Can Soil Amendments Reduce Disease Severity in Trees?

2016 | Dr. Glynn Percival, R.A Bartlett Tree Research Laboratory, University of Reading, United Kingdom

During their life cycle, urban trees are susceptible to attacks by many pathogenic fungi and bacteria that, if uncontrolled, can result in high mortality rates. Control of these diseases is primarily through the use of synthetic agrochemicals. Increased tolerance to commercially available agrochemicals, failure of many chemicals to adequately control diseases once a tree is infected and increased legislative restrictions regarding the use and application of agrochemicals means new techniques of disease control are now of fundamental and economic importance. It is widely known that trees can defend themselves against pathogen infection through a wide variety of mechanisms that can be either local, constitutive or inducible. Developments in plant protection technology have led to the formulation of several soil amendments that have been shown to induce or “switch on” a plant’s own defence mechanisms. These include chitin, phosphites, biochar (a form of activated charcoal) and pure mulches i.e. a mulch made from a single tree species such as willow or eucalyptus. Preliminary studies have found a single soil application of these amendments provides long lasting, broad spectrum control of several fungal, bacterial and viral pathogens. Importantly these amendments act by organic means so are not subject to government legislative restrictions that relate to synthetic agrochemicals. Aims of this investigation are to investigate the efficacy of a range of commercially available soil amendments singly and in combination on controlling two worldwide economically important tree diseases i) apple scab (Venturia inaequalis) a foliar biotrophic pathogens and ii) Phytophthora root rot (a root invasive pathogen).

Final results

Several containerised pot and field trials were conducted over a five year period to investigate the efficacy of soil amendments singly and in combination on reducing disease severity at the whole plant level against apple scab (Venturia inaequalis) and Armillaria root rot, important foliar and root biotrophic pathogens frequently encountered in urban landscapes. Tree species used in these studies include Alnus glutinosa (susceptible to Armillaria root rot) and Malus sylvestris (susceptible to apple scab). Soil amendments tested include: 

  1. Granular phosphite, (crystalline rock form, H2KO3P). 
  1. Chitin, (granular form, C8H13O5N, a long-chain polymer of an N-acetylglucosamine) 
  1. Biochar, (100% crushed pyrolysed hardwood biochar) 
  1. Willow mulch made from Salix alba, 4-6 cm (1.6 – 2.4 in) long chips  

Containerised pot and field trials have demonstrated: 

  1. That disease severity of apple scab can be reduced by circa 60% following application of an appropriate soil amendment. However, synthetic fungicides such as myclobutanil or penconazole used as commercial comparative analysis always resulted in the greatest levels of apple scab disease reduction. Experiments using Armillaria root rot, have to date, produced similar results in that root rot severity can be reduced by, on average, 60% following application of an appropriate soil amendment. However, a comparative evaluation of the fungicide Aliette 80 WG (fosetyl-aluminium) commercially used for Armillaria control provided the greatest degree of control. 
  1. Combination of soil amendments can enhance the degree of disease control by 5-10%. For example, soil amendment with either chitin or willow mulch reduced apple scab severity by 60%. Soil amendment with a chitin + willow mulch reduced apple scab severity by 65-70%. Given the small increase in the reduction of scab severity by combining amendments means careful consideration, should be given regarding the cost effectiveness of combining amendments.
  1. Use of a soil amendment with a reduced dose of synthetic fungicide can be as effective at reducing symptom severity of apple scab compared to a fungicide applied at full strength. This suggests that an appropriate soil amendment plus a reduced dose of fungicide offers a way to reduce fungicide usage and the risk of fungicide resistant pathogen strains developing by decreasing fungicide selection pressure.
  1. The efficacy of soil amendments lasts for at least one full growing season. For example, soils amended with chitin in October 2019 will provide protection against scab diseases until October 2020. Protection will still be provided the following year (October 2020-2021) but the degree of scab control will be lower. By year 3 no scab protection will occur and amendments will need to be replenished. 
  1. Soil amendments confer maximal protection when applied preventatively i.e. before the visible appearance of disease symptoms. Limited efficacy as disease scab protectants occurred when amendments are applied curatively i.e. after symptoms of disease are visibly observed. 

Year: 2016

Funding Duration: 3-5 years

Grant Program: Hyland R. Johns Grant

Grant Title: Can Soil Amendments Reduce Disease Severity in Trees?

Researcher: Dr. Glynn Percival

Key words:

Peer Reviewed Publications from Grant:

General Audience/Trade Publications:

  • Percival, Glynn. 2017. Can We Vaccinate Trees to Protect Against Diseases? Part 1: The Science Behind the Theory. Ontario Arborist Magazine. June/July 2017, pp. 22-24.
  • Percival, Glynn. 2017. Can We Vaccinate Trees to Protect Against Diseases? Part 1I: Putting Science into Practice. Ontario Arborist Magazine. August/September 2017, pp. 20-22.
  • Percival, Glynn. 2017. Controlling Tree Diseases: Thinking Outside the Box. ARB Magazine. Issue 176, pp. 42-46.

Short Discussion Articles/Conference Proceedings:


For more information on this project, contact the researcher via TREE Fund at