Effects of Glyphosate on Soybean Nutrition, Endophytic Colonization by Cercospora cf. flagellaris and Development of Cercospora Leaf Blight

• Main Author: Garcia Aroca, Teddy
• Other Authors: Schneider, Raymond W
• Format: Others
• Language: en
• Published: LSU 2016
• Subjects: Plant Pathology & Crop Physiology
• Online Access: http://etd.lsu.edu/docs/available/etd-11072016-093859/

Glyphosate (Roundup®, Monsanto, Inc., St. Louis, MO) is the most widely used herbicide in the world because of its broad spectrum and its efficacy in controlling annual broadleaf weeds and undesired grasses. The effect of glyphosate on mineral nutrition and plant diseases has been an important topic during the past decade, because of its controversial effects on plant mineral nutrition. In order to test the hypothesis that glyphosate affects soybean mineral nutrition, and therefore predisposes soybean to Cercospora leaf blight, six glyphosate-resistant (GR) soybean varieties were either not treated or treated with glyphosate in field experiments. Plants were then evaluated for leaf concentrations of 13 nutrients, foliar disease symptoms, and biomass of Cercospora cf. flagellaris as assessed with real-time PCR assays. Experiments were conducted at three locations in Louisiana. Three sets of soybean GR varieties were used in the experiments, six each year at each location, for a total of 18 varieties. These varieties corresponded to maturity groups III, early and late IV, and V. Control plots received no glyphosate application on the soybean foliage. Disease assessments were performed at R6 growth stage at one location using predefined scales for purple symptoms in 2014 and 2016. No symptoms of CLB were observed at any other location during the three-year period of the study. Differences were detected in nutrient uptake among research stations and years, and there were variations in fungal biomass across varieties. The effects of glyphosate on leaf concentrations of Al, Fe, Mn, N, Na, and K were location-dependent. Glyphosate enhanced uptake of Zn in all experiments. Real-time PCR analyses of the CTB6 gene of C. cf. flagellaris consistently detected higher fungal biomass in glyphosate-treated samples compared to controls, indicating that glyphosate affected colonization of host plants during the latent period of infection. Enhanced uptake of of Zn, which is a key part of a transcriptional activator (CTB8) in the cercosporin biosynthetic pathway, could be associated with higher concentrations of C. cf. flagellaris DNA in glyphosate-treated leaves. However, CLB purple leaf symptom severity was significantly lower in glyphosate-treated plots compared to controls in 2014 and 2016. Therefore, colonization by C. cf. flagellaris was not associated with disease severity. These results suggest glyphosate may affect colonization by C. cf. flagellaris only while the pathogen is in its endophytic stage of development.