Characterization of turcicum leaf blight epidermics and pathogen populations in the exserohilum turcicum-- sorghum pathosystem of Uganda
Turcicum Leaf Blight (TLB) of sorghum incited by Exserohilum turcicum is a major threat to sorghum production globally. This pathogen has been reported to attack both maize and sorghum resulting in yield losses as high as 70% on susceptible cultivars. Studies so far conducted on TLB have mainly focused on maize host. Little or no research has been conducted yet to try to understand the epidemiology and population structure of TLB in Exserohilum turcicum-sorghum pathosystem. This aspect is very important in the development of long lasting management practice. This study was therefore designed to (a) investigate the occurrence of TLB in major sorghum growing regions, (b) characterise sorghum accessions for resistance to TLB, (C) carry out epidemiological studies on temporal and spatial attributes of TLB in sorghum, (d) determine the mating types and races of E. turcicum that occur in Uganda as well as (e) establish variability of sorghum derived E. turcicum isolates in Uganda. Surveys were conducted in eight major sorghum growing agro ecologies of Uganda in order to establish the occurrence of TLB in these regions. During the survey, sorghum fields were sampled for TLB incidence and severity. Diseased leaf samples from over 200 sorghum accessions were collected from farmers’ field for further studies in the field and screen house experiment. To understand the epidemiology aspect of TLB in sorghum, the collected accessions were planted in the field and challenged with Exserohilum turcicum isolates. The varieties that showed resistance were selected and further challenged with Exserohilum turcicum isolates at different locations to confirm their resistance. To establish the population structure of this pathogen, from the isolates collected from different parts in Uganda. Exserohilum turcicum isolate DNA was subjected to PCR using selective markers for mating types and genetic variability. The pathogen isolates were also characterised using race differentials. The survey study found out that TLB occurred in all agro-ecologies, albeit at lower levels (20-40%) than in maize (60-100%). Disease severity ranged from 24.6 to 37.8 % and were quite low on land races grown by farmers, while incidence values ranged from as high as 100% in Pallisa to as low as 20% in Tororo. The effect of agro-ecology on TLB epidemics was highly significant (P≤0.05). Areas with high levels of humid and moderate temperature had the highest severity and incidence. Additionally cropping systems significantly influenced the patterns of spread of TLB across the region, suggesting that the upsurge in the TLB on sorghum was highly influenced by wide-spread use of susceptible genotypes such as Epuripuri and Sekedo. Indeed among the 202 sorghum accessions collected from farmer’s fields, the majority (over 99%) were resistant to TLB. The fact that most land races grown by farmers had moderate to high level of resistance to TLB suggests that they could be used in sorghum breeding programmes to improve resistance to TLB in the improved cultivars such as Epuripuri and Sekedo. The study on temporal aspect of TLB epidemics indicated that disease development in sorghum was delayed by about three weeks compared to maize. The data further indicated that the severity and incidence of TLB on sorghum were quite low as compared to maize, suggesting possible role of physiological adaptation of E.turcicum on maize than on sorghum. Rates of disease development were relatively low among sorghum accessions and high in maize. The data indicated that the nature of resistance being expressed by sorghum is quite different from that of maize. It also further presupposes the role of pathogen physiological specialization. The analysis of spatial attributes of TLB epidemics in sorghum showed that disease severity and gradients were affected by wind drift. Differences in crop residues where significant in plants close to crop residue than plant found far away from the residue source. The above results where more pronounced in Epuripuri a susceptible plant variety as compared to selected sorghum accession MUC/007/009 and MUC/007/029. In this study flattening of the disease gradient was observed confirming the polycyclic nature of TLB of sorghum. Mating type analysis revealed the occurrence of MAT 1, MAT 2 and MAT 1, 2 in Uganda on sorghum. Furthermore in Soroti, mating types MAT 1 and MAT 2 were found to occur in equal proportions indicating a great potential of sexual recombination with a possibility of the emergence of new races of E.turcicum. In other locations MAT 2 was more common that MAT 1. The race differential study revealed occurrence of races 0, 13, 2, and 3 in Uganda. Both the mating type and race differential study suggest a great potential of having more virulent races of E. turcicum in the future. Also, the genetic variability studies based on molecular tools indicated high genotypic diversity among the sorghum derived E.turcicum isolates. However, the similarity coefficient was high (0.64) indicating a low genetic variation among sorghum E.turcicum isolates collected from Uganda. A high similarity coefficient presupposes the occurrence of gene flow between populations. Therefore for disease management, the data presented here suggests that if no improvements are made on the elite released sorghum lines such as Epuripuri an epidemic is bound to happen. The variety screening study suggests that farmers grown varieties that might have alternative sources of resistances for TLB breeding. The epidemiology studies imply that the resistance mechanisms exhibited by sorghum differ from that of maize. This work further suggests that management practices should focus on reducing crop residue and use of resistant crop varieties.