Identification and characterization of colletotrichum species associated with mango and citrus diseases in the Ashanti region of Ghana

The purposes of the study were to identify Colletotrichum species, detect their variability, to investigate their cross infectivity potential and determine farmers' awareness about the diseases that are associated with mango and citrus. Culture characteristics, size and shape of conidia and mycelial growth rates were used to characterize the isolates into different morphological groups. Those grouped isolates were further studied for their genetic similarity and differences, using multigene loci primers and restriction enzymes. ITS region, β- tub gene and GPDH primers were also used to characterize the isolates. Farmers were aware of mango anthracnose but did not know any such disease on citrus. Colletotrichum species isolated from mango and citrus had overlap in their cultural characteristics. All of the isolates were assigned into ten morphogroups. Seven of the morphogroups were characterized by their cylindrical conidia and three of the morphogroups (G1, G3 and G6) had curved conidia. Apart from the conidial shape, most of the morphogroups had unique characters to stand as a group on their own. However, G2, G5 and G9 showed overlap in many of their characters. The results from molecular analysis were different, depending on the gene/region targeted. The ITS region amplification showed high genetic similarity among the ten morphogroups, irrespective of their conidial or other cultural characteristic variations. Further investigation on the same ITS region, using Colletotrichum species-specific primer, CgInt, confirmed the similarity for six of the morphogroups such as (G1, G2, G4, G5, G8 and G9) but the other four morphogroups were not picked. The overall results of the ITS region were very helpful in understanding the close relationship of the different morphogroups. Further, analysis of the same group of isolates, using β-tubulin primer, clearly showed existence of genetic variability in between the groups. The variability expressed on β-tub gene analysis was in line with the characterization made, based on conidial shape. It gave two distinct groups; one group having cylindrical conidia and the other having curved conidia. This result is an indicator that conidial shape is governed by a genetic factor. The third gene, GPDH analysis gave high genetic variability results. It amplified nine distinct groups from the ten morphogroups studied. Eight of them were in line with eight of the morphogroups. Therefore, GPDH results were more detailed than the other studied regions of Colletotrichum species in showing genetic variability. However, the combined results of the morphological characterization and the multigene analyses played their own role for a better understanding of species variability. The pathogenicity tests on mango and citrus fruits confirmed most of the morphogroups as pathogenic. However, all the curved morphogroups isolated from diseased mango tested negative on mango. The cross-infectivity potential of citrus and mango isolates of Colletotrichum were confirmed. The cross tests on papaya also showed that most of the morphogroups were pathogenic. From the study, it is understood that critical investigation of Colletotrichum spp., using the traditional techniques of morphological characterization, can assist in determining the genetic variability among isolates but it is tedious, time consuming and wrong conclusions could be drawn if it is first time study. Once the organisms are sequenced and named, the conventional investigation could play a great role to detect the variability. Therefore, it is recommended that the identified groups of isolates be sequenced for the purpose of accurately naming them to the species level.

Source: KNUST