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Organic Agriculture Centre of Canada

Replant disorder has plagued farmers for hundreds of years.聽 It was studied in the Okanagan and Similkameen Valleys of British Columbia during the 1980s and has recently regained a high profile in fruit growing regions of BC and other orchards, worldwide.

To increase fruit yields, farmers are pulling out their old fruit trees and planting dwarfing rootstocks that are smaller, closer together and produce more fruit at a higher efficiency.

Planting new trees where an established orchard once was, however, can result in trees that lack vigour when compared to the former orchard block - a term referred to as 鈥榬eplant disorder鈥�.聽 Characterized by poor growth and establishment, the detrimental impact on fruit yield can cost British Columbian farmers up to $10,000/hectare.

Traditionally treated by chemical fumigation, replant poses a particular challenge for organic farmers.聽 Dr. Louise Nelson of the University of British Columbia in Kelowna, BC and Dr. Gerry Neilsen of the Pacific Agri-Food Research Centre in Summerland, BC are searching for organic alternatives to combat replant disorder.聽 They have selected a number of naturally-occurring soil bacteria that may offer a solution to this complex and devastating problem.

The cause of this highly variable disorder is unknown, can differ from orchard to orchard and cannot be attributed to one single agent or causal factor.聽 Some explanations include the presence of pathogenic fungi, bacteria, or microscopic worms called nematodes.聽 Others suggest soil factors such as improper pH, unavailability of phosphorous for plant growth or a lack of moisture.聽

Molly Thurston is a graduate student working on solutions to apple and cherry replant for Drs. Nelson and Neilsen.聽 She is also a horticulturist with the Field Service at the Okanagan Tree Fruit Cooperative who provides advice to over 100 growers in the Okanagan Valley.聽 From Nelson鈥檚 collection of soil bacteria, Thurston has narrowed down her search for the most promising strains from a selection of 100 to 15 candidate organisms.聽 In the lab, she determines their ability to control potential pathogens in the soil and their capacity to act as a biofertilizer by making phosphorous more available to the plant.

Rock phosphate is one of the only natural phosphorous fertilizers available to organic farmers.聽 In Thurston鈥檚 experiments, it is hypothesized that the beneficial bacteria applied in combination with rock phosphate will make phosphorous more available to the plant.聽 In doing so, this added phosphorous will hopefully promote healthy growth and development with newly planted seedlings.

鈥淩egardless of whether growers are organic or conventional, there is a need to explore alternatives,鈥� says Thurston, an organic farmer herself, 鈥渁nd there is a strong interest in biological controls.鈥澛� Although the chemical, methyl bromide, was originally recommended for use against replant, it has now been declared an ozone depleting substance.聽 In 1994 it was removed from the market in Canada and other countries because of its toxic effects.

Currently, Thurston observes how her selected bacteria affect root architecture of young apple seedlings in experimental growth pouches. 聽She also conducts small-scale greenhouse trials and this spring, hopes to apply 4 or 5 of the best performing biological treatments to newly replanted apple seedlings in commercial, organic orchards that have shown symptoms of replant in the past.

Commenting on their results thus far, Nelson states that 鈥渨e see some [bacteria] with good phosphate solubilizing activity on plates in vitro, but now need to see if they are active in the rhizosphere and if they enhance plant growth.

鈥淲e are looking at a complex problem, one which may involve P[hosphorous] availability as well as control of fungal pathogens.聽 It may be difficult to find the optimal combination of P solubilization and fugal inhibition in a single bio fertilizer/biocontrol agent.鈥�

When asked if biological control agents are a viable option for controlling replant disease, Thurston states that it鈥檚 鈥渃ertainly an alternative that needs to be explored.聽 As time goes on, there are more and more barriers to the application and registration of chemical fumigants鈥�, specifically those used on food products.

Consumers are open to the idea of using live organisms, in particular, bacteria on their food crops; they offer a safe, environmentally friendly alternative for both organic and conventional growers.聽 There are a number of microbial products registered and available to treat other orchard problems such as fire blight or apple scab.聽 Their application illustrates that alternatives to chemical control agents are gaining acceptance with both farmers and the Canadian horticultural sector.

This article was written by Daylin Mantyka on behalf of the OACC with funding provided by Canada鈥檚 Organic Science Cluster (a part of the Canadian Agri-Science Clusters Initiative of Agriculture and Agri-Food Canada's Growing Forward Policy Framework).聽 The Organic Science Cluster is a collaborative effort led jointly by the OACC, the and industry partners. For more information: oacc@dal.ca or 902-893-7256.


Posted September 2011