Studies on a pathogenicity assay for screening cotton germplasms for resistance to Fusarium oxysporum f. sp. vasinfectum in the glasshouse

Abstract

Summary. Fusarium wilt, caused by Fusarium oxysporum f. sp. vasinfectum, is a new and important disease of cotton in Australia. Some factors affecting either the infection process or the subsequent development of symptoms under glasshouse conditions were examined in this study. The pathogenicity of inocula was significantly affected by the media in which they were produced. The most severe symptoms developed in the plants inoculated with the inoculum produced in Komada-Ezuka liquid medium, in which glucose and L-asparagine were used as the carbon and nitrogen source, respectively. Symptoms were significantly more severe in plants inoculated with the inocula suspended in culture filtrates than in those inoculated with the inocula suspended in distilled water, indicating that fungal metabolites played an important role in the infection process. The disease was enhanced by high conidial concentration (>1.0 x 10⁶ conidia/mL), slightly acidic inoculum (pH 4.0–5.5) and longer inoculation period (5–25 min). One-week-old seedlings were most susceptible, regardless of cultivar, and the resistance of plants increased with their age. The development of symptoms was enhanced at a moderate temperature range (18–23°C), but suppressed at a higher temperature range (28–33°C). Based on these results, an optimised procedure of pathogenicity assay is described.