Issues Magazine

Rotting to the Crown: How a Local Fungus Is Controlling an Introduced Weed

By David Officer

A fungus is providing a biological alternative to chemical methods for weed control.

Who doesn’t like a good movie mystery? You cheer for the good guys and are relieved when the bad ones get their comeuppance.

But who are the good guys and who are the bad? Who do you trust and who do you mistrust? In real life, as at the movies, the more we learn the more we realise we need to know!

In 2000 I was undertaking research into various means of managing an introduced perennial unpalatable weed called giant Parramatta grass (GPG; Sporobolus fertilis) on the NSW north coast. GPG is one of a suite of introduced grasses (e.g. Coolatai grass, gamba grass, serrated tussock, Chilean needle grass) affecting productive grazing land, some of which are posing a threat to the Australian natural environment.

The problem for agriculturalists and environmentalists alike is that removing an unwanted grass from a mixture of grasses is not an easy task. Until 2000 we had been relying mostly on a single rate-selective herbicide for grass weed control in land unsuitable for cultivation. Rate-selective herbicides kill the target grass when applied in the right amount but won’t kill all grasses in the treated area. This herbicide type is not suitable for all situations nor is it always effective, which means that, despite ongoing concerted effort, control of GPG has often not been achieved.

The rate-selective herbicide strategy has inherent risks: the efficacy post-application is dependent on rainfall – both when it falls and how much. This is because to work effectively the herbicide needs to be washed into the root zone and absorbed by the roots.

Anyone living in Australia knows rainfall is not reliable; average rainfall for an area gives you little clue to variability in timing, location and quantity. Therefore, any new management strategy that allows us to selectively control unpalatable grasses needs to be able to cope with our variable environment and preferably reduce our ongoing reliance on herbicides.

Back in 2000 I was conducting an herbicide experiment with an unselective herbicide. I was looking at ways of using it in a selective way. The wick-wiper I was using was designed to apply the chemical only to the tall GPG but not the well-grazed shorter species.

Part of this work involved measuring the size of GPG tussocks before and after treatment. I discovered that the plants in the control plots, which had received no chemical, were shrinking in size. They were not shrinking or disappearing at the rate of the treated plants but were still shrinking! I had a mystery that needed to be solved.

My first thought was that my method of measuring the tussock size was inaccurate and therefore I needed to improve my technique. By chance, several months later I happened to be driving past the trial site and stopped to see what was happening to my treatments. To my surprise the whole trial area and even well beyond the trial area had relatively few GPG plants remaining, and they were small and sickly. Something was killing the GPG and it had nothing to do with any treatment I had imposed.

I sent some of the remaining plants to some plant pathologists to try to determine what was causing the decline. Unfortunately nothing definitive was found. Eventually I sent some samples to a PhD student, Sethu Ramasamy at RMIT, and he was able to isolate a fungus from the diseased plants. In turn, when he inoculated healthy GPG plants with the fungus they developed disease symptoms similar to what I was now able to observe in the field. The loop was complete when Sethu was able to reisolate the same fungus from the plants he inoculated.

The fungus in question, Nigrospora oryzae, is a local fungus known to be a weak pathogen. Most often it is not the cause of disease because it is a saprophyte, meaning it lives off dead plant tissue.

This discovery of a causal agent for the decline in GPG began a series of experiments to further understand how the fungus affected plants, how quickly this happened, and what, if any, other species of plants were showing or were likely to show symptoms of this disease.

Sethu and I also started work to understand how to cultivate the fungus in the laboratory because we realised we had a potential biological control agent for GPG if it passed muster in a series of host specificity tests. The testing showed that the crown rot symptoms observed in GPG were not found in plants outside the Sporobolus genus, and none of the native Sporobolus tested showed symptoms. This meant that only the weedy introduced Sporobolus grasses were affected.

We observed infestations of GPG where the disease symptoms were present, and monitored the decline in size of and number of tussocks over time. These “natural” infections showed that the rate of decline, even within a property, was variable and quite slow. In most cases the time from infection through decline to a level where the GPG was having no effect on animal production was a matter of 2–3 years rather than the days or weeks you would normally expect from a chemical herbicide.

This slow decline turns out to have an unexpected benefit: other pasture species have time to fill the gaps created by the dying GPG. Thus far fewer broad-leafed weeds replace the GPG. Broad-leafed weed infestation is a common problem resulting from the use of chemical herbicides.

The variable rate of the decline means there is more of this mystery to be revealed. We still need to know more about the conditions that do and do not favour the disease.

Most of GPG’s yearly growth occurs between spring and midsummer when there is usually plenty of moisture and warmth for growth in our region. The disease affects the tillers (leafy shoots of tussock grasses) when they are new, and often stops their growth, preventing them from elongating and setting seed.

When the fungus is present in the crown of the plant, right at ground level, it turns new tillers a pale orange colour that is only evident as this shoot tries to grow. Disease symptoms usually become evident a week after the first significant spring rainfall event, and only last for a couple of weeks for each tiller.

Therefore, one of the reasons that this “crown rot”, as we have called it, is hard to identify is that the symptoms of the disease tend to be present only during this growth period. Over autumn and winter the GPG plant, which is made up of numerous tillers, naturally senesces (matures), turning from an olive green to straw-coloured seedless stalks. A diseased and a naturally senescent tiller have the same colour during autumn and winter. The only difference is that the diseased tiller is easier to pluck from the crown. Like mystery movies, sometimes it is hard to pick the good from the bad.

Because the disease works progressively through a plant, not all the plant is symptomatic at the same time. Trying to observe one or two diseased tillers among many in the same tussock during the early stage of the disease means it is easy to miss the presence of disease, which is why many people do not know if they have disease present or not. (Help with identification of symptoms and tips on relocating diseased plants can be found online by typing “Nigrospora crown rot” into a search engine.)

Recent research has shown that Nigrospora crown rot disease can be introduced to new areas by either relocating or transplanting diseased plants or by inoculating healthy plants with spores produced in the laboratory. Because commercial production of spores is not yet available, local farmers who want to get rid of GPG are going to sites where the disease is present and are digging up diseased plants and relocating them to their own properties. To assist this process, regional council weeds officers are organising opportunities for farmers to come, dig and then transplant diseased plants.

The initial identification of the disease was in the Clarence Valley in coastal northern NSW a little over 10 years ago. Diseased weedy Sporobolus plants have now been found between Newcastle in NSW and Mackay in Queensland.

The area of diseased plants continues to increase as farmers learn about its existence and relocate it to new locations. It is hoped that our local biological control agent, Nigrospora oryzae, will prove resilient and effective over the current range of the weedy Sporobolus grasses.

Already farmers are benefiting from the research conducted so far, even though parts of this mystery are yet to be solved. Research is ongoing to develop techniques for the production of commercial quantities of Nigrospora oryzae spores in the laboratory.

Different management techniques are being evaluated to control the speed with which the crown rot spreads. An optimal time frame between the often rapid plant loss using herbicide which leads to broad-leaf weed infestation and the sometimes too slow spread of the crown rot is desired. Work has commenced in central Queensland on the inoculation of the other major weed in the weedy Sporobolus group, giant rat’s tail grass.

Not often does a significant breakthrough occur in the management of an introduced perennial grass weed that does not include a new or improved chemical herbicide. On this occasion, the breakthrough comes from a natural source.

The good guy in our mystery turns out to be a common and generally harmless fungus that is proving to be a hero to our local farmers.