Premier Press

Summer 2000 vol. 7/no. 2

A Comparison of Peat/Bark Mixes -
PRO-MIX 'BRK' and PRO-MIX 'GSX'

Disease Suppression Associated
With Sphagnum Peat Moss

GROWER PROFILE:
Hole’s Greenhouse and Gardens

Various reports have confirmed that Sphagnum peat moss has disease suppressive qualities against certain root-rot pathogens. This is due to the presence of beneficial microorganisms. Contrary to popular belief, most peat moss producers do not sterilize or pasteurize their peat-based products for three reasons. First, Sphagnum peat moss is essentially free of pathogens and pests (Tahvonen, 1993). Second, it kills disease suppressive microorganisms found in Sphagnum peat moss (Tahvonen, 1993). Third sterilization is expensive.

Microorganisms Involved in Disease Suppression

Sphagnum peat moss bogs contain many microorganisms including Bacillus, Arthrobacter, Actinomyces, Streptomyces, Penicillium, Cladosporum, Trichoderma, Mucor, etc (Belanger, 1988 and Tahvonen, 1993). Of these, Trichoderma and Streptomyces are quite effective at suppressing certain root disease organisms (Tahvonen, 1993) due to their synthesis of antibiotics. Their presence in Sphagnum moss has been found to suppress Fusarium, Rhizoctonia solani, Phythium, (Tahvonen, 1993 and Chen, 1986) and Alternaria brassicicola (Tahvonen, 1993). The remaining microorganisms suppress root rot pathogens through competition. Simply stated, there is a limited amount of space and resources therefore, if a lot of beneficial organisms are present it is difficult for root rot organisms to establish themselves.

How Reliable is Sphagnum Moss for Disease Suppression?

Sources of Sphagnum peat moss vary in microbial populations and composition. Therefore, disease suppression is not always predictable (Tahvonen, 1993). Blonde, fibrous peat from the surface of the bog has higher microbial populations versus darker, decomposed peat from deeper layers in the bog (Hoitink, 1991). Only blonde, fibrous peat, classified as H2 to H3 on the von Post decomposition scale (e.g. PRO-MOSS 'TBK') provides enough beneficial microorganisms to promote disease suppression. Disease suppression lasts about 6-10 weeks (Hoitink, 1997). However, when a plant is planted into a Sphagnum peat-growing medium, populations of disease suppressive microorganisms sometimes increase (Tahvonen, 1993).

Caution: Preventative Fungicide Applications

Since disease suppression comes from mostly Trichoderma and Streptomyces, fungicide drenches, fungicide granules or steam sterilization can reduce or eliminate their populations. If there are few beneficial microorganisms to compete with root disease pathogens, then the pathogens quickly take over the growing medium. Now preventative fungicides have to be applied to prevent disease problems.

Aerial view of a bog operated by Premier Horticulture
Pointe-Lebel (Quebec) Canada

To utilize the disease suppressive qualities of Sphagnum peat, only apply preventative fungicides on crops that have had problems in the past. For the remaining crops, do an experiment. Apply preventative fungicides on half of the crop (as done in the past) and do not treat the other half. Monitor the untreated plants closely and only apply fungicides if root disease is found. You might be surprised with the results. Crops treated with preventative fungicides could become stunted and have more disease problems than untreated plants, especially if these fungicides are applied too frequently or at excessive rates.

Literature Cited
— Belanger, A., et.al. 1988.
Peat A Resource of the Future.
Centre Quebecois de Valorisation de la Biomasse, Sainte-Foy, Quebec.

— Chen, Y. and Y. Avnimelech. 1986.
The Role of Organic Matter in Modern Agriculture.
Martinus Nijhoff Publishers, Netherlands.

— Hoitink, H.A.J., Y. Inbar and M.J. Boehm. 1991.
Status of Compost-Amended Potting Mixes Naturally Suppressive to Soilborne Diseases of Floriculture Crops. Plant Disease 75(9): 869-873.

— Hoitink, H.A.J., A.G. Stone and D.Y. Han. 1997.
Suppression of Plant Diseases by Composts. HortScience 32(2): 184-187.

— Tahvonen, R. 1993. The Disease Suppressiveness of Light Coloured Sphagnum Peat and Biocontrol of Plant Diseases with Streptomyces sp.
Acta Horticulturae 342: 37-42.