Etiology of Fusarium crown and root rot of grain sorghum in South Africa

English: This study investigated the interaction between grain sorghum and Fusarium spp. associated with the crop in field soils and on sorghum roots as causal agents of crown and root rot. The effect of disease on plant growth and development was investigated as was the efficiency of various inoculation techniques on disease severity. Factors affecting disease susceptibility such as plant age, soil type, soil moisture, soil pH, soil fertility and possible chemical and bio-control control tactics were also investigated. Fusarium spp. isolated from sorghum roots and field soils at Cedara (Kwazulu- Natal province), Bethlehem (Free State province) and Potchefstroom (North West province) indicated that F. oxysporum, F. solani, F. verticillioides and F. thapsinum were the most frequently recovered species. Other Fusarium spp. recovered were F. equiseti, F. nygamai, F. pseudonygamai, F. proliferatum, F. subglutinans and F. polyphialidicum. The most aggressive spp. were F. equiseti, F. thapsinum and F. solani while F. proliferatum, F. subglutinans, F. Verticillioides and F. nygamai were moderately aggressive and F. oxysporum, F. polyphialidicum and F. pseudonygamai least aggressive. Population densities of Fusarium spp. in field soils and on sorghum roots were affected by genotype resistance and initial inoculum. Population densities were higher for susceptible than for resistant sorghum genotypes. A study on the effect of Fusarium spp. on plant growth and development indicated that shoot and root mass did not always correspond with the severity of the disease. Various inoculation techniques for determining susceptibility of sorghum genotypes to crown and root rot and the virulence of Fusarium species were investigated. Wounding the crown and roots and inoculating them with ground colonized oat seeds and drenching soil with a conidial suspension were both very effective. An inoculum concentration of 1 x 106conidia per ml consistently reproduced the disease on inoculated sorghum plants. Crown and root rot severity increased and plant mass decreased with an increase in inoculum concentration. Sorghum genotypes differed in their level of resistance in accordance with the inoculation techniques used. Plant age was shown to affect resistance with two and four-week-old plants being more susceptible than six-week-old plants. Sources of partial resistance to Fusarium crown and root rot were present in some genotypes. Although immunity to Fusarium crown and root rot was not found, Trichoderma harzianum induced systemic resistance in sorghum through the reduction of crown and root rot severity. Fusarium spp. can survive in a wide range of soil types. Certain soils were suppressive while others were more conducive to crown and root rot development. Soil moisture studies indicated that Fusarium spp. causing crown and root rot can survive over a wide range of soil moisture levels ranging between 25 and 100 percent. Low 25% and high 100% moisture levels were suppressive to crown and root rot severity compared to conducive at 50% and 75%. Soil fertility studies indicated that nitrogen applied at normal and high rates significantly and differentially (by genotype) increased crown and root rot severity. Soil amendments also significantly reduced crown and root rot severity with chicken manure being most effective. Studies on the efficacy of fungicides indicated that four fungicides (carboxin / thiram (Vitavax ®Plus FS), tebuconazole/triflumuron (Raxil® 015 ES), tebuconazole, (Ingwe® 6FS), difenoconazole / metalaxyl-m (Dividend ® 030 FS) and a bio-control agent (Trichoderma harzianum) significantly reduced colonization of seedlings. Fungicides also improved plant growth and development by increasing the shoot mass, root mass and enhancing root health. Major Fusarium spp. responsible for crown and root rot were F. equiseti, F. nygamai, F. oxysporum, F. proliferatum, F. pseudonygamai, F. thapsinum, F. solani, F. subglutinans, and F. verticillioides based on the isolation frequencies and pathogenicity tests results. The present study revealed the wide occurance and distribution of Fusarium spp. associated with crown and root rot of sorghum in South Africa. It is hoped that these findings may motivate more research on variation in virulence of these Fusarium spp. Secondly work on the defense mechanisms present in the sorghum genotypes widely grown in in South Africa need to be evaluated in relation to crown and root rot development. The most useful parameters for assessing the disease would be crown and root rot severity and root mass.