Glyphosate Control of Orange and Brown Rusts in Glyphosate-Sensitive Sugarcane Cultivars.

Brown and orange rust diseases cause substantial yield reductions on sugarcane (Saccharum spp.) in Florida and other regions where sugarcane is grown. Brown rust caused by Puccinia melanocephala Syd. & P. Syd has been present in Florida since 1978 and orange rust caused by Puccinia kuehnii E.J. Butler was first identified in Florida in 2007.  Florida growers have relied on genetic resistance to control brown rust, but this has been problematic because the fungus often overcomes resistance by forming new races. It is expected that orange rust will further add to the difficulty of achieving durable resistance genetically.  Others reported that glyphosate [N-(phosphonomethyl) glycine] effectively controlled stripe rust in wheat (Triticum aestivum L.) that was genetically modified for glyphosate resistance. Our objective was to determine if low doses of glyphosate applied to glyphosate sensitive sugarcane cultivars would control the rusts without damaging sugarcane. In-vitro experiments showed significant control of germination and tube elongation in spores of both rusts. In field tests, a dose of 2.34 x 10³ μmole per L applied to young sugarcane plants about 100 cm tall, reduced disease and did not visually harm sugarcane. Low rates of glyphosate applied to glyphosate sensitive sugarcane has promise as a chemical control of sugarcane brown and orange rusts. However, more research is needed to determine duration of control, if there is some systemic effect beyond the observed contact mode of action, and if there are effects on sugarcane yield.