Combined effects of leaf spot diseases caused by Stemphylium vesicarium (Wallr.) Simmons and Leptosphaerulina trifolii (Rostr.) Petr. on lucerne cultivars, and the efficacy of some fungicidal control methods

Relative leaf damage caused by the combined effects of 2 leaf diseases, stemphylium leaf spot and leptosphaerulina leaf spot (caused by Stemphylium vesicarium [Wallr.] Simmons and Leptosphaerulina trifolii [Rostr.] Petr.), was monitored in 2- and 3-year old stands of commercial and experimental lucernes (Medicago sativa L.) in subcoastal south-eastern Queensland. A range of fungicides was evaluated for efficacy in reducing damage caused by both diseases. None of the 38 lucerne cultivars tested showed substantial resistance to these diseases. The level of damage resulting from a severe infection was significantly (Pt0.001) and positively related to the relative winter activity level of the cultivar. Of the 8 fungicides evaluated, iprodione and propineb were the most effective, reducing the amount of leaf damage relative to the untreated areas. The most effective rate of application was a spray concentration of 0.1% product, applied at 300 L/ha. All rates were most effective when applied every 10 days. Although the increase in yield in response to fungicide application was less consistent than that of lesion suppression, up to a 90% yield increase over that of untreated areas was measured.