Sorghum root rot and grain mold pathogen responses to legume rotation systems

Abstract

This dissertation has four chapters with literature review included. The overall aim of this dissertation was to investigate how sorghum root rot and grain mold pathogens respond to legume rotation systems. This study looked at the cultural benefits obtained through rotation systems in promoting root and plant health and sustainable food security through the reduction of mycotoxin production. The first chapter is the literature review, which introduces the main legume crops that were used to control root rot, grain mold and occurrence of specific mycotoxins in sorghum. It detailed why there is a need to control these constraints affecting crop growth because of its reliance as a staple food in South Africa and many other developing countries. The introduction included the origin and distribution of sorghum, its production level globally and locally and its economic value. It further includes the constraints such as grain mold, root rot and mycotoxins and their cultural management strategies. In Chapter 2 the impact of various crop rotation systems on the nutrient status of soil and the relationship between soil health and root rot severity in subsequent sorghum crops were established. Field trials were planted at Potchefstroom (ARC-Grain Crops, South Africa) during 2015/16, 2016/17 and 2017/18 respectively. The six main plots were established during the first season (2015/16) by either fallow or planting of sorghum, dry bean, soybean, cowpea and bambara. Plants were randomly collected from sorghum plots at Potchefstroom to quantify root colonization using ergosterol content and test for 12 root pathogens in root tissues using qPCR analyses. Soil samples were collected at the beginning of every season to test for soil nutrient elements ie. N (NO3 and NH4), P, K, Ca, Mg, Na, Fe, Cu, Zn and Mn and soil pH that contribute to improved soil and plant health. In Chapter 3 colonization of F. graminearum Species Complex (FgSC) on three sorghum cultivars and their response to grain mold pathogens under fallow, monoculture and legume rotation systems over a three season period (2015/16; 2016/17 and 2017/18) was determined. Ten plants were collected each season from a 10 m designated inner row per cultivar, per treatment for threshing. The threshed grains were visually rated for grain mold on a 1-5 scale. Total biofungal mass as a measure of colonization was quantified through ergosterol quantification, while FgSC was quantified using qPCR analyses. In Chapter 4 determination of whether legume based rotational systems assist in reducing deoxynivalenol (DON), nivalenol (NIV) and zearalenone (ZEA) concentrations in sorghum grain and establishing whether the mycotoxin concentration in sorghum grain might be due to translocation of mycotoxins from the root system to sorghum grain was assessed. Ten plants were collected during each season from a 10 m designated inner row per cultivar, per treatment for threshing and assessment of root rot severity. The threshed grains were visually rated for grain mold on a 1-5 scale and roots were visually assessed on a scale of 0-4. Total biofungal mass as a measure of colonization was quantified through ergosterol quantification in grains and roots, while FgSC was quantified using qPCR analyses. Mycotoxins were quantified using LC-MS/MS method. Relationships between FgSC and mycotoxins were also determined.