Value of Silver Leaf Fungus Explored

Dr Von Johnson of EnformTech surveying glasshouse trials.

No-one wants their drinking water contaminated with chemical herbicide residues. Nonetheless, we need to control weeds that grow in and beside water (riparian weeds).

Control of willows growing in wetland areas of New Zealand is typically done by cutting them down and then applying glyphosate to the cut stumps to prevent resprouting. While this method undoubtedly reduces the amount of glyphosate entering waterways, when compared with aerial application, some of the chemical is still likely to be washed into catchments. Consequently, we have been exploring non-chemical alternatives for willow control in Auckland water-catchment areas, where there has been strong opposition to the use of chemicals by some residents. We have found a living organism (the silver leaf fungus, Chondrostereum purpureum) that can damage willows and are investigating its potential to be used against them. A herbicide having a plant-pathogenic microbe as its active ingredient is called a bioherbicide. If the microbe is a fungus, as is the case here, then it belongs to a subset of bioherbicides called mycoherbicides.

There are many steps to the successful development of a mycoherbicide, and with respect to using silver leaf fungus against riparian weeds we are at the early stages. So far we have done some work on step 1 (finding a suitable pathogen), step 2 (finding a suitable isolate of that pathogen), and step 3 (developing a method for mass producing the organism). Some research has also been done on step 6 (developing an appropriate formulation and application technology) by our colleagues at EnForm Tech.

Silver leaf fungus was identified as having potential for control of woody riparian weeds in Auckland water-catchments for two reasons: it has a wide host range among woody plants; and, it has already been developed successfully overseas as a cut-and-paste mycoherbicide, or rather as a “wood rot promoter”, a product that is much easier to register. The fungus is used to control red alder (Alnus rubra) and other woody weeds in utility rights-of-way (i.e. areas of forest cleared to make way for power lines etc.) in Canada and to control black cherry (Prunus serotina) and other woody weeds in plantation forests in the Netherlands. The products used in these two countries are “ChontrolTM” or “EcoclearTM” and “BioChonTM” respectively.

There are many isolates of silver leaf fungus stored in Landcare Research’s International Collection of Micro-organisms from Plants (ICMP) at Tamaki. Three of these were grown in several different artificial media (liquid broths) to determine which, if any of them, could be easily grown in this way. While all three isolates grew in the broths, the amount (biomass) of silver leaf fungus produced varied significantly according to which broth was used and which isolate was grown. “Happily, there was one clear winner in each case,” explained Stan Bellgard, who is leading this project. “The best broth was one containing malt extract, and the best isolate was one recovered from a Prunus (plum, peach or cherry) tree.”

Now that we had a silver leaf isolate that we could grow easily in the lab we applied it to two species of willow in glasshouse experiments, to see if it could damage these weeds. The two willow species targeted were crack willow (Salix fragilis) and grey willow (S. cinerea).

Stems of both crack and grey willow were harvested from a site in the Hunua Ranges – a southern water-catchment area of Auckland. Three- to four-metre lengths were cut in the field and transferred back to our laboratory at Tamaki where they were cut into smaller, 20-cm-long lengths for the experiment. One end of each stem was dipped in rooting hormone gel and planted in sterile potting mix. Each stem was then covered with one of the following four treatments: the silver leaf isolate in one of two different gel formulations, glyphosate 360 (full strength) in a commercially available gel formulation, or water (as a negative control).

After 22 weeks in a glasshouse the willow stems were harvested. We weighed them and counted the number of healthy and dead shoots. Small pieces of tissue were taken from each stem, surface sterilised (to remove contaminating organisms on the surface) and plated on to agar so that any fungi present within the willow tissue would grow. We analysed the sequences of any fungi resembling the silver leaf fungus that grew from the tissue fragments and compared them with the sequence of the isolate we applied. This allowed us to tell whether the fungus we applied had successfully infected the woody stems and check that it wasn’t already present (through infection in the field) on the stems treated with water or glyphosate.

The results of the glasshouse trial were promising. Our selected silver leaf isolate was found infecting the stems of both crack willow and grey willow that had been treated with it, while it wasn’t present on the stems treated with water or glyphosate. Best of all, there were significantly more dead stems in the groups that were treated with one of the silver leaf fungus formulations when compared with the water treatment, for both species of willow. In fact, this silver leaf fungus formulation was just as effective as full-strength glyphosate when it came to killing cut stems. “When applied in the other formulation the fungus was just as effective as glyphosate on crack willow, but was significantly less effective than glyphosate (and statistically speaking, no better than water) on grey willow,” observed Stan. “Since we want to develop a mycoherbicide that can be used against several different woody weeds, the formulation that worked well on both willow species seems to be the better choice at present.”

With these promising results under our belt we applied the same four treatments to cut stems of crack and grey willow in the field. The field site was the same spot in the Hunua Ranges where the cut stems had been collected. After 32 weeks, 50% of all the cut and treated willow stumps had been colonised successfully by the silver leaf fungus. In the field, both of the gel formulations tested worked equally well with respect to allowing the fungus to colonise the cut stems. Data have been collected, but not yet analysed, on shoot regrowth and stem death. Once the analysis has been completed we will be able to compare the silver leaf treatments with the glyphosate and water treatments.

Our next steps towards developing a mycoherbicide for riparian weeds will be to “scale-up” our production and ensure we can produce stable inoculum with a long shelf-life, and to test the efficacy of our selected isolate and formulations on other riparian weeds including basket willow (Salix viminalis) and privet (Ligustrum spp.). We also plan further fieldwork to test the impact of weather conditions on efficacy (i.e. application in summer vs winter) and to develop “liquid” formulations that could be applied through drilling and/or injection into woody weeds. If results continue to be promising, the final steps will include identifying potential users and markets and finding a commercial sponsor so that a product can be registered and commercialised.

Hopefully one day in the not too distant future, Auckland residents will sleep a little more soundly because riparian weeds are being controlled biologically, rather than chemically, in their water catchments.

This research was funded by the Ministry of Business, Innovation and Employment as part of the Beating Weeds Programme.