Cotton growers across northern and central Queensland are grappling with a significant new challenge as a previously little-known fungal disease, Ramularia leaf spot, takes hold. This aggressive pathogen, which causes premature leaf defoliation and can severely impact crop yield and fibre quality, has sparked an urgent, industry-wide response.

First identified in the region over the past two seasons, the disease has surprised experts with its rapid spread and potential for widespread damage. Plant pathologists describe Ramularia as a fungal disease that thrives in hot, humid, and wet tropical conditions, making parts of Queensland an ideal breeding ground. It is known by several names, including areolate mildew, grey mildew, or false mildew, due to its visual similarities to other common mildews, despite being distinct.

A Growing Threat to Local Crops

While Ramularia leaf spot has been observed in other cotton-growing regions globally, particularly in the south-eastern United States and Brazil, its widespread presence in Queensland marks a concerning development. The disease causes cotton leaves to yellow, develop necrotic lesions, and ultimately fall off prematurely. This defoliation can lead to substantial reductions in both the quantity of cotton produced and its market value.

Agronomists like Simon Struss, based in the vital Callide-Dawson region, have witnessed the disease's pervasive nature firsthand. “It really was right throughout the whole area,” Struss noted, describing how every farm and field displayed symptoms. Initially, many, including Struss himself, confused it with other fungi like Alternaria until samples sent to laboratories confirmed the presence of Ramularia, highlighting the diagnostic challenge it poses.

Limited Treatment Options Spur Innovation

The immediate challenge for growers is the severe lack of registered chemical treatments specifically targeting Ramularia leaf spot in Australia. This scarcity leaves producers with limited options to protect their crops once the disease takes hold. Growers are currently advised to implement cultural control measures, such as planting crops early—ideally before December—to avoid peak disease conditions. Additionally, best practices include burying infected crop residues after harvest to eliminate volunteer cotton plants, which can act as a host for spores, and avoiding back-to-back cotton planting in the same fields.

Industry Unites for Long-Term Strategy

In response to this emerging threat, the broader cotton industry is mobilising swiftly. Key industry groups and agricultural experts are collaboratively developing a comprehensive control strategy. This includes fast-tracking access to effective chemical treatments through the necessary permits for fungicides, ensuring growers have a broader arsenal at their disposal.

A critical focus of this strategy is the implementation of resistance management programs. Learning from international experiences, particularly in Brazil where Ramularia has developed resistance to commonly used chemicals, Australian industry leaders are committed to adopting diverse treatment methods. This multi-pronged approach aims to prevent the fungus from developing resistance locally, ensuring the long-term sustainability of control measures.

The origin of the disease in Australia remains somewhat of a mystery, with some experts speculating it may have been endemic for some time, only recently manifesting as a significant problem. Regardless of its provenance, the industry's immediate priority is understanding its full impact and developing robust countermeasures.

Early Detection is Key

Research efforts are underway to gain a deeper understanding of Ramularia's specific effects on Australian cotton and to develop rapid diagnostic tools. Fortunately, a highly effective molecular diagnostic tool, known as LAMP (loop-mediated isothermal amplification), has been developed. This advanced tool allows for the rapid identification of Ramularia from a leaf sample within just three hours, enabling growers to respond quickly and prevent potentially significant yield losses. Early detection and swift action are crucial in mitigating the impact of this new and serious threat to Queensland’s cotton industry.