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

[摘要] English: This study investigated the interaction between grain sorghum and Fusariumspp. associated with the crop in field soils and on sorghum roots as causal agents ofcrown and root rot. The effect of disease on plant growth and development wasinvestigated as was the efficiency of various inoculation techniques on disease severity.Factors affecting disease susceptibility such as plant age, soil type, soil moisture, soilpH, soil fertility and possible chemical and bio-control control tactics were alsoinvestigated.Fusarium spp. isolated from sorghum roots and field soils at Cedara (Kwazulu-Natal province), Bethlehem (Free State province) and Potchefstroom (North Westprovince) indicated that F. oxysporum, F. solani, F. verticillioides and F. thapsinumwere 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. solaniwhile F. proliferatum, F. subglutinans, F. Verticillioides and F. nygamai weremoderately aggressive and F. oxysporum, F. polyphialidicum and F. pseudonygamaileast aggressive.Population densities of Fusarium spp. in field soils and on sorghum roots wereaffected by genotype resistance and initial inoculum. Population densities were higherfor susceptible than for resistant sorghum genotypes. A study on the effect of Fusariumspp. on plant growth and development indicated that shoot and root mass did not alwayscorrespond with the severity of the disease.Various inoculation techniques for determining susceptibility of sorghumgenotypes to crown and root rot and the virulence of Fusarium species wereinvestigated. Wounding the crown and roots and inoculating them with groundcolonized oat seeds and drenching soil with a conidial suspension were both veryeffective. An inoculum concentration of 1 x 106conidia per ml consistently reproducedthe disease on inoculated sorghum plants. Crown and root rot severity increased andplant mass decreased with an increase in inoculum concentration. Sorghum genotypesdiffered 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 plantsbeing more susceptible than six-week-old plants. Sources of partial resistance toFusarium crown and root rot were present in some genotypes. Although immunity toFusarium crown and root rot was not found, Trichoderma harzianum induced systemicresistance in sorghum through the reduction of crown and root rot severity.Fusarium spp. can survive in a wide range of soil types. Certain soils weresuppressive while others were more conducive to crown and root rot development. Soilmoisture studies indicated that Fusarium spp. causing crown and root rot can surviveover a wide range of soil moisture levels ranging between 25 and 100 percent. Low25% and high 100% moisture levels were suppressive to crown and root rot severitycompared to conducive at 50% and 75%.Soil fertility studies indicated that nitrogen applied at normal and high ratessignificantly and differentially (by genotype) increased crown and root rot severity.Soil amendments also significantly reduced crown and root rot severity with chickenmanure 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-controlagent (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 androot rot were F. equiseti, F. nygamai, F. oxysporum, F. proliferatum, F.pseudonygamai, F. thapsinum, F. solani, F. subglutinans, and F. verticillioides basedon the isolation frequencies and pathogenicity tests results.The present study revealed the wide occurance and distribution of Fusariumspp. associated with crown and root rot of sorghum in South Africa. It is hoped thatthese findings may motivate more research on variation in virulence of these Fusariumspp. Secondly work on the defense mechanisms present in the sorghum genotypeswidely grown in in South Africa need to be evaluated in relation to crown and root rotdevelopment. The most useful parameters for assessing the disease would be crownand root rot severity and root mass.

[发布机构] University of the Free State