Effects of soil-applied fungicides on sugarcane root and shoot growth, rhizosphere microbial communities, and nutrient uptake

Abstract

Sugarcane (Saccharum spp. Hybrid) successive planting (also called monoculture) causes serious yield losses and its management is not well studied in Histosols. Based on very few studies in other sugarcane regions, root colonization by harmful soil fungi is considered as a major cause of this yield decline, but there is lack of knowledge on its management in Histosols. A two-year greenhouse study was conducted with soil-drench application of mancozeb, mefenoxam, and azoxystrobin fungicides to determine their effects on early root and shoot growth, soil microbial communities, and nutrient uptake by plants. The study indicated that mancozeb soil application improved sugarcane-shoot and-root dry matter by 3-4 times and shoot-root length, fine-root length and root surface area by 2-3 times compared to untreated soil. Phospholipid fatty acid (PLFA) analyses of sugarcane rhizosphere soil showed significant reduction in fungal-biomarker abundance with mancozeb and azoxystrobin in comparison to the untreated check or mefenoxam treatments. Bacterial functional-group abundance was reduced by mancozeb and mefenoxam. All fungicides significantly reduced mycorrhizal colonization but not mycorrhizal spore counts. There was a functional relationship between fine-root systems and higher tissue concentration of nitrogen and silicon. The study indicated that application of fungicides to the soil may improve early root and shoot growth and plant-cane establishment that can potentially reduce the yield decline in successively planted sugarcane in histosols. Additional field research is needed in the future to determine the fungicide soil application method, sugarcane growth response in whole crop cycles, and any environmental effects.