The resistance of grain sorghum to the root rot pathogen complex

[摘要] English: Crop losses are estimated at 30% for sorghum annually as a result of invasion of pathogens or pests. Root rot is usually the result of a complex of soilborne fungi such as Fusarium spp., Pythium spp., Macrophomina phaseolina, Colletotrichum graminicola and Periconia circinata and other fungi. Colonization of host plant tissues are dependant on the environment that favours a specific pathogen at a particular time and host predisposition. Fungi were isolated and tested for pathogenicity on two sorghum cultivars. Root discolouration as a measure of root infection indicated a range of host responses, from severe root rot to some isolates that suppressed root rot and stimulated host growth. Isolate Bet9-3 (unidentified) resulted in the highest percentage root rot on both PAN 8420 and PAN 8706W. In most cases an Isolate x Cultivar interaction was recorded indicating that cultivars differ in their responses to pathogens although overall, PAN 8706W was more resistant to root rot than PAN 8420. All the isolates were identified by means of sequencing. A number of isolates that proved capable of causing severe root damage could not be identified suggesting that species, not previously identified, may be associated with root rots locally. Similarly, a number of species were isolated that have not previously been recorded on sorghum. Ergosterol concentrations in sorghum roots were used as a measure of root colonization by the test isolates. Isolate Bet26-4 (Acremonium strictum) resulted in the highest ergosterol level but with a moderate root rot discolouration. This disparity between the two evaluation criteria has resulted in the reliability of the evaluation criteria generally used to evaluate root rot, being questioned. Root rot severity, flag leaf length, total plant length, root volume and effective root volume of 26 sorghum cultivars from the National Cultivar Trials, planted in a naturally infested field trial at Cedara, were measured. All the cultivars were susceptible to root rot with PAN 8389 being the most susceptible (45.60%) and PAN 8534 the most resistant (29.23%). No significant relationship between root rot, root volume or effective root volume and plant growth parameters was recorded. A study using root extracts from the different cultivars showed only limited suppression of test fungal growth. Similarly, extractable phenol content from roots showed a tendency with reduced root rot severity in the field but this relationship was not significant. Some of the highest phenol content was observed in roots of PAN 8706W and PAN 8648W (white tan cultivars) and these cultivars had the second and third lowest root rot severities respectively. The high phenol content in roots of these cultivars is contrary to the low phenol contents normally reported in white sorghums and further studies of root physiology in relation to root rot resistance, as well as other mechanisms of root rot resistance, are warranted. The negative effects and risks associated with the use of agricultural chemicals have led to the search and discovery of new methods of disease control in plants. The potential of Trichoderma spp. associated with field sorghum as biological control agents was considered. Trichoderma spp. isolated from sorghum roots were tested for their effect on the growth of root sorghum isolates. In vitro inhibition of these isolates of up to 80 % was recorded due to modes of action including direct colonization and the production of volatile and non-volatile substances by Trichoderma spp. Trichoderma spp. differed in their ability to colonize sorghum seed, rhizospheres and roots. In a greenhouse study, in soil infested with sorghum root pathogens, Ced3-1 (T. hamatum) resulted in the lowest root rot severity while Kl23-1 (T. virens) had the least suppressive effect on disease development. Although Trichoderma spp. differed in their efficacy to suppress root rot, none was sufficiently so to be of significant commercial value.

[发布机构] University of the Free State