Ondersoek na die wortelvrotkompleks by sorghum bicolor (l. ) moench

Abstract

Various fungi are associated with the root rot disease complex of grain sorghum. Little at tent ion has been given to the relative importance of these fungi with respect to their effect on the host. A possible reason is that root rot seldom results in distinct above-ground symptoms such as wilting and plant death. The objective of this study was to determine the root rot complex and identify the pathogens involved, and to determine their relative importance in the field as well as the greenhouse. Fungi were isolated from plant roots planted at population of 50 000, 100 000, 150 000 and 200 000 plants ha-1 and identified. From 4 weeks after planting, root isolations were made at biweekly intervals up until flowering. F- oxysporum was most frequently isolated, followed by F- subglutinans. This study was carried out the following season at Potchefstroom with plant populations of 100 000 and 200 000 plants ha-1, and at Bethlehem at plant populations of 100 000 plants ha-1. At Potchefstroom isolations were made weekly from 4 weeks after planting until flowering, and at Bethlehem every two weeks from 5 weeks after planting until flowering. F. oxysporum was most frequently isolated which was in agreement with the results of the previous season. The presence of this fungus was correlated with a positive effect on plant growth in both seasons at Pothefstroom and Bethlehem. The effect of two soil types (clay and sandy loam) and their effect on fungus re- isolations from roots, plant mass and root discolouration were studied in the greenhouse. Isolations were made every week from 4 weeks after planting until flowering. Results show that plant growth in the sandy loam soil (Clovelly) was poorer than in the clay soil (Avalon). F- solani was most frequently isolated, followed by F. oxysporum. F. solani and F. moniliforme was correlated with reduced plant mass and F-oxysporum with increased plantmass in both soil types. Pathogenici ty tests were done with various fungal isolates in sterilized and unsterilized clay soil (Avalon) to determine the effect of the fungus on plant growth and root discolouration. Isolations were done 8-9 weeks after planting. Sterilized soil showed higher percentage re- isolations (>80%) for E- solani ( > 80%), E. graminearum and M. phaseolina in the unsterilized soil than E- solani (40%), E- graminearum (48%) and M- phaseolina. Root mass, stalk mass and root discolouration were hi gher in the sterilized than in the unsterilized soil. Rhizoctonia sp. had the greatest adverse effect on all growth parameters evaluated in this test. (28%). It is concluded that it is ~irtually impossible to single out any fungus as the most important in the root rot syndrome.