no differences were found in nitrogen use (100 to 200 ppm nitrogen allowed for normal growth of zinnia and marigold), liming practices, or plant growth regulator efficacy/response compared to the same rates of perlite.

Some knowledge is available from published scientific studies on beneficial organisms (antagonists) involved in suppression of plant pathogens in media amended with composts, including those with wood components. For example, Trichoderma spp. and Gliocladium virens are among the most abundant fungal taxa associated with suppressiveness in composted hardwood bark (Hoitink and Fahy 1986). Unfortunately, biocontrol organisms associated with fresh wood fiber and other wood components in container media is little studied and there are few references to it in the literature (Brian Jackson, personnel communication, 2017).  Research on SMDC and the ability of non-composted wood amendments to regulate the type and quantity of biological control organisms in container media is needed.

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Jim Downer is Environmental Horticulture Farm Advisor, UC Cooperative Extension, Ventura County

References

Downer AJ, Crohn D, Faber B, Daugovish O, Becker JO, Menge JA, Mochizuki MJ.  2008.  Survival of plant pathogens in static piles of ground green waste. Phytopathology 98: 547–554.  http://apsjournals.apsnet.org/doi/pdf/10.1094/PHYTO-98-5-0547.

Filmer CL, Macdonald JD, Paul JL, Leiser AT. 1986. Influence of air-filled porosity of container media on Phytophthora root rot of toyon. HortScience 21(4):1010–1011.

Gravel V, Menard C, Dorais M. 2009. Pythium root rot and growth responses of organically grown geranium plants to beneficial microoganisms.  HortScience 44:1622–1627.

Hoitink H, Fahy PC. 1986. Basis for control of soilborne plant pathogens with composts.  Ann. Rev. Phytopathol. 24:93–114.

Jackson BE. 2016. The evolution and revolution of wood substrates.  Greenhouse Grower 34:36-40. https://projects.ncsu.edu/project/woodsubstrates/documents/press/evol-revolution-wood-substrates.pdf.

Kaderabek LE, Lookabaugh EC, Owen WG, Judd LA, Jackson BE, Shew HD, Benson DM. 2013. Measuring disease severity of Pythium spp. and Rhizoctonia solani in substrates containing pine wood chips. In: Gawel N (Ed). Proceedings of the Southern Nursery Association Research Conference, Atlanta, GA; p 135–142. https://projects.ncsu.edu/project/woodsubstrates/documents/research/measure-disease-severity-pythium.pdf.

Owen WG, Jackson BE. 2014. The processing and properties of pine wood chips. Greenhouse Grower. 32(6):48–54. https://projects.ncsu.edu/project/woodsubstrates/documents/press/processing-properties-pine-wood-chips.pdf.

Owen WG, Jackson BE, Whipker BE, Fonteno WC. 2014. Fertilization requirements for pine wood chips as an alternative to perlite. Greenhouse Grower. 32(8):64–70. https://projects.ncsu.edu/project/woodsubstrates/documents/press/fertilization-requirements.pdf.

Owen WG, Jackson BE, Whipker BE, Fonteno WC. 2016. Pine wood chips as an alternative to perlite in greenhous substrates: Nitrogen requirements.  HortTechnology 26: 199–205.