The potential of a Novel Green Manure as a treatment for Replant Disease on Sorbus aucuparia

Trees have been an integral part of our environment for millennia by providing many ecosystem services to the surrounding populous. Trees add to their environment through the provision of ecosystem services and are a positive contributor to public health which financially equates to £1.8 billion per year. The importance of trees is recognised at government level and their health and resilience is considered a priority. Tree and shrub species in the Rosaceae are susceptible to replant disease (RD), which is expressed in the form of necrotic roots, stunting and reduced yield, and are thought to be the result of a build up of soil borne pathogens that are associated with root rotting, and can be exacerbated by abiotic factors such as poor soil condition, the presence of phytotoxins and a lack of available nutrients. Pathogens detected in replant soils cover a broad range of microorganisms often in the form of a complex consisting of organisms from bacteria, fungi, and stamenopiles to nematodes. As a result of this disease being incited by a complex of various pathogens it was treated with a broad spectrum pesticide called methyl bromide which is non-selective in its target species. This chemical treatment was revoked under the food and environment protection act 1985 and the control of pesticides regulations 1986 due to being a stratospheric pollutant. RD was identified as a specific problem on Sorbus aucuparia (rowan) which was previously associated with a complex of Cylindrocarpon, Fusarium, and Pythium. This species was found to be particularly sensitive to RD and was demonstrated as being a model species for soil susceptibility bioassays. S. aucuparia is an ornamental species which is used widely throughout Europe in urban forest plantings due to its tolerance to disturbance and poor conditions. Due to these factors this species was used to determine the effectiveness of green manure as a treatment for RD. It was proposed that the use of novel green manures with specific antimicrobial properties as an organic soil addition had the potential to alter the condition of the soil to one that is conducive to tree growth. Phytolacca americana (American pokeweed) was selected for its potential as a biofumigant, with Brassica juncea (mustard) Tagetes patula (French marigold), Triticum aestivum (wheat), and Allium sativum (garlic) studied as comparative treatments. The research was split into three streams of work involving a three year bioassay running alongside microbial inhibition tests and DNA analysis. In-vitro studies (n=9) were conducted to determine the potential of green manure leaf matter to inhibit Pythium ultimum and Rhizoctonia solani, which were previously associated with RD. At the same time, bare root S. aucuparia saplings were arranged in a randomised block design in the research area of the Moulton College estate in Northamptonshire. After three months of establishment trees were categorised into 16 treatment groups of single tree replicates (n=25). Eight groups were potted in sterile soil, whilst eight were potted into diseased soil, obtained from Burbage Common in Leicestershire. P. americana was applied in three concentrations (1.67g/l, 8.33g/l and 15.00g/l), whilst comparative green manures were applied in one concentration (8.33g/l). During a period of three years trees were monitored initially after three months and then yearly, with growth variables including primary and secondary growth, leaf count, active bud count, and chlorophyll content. At the end of the three years the trees underwent destructive analysis which consisted of dry weight measurements of stems, roots, fruit and foliage. Roots were examined for nematode cysts. Soil samples were analysed by Scientia Terrae for a prescribed list of pathogens and beneficial microorganisms using a polymerase chain reaction and DNA Multiscan. The in-vitro laboratory study demonstrated that application method, concentration, and green manure species used can influence pathogen growth. All P. americana treatments significantly inhibited R. solani and P.ultimum. P. americana has properties which induce inhibition of pathogen growth, with these properties being liberated from the plant matter directly or as a water based extract. Of the two pathogens the former was found to be more sensitive to treatments than the latter. Application of comparative green manure species as plant matter in agar inhibited R. solani and P.ultimum growth. T. patula was the only plant species that did not inhibit R. solani and only temporarily inhibited P. ultimum growth. When the treatment was applied as a cold water extract P. americana successfully inhibited more than other treatments. The in-vivo field study determined that the influence of the sterile soil sub-set treatments on tree growth was no different to the control. Treatments did not have an influence on height, secondary growth, foliage and fruit production throughout the three years. There was also no significant difference in root and stem biomass. The use of green manures appears to produce variable results, which is especially true in terms of P. americana where improvement in growth parameters was not correlated with concentration. T. patula (8.33g/l) and P. americana (15.00g/l) were linked to increased height and vigorous roots with greater biomass, whilst B. juncea and A. sativum caused stunting in comparison to the diseased control. T. aestivum treated trees were associated with variable vigour. Molecular DNA results in relation to tree growth data indicated that the disease in this particular instance was exhibited in the presence of soil borne pathogens Rhizoctonia solani, Pythium spp., and Pythium sylvaticum. Results demonstrated that, regardless of concentration, P. americana does not have a consistent affect on soil microbes. However, P. americana is successful in eliminating R. solani when used at a rate of 15.00g/l. Overall results suggest that P. americana and T. patula soil amendments improve root and shoot growth relative to the control. P. americana was consistently fungistatic in vitro whilst displaying variability in its activity in field conditions. T. patula demonstrated poor ability to suppress pathogens in the laboratory however was much more successful when added as an amendment. Surprisingly, two species known for their antimicrobial activity, A. sativum and B. juncea, were not found to be beneficial in this instance. T. patula may have potential as a green manure; however the other species are unreliable treatments. It is recommended that active ingredients of P. americana and T. patula are investigated further to deduce their activity against replant disease on S. aucuparia.