Rapid assessment of early-stage herbicide-rust compatibility to support integrated management of Cryptostegia grandiflora

Abstract

Cryptostegia grandiflora (rubber vine) is an invasive weed in tropical and subtropical regions worldwide and is listed as a Weed of National Significance in Australia. The rust Maravalia cryptostegiae, deliberately introduced to Australia as a biocontrol agent, contributes to long-term suppression, but does not provide complete control, making herbicide intervention necessary in many land-use contexts. For integrated weed management (IWM) to be effective, chemical and biological controls must complement rather than conflict, yet herbicide-rust compatibility has rarely been quantified. We developed a 14-day laboratory workflow to assess compatibility by combining in vitro spore germination and detached-leaf infection assays with image-based analysis. Treatments included metsulfuron-methyl (MM), a recommended foliar herbicide for rubber vine, and a novel herbistatic analogue (COT-MM). Both the herbicide and the herbistatic analogue reduced spore germination and leaf infection relative to the blank, but neither differed from the surfactant, indicating strong adjuvant-mediated inhibition. No detectable difference between MM and COT-MM was demonstrated. This rapid workflow detected early antagonism between foliar herbicide formulations and M. cryptostegiae and identified the adjuvant as a major contributor. Integrating such assays into herbicide discovery and management planning will help ensure that chemical and biological tactics reinforce one another, supporting more sustainable IWM of invasive weeds.