Interactive effects of drenched triterpenoid phytonematicides on sugar content and plant growth of sweet sorghum (Sorghum bicolor) under microplot conditions

Globally, sweet sorghum is being used as source of ethanol. However, its sugar content is highly variable, and nematodes are suspected as the potential cause of the variability. Two cucurbitacin-containing phytonematicides were on various occasions shown to improve sugar content and plant growth of sweet sorghum cultivars, with cucurbitacins chemically classified as terpenoids. The terpenoids being tested during 2020 and 2021 as phytonematicides at the University of Limpopo, South Africa, include Nemarioc-AL, Nemafric-BL and Mordica. Although the interactive effects of Nemarioc-AL and Nemafric-BL on sugar content and plant growth had been widely investigated, the interactive effects of the products with Mordica on the test plant variables had not been documented. The eight factorial treatments, namely, untreated control, Nemarioc-AL alone Nemafric-BL alone, Mordica alone, Nemafric-BL + Mordica, Nemarioc-AL + Mordica, Nemafric-BL + Mordica and Nemarioc-AL + Nemafric-BL + Mordica, arranged in a randomised complete block design, were replicated six times in a microplot trial. At 150 days after initiating the treatments, the second order interaction increased degree Brix by 66 and 48% in the middle and bottom internodes, respectively, but without affecting plant growth variables. The latter were significantly affected by the first order interactions which excluded Mordica. In conclusion, Mordica should not be included in cropping system of sweet sorghum, where the intention includes increasing sugar content.