In 2018, several seed companies that produce tropical grass seeds for the grazing industry approached AgEtal to figure out what was happening with Pasture Dieback (PD). Reports at the time were that some 200000 acres of forage had succumbed, and that science didn’t seem to have any answers.

We visited known PD sites around the Sunshine Coast hinterland - with a completely open mind. I had heard that significant research was directed at Mealy Bug as a possible cause. I confess that I had some doubts.

Initial observations revealed large areas (up to about 5 acres) of browned off pasture within an otherwise healthy looking paddock. On close inspection, it was clear that the grass (Bisset Creeping Bluegrass) had died completely and in its place, a range of legumes and broad-leaved weeds had emerged. There was some white mycelia on the surface of the dead grass that could have been saprophytic or parasitic.

We put a spade as deep as we could and lifted the soil to take a sample for laboratory inspection. Immediately, we noticed small golden pearly objects lodged in the clod to some depth (subsequently identified as White Ground Pearl; Margaroides australis). I enquired of the paddock history. It had been established some 25 years previously. Since then, some urea had been applied, but in 25 years, no phosphate had been applied whatsoever. Climate had been gradually changing - warming and less consistent rainfall patterns.

Once we got the soil samples back into our laboratory, we set about searching for the unseen. Firstly, we were unable to identify the golden pearl objects. These, we sent to a government pathologist, who was also unable to identify them. Ultimately, Anthony Young and Melody Thomson heard about our discovery after they had been independently working on PD and formed an interest in Ground Pearl. Our relationship has built from there. There is immense value in the coincidence of independent research.

A microscopic examination of dust from the soil surface revealed the minute seeds of Mercadonia procumbens, a small herb in the Scrophulariaceae family. 

At this point, we knew the following:

•    Possible nutritional stress, particularly phosphate

•    Longer, hotter and drier summers

•    Golden coloured pearls - identification unknown

•    Presence of Mercadonia procumbens

•    Presence of fungal mycelium.

What we did not know:

•    Soil nutrition status

•    Status of pathogenic fungi

•    Status of soil nematodes.

The soils that we tested from PD areas generally had very low Phosphate levels.

We found a multitude of fungivorous nematodes but very few plant parasites - certainly not in sufficient numbers to cause catastrophic damage to grass pastures with no simultaneous damage to broad-leaved plants.

We understand that some 20 fungal pathogens have been isolate from PD sites. Our interest was drawn to a group of fungi that have a high level of host specificity for grass species - Bipolaris, Exserohilum, etc.

The seeds of Mercadonia procumbens are just 0.2mm long. Generally seeds that are this small, rely on a symbiotic relationship with mycorrhizae in the soil to germinate and emerge.

Plants that are under nutritional stress are known to exude hormones (Strigolactones) from their roots. These chemicals stimulate the proliferation of mycorrhizae, which can form a symbiotic relation with the roots, effectively increasing the surface area of contact between the plant and soil particles. This enables the plant to acquire nutrients like phosphate that would otherwise be unavailable. It is possible that Strigolactones also stimulate the proliferation of pathogenic fungi and some other organisms too. It is possible that they are implicated in the emergence of Ground Pearl adults from the cysts.

Whatever the cause/s are for PD, they are likely to be highly complex, involving climatic, abiotic and biotic factors. There are several crucial questions that need answering...

1. Is Ground Pearl a key causal agent of PD? We believe that the known biology of this organism makes it a prime candidate.
   
   
2. What are the complex interactions between climate, abiotic and biotic factors that lead to PD?
   
   
3. What control measures can be applied to prevent or remediate PD?