Methods for disposal of Phytophthora infected material and site remediation

This work area tests and evaluates four innovative methods:

1. Disposal through production of biochar

Portable charcoal reactor
Portable charcoal reactor

Current methods of disposal of infected material are on-site burning or deep burial in landfill sites.  These methods are restricted and expensive.

A review of the extent of Phytophthora ramorum and P. kernoviae infection, current methods of clearing infected material, uses of infected timber, distribution and persistence of infection within the soil, and persistence of infection within felled material has been carried out. 

On-site charcoal production using modern portable reactors could be a cost-effective and safe method of disposal of infected material.  Optimum charring conditions sufficient to destroy spores and create good quality charcoal have been established at the pilot scale. Yields of 25-33% of charcoal from the original material, releasing 59-78% of the calorific value as heat have been determined for different sections of R. ponticum and L. kaempferi material.  Data on the physical and chemical properties of the chars produced from each material type have also been collected. The impact of biomass moisture content on net energy yields, and the potential of net energy produced from charring infected materials to feed soil sterilisation systems are also under investigation.

The potential to use the heat generated to sterilise infected soils is also being investigated, see below.

2. Use of the steam generated for on-site soil sterilisation

The heat produced by the charring process has the potential to be converted to steam and be used on-site to sterilise Phytophthora contaminated soil and litter layers.

A review of the range of techniques available for sterilisation of soils has determined that steam injection is likely to be the most effective method.  However, although this technique may prove useful in high value, easily accessible sites, such as gardens and nurseries, it may be less practical at the forest scale. Application methods, effects on existing flora and fauna, and energy requirements have been researched. Conflicting evidence on the temperature and duration of steaming required to eliminate P. ramorum from infected soils has been found, and will be summarised and evaluated in the final reports.

3. Accelerated decomposition of infected biomass using urea and chicken manure

Levels of Phytophthora ramorum and P. kernoviae inoculum in naturally infected leaf litter decline naturally as the leaf litter decomposes.  Application of nitrogen rich materials such as chicken manure and urea may accelerate the substrate decomposition rate by supporting populations of soil mites and microbes.  Some of these microbes may also have a Phytophthora suppressing role.

Replicated trials have been set up to examine the use of chicken manure and urea to treat infected biomass (in the form of woodchips) which may aid disposal of the material and may reduce survival time of the pathogens.

Despite earlier heavy Phytophthora infection at the trial sites, inoculum levels in the initial trial were found to be very low, probably due to dry weather conditions.  New trials established in Autumn 2011 are now underway to monitor the impact of the treatments on decomposition rates and inoculum levels of infected woodchips. Early results indicate that both urea and chicken manure can increase the rate of woodchip breakdown; however, as infection rates were again very low, no significant impact of the treatments on Pr or Pk infection was observed. Larch woodchips have been found to yield higher levels of Pr compared with rhododendron woodchips, but these differences may reflect higher levels of infection and sporulation on larch foliage compared with rhododendron.

4. Anti-microbial treatment using  heated wood shaving material

Chipped woody material can be heated and used as a mulch
Chipped woody material can be heated and used as a mulch

Mild heating of wood shavings has recently been found to confer short-lived anti-microbial properties, and may be suitable for treating infected rhododendron and larch material. It is thought that heating eliminates Phytophthora from infected wood, and that after chipping, incorporation of the treated chips into the soil would reduce the amount of inoculum present in the soil.

Initial experimental work has demonstrated a highly significant inhibitory effect of heated rhododendron, Scots pine and larch wood shavings on the growth rates and infection rates of Phytophthora in lab-scale microcosm studies. Further work has been carried out to identify the inhibitory compounds and combinations of compounds, and to establish their relative inhibitory activity against Pr and Pk. 

Minimal inhibitory concentrations of the active compounds have been determined for different isolates of Pr. Several active compounds have been identified, with conifer aldehyde showing the most biological activity against both isolates of Pr. In addition, the compounds vanillin and acetovanillone showed activity against Pr when in combination with either abietic or dehydroabietic acid. 

The next stage of the project will focus on evaluation of inhibitory effect of heat treated woodchips in microcosm and field studies.