Studies on population genetic structure and host specificity of Alternaria species causing brown spot of Minneola tangelo and rough lemon
The population genetic structure and host specificity of Alternaria species causing brown spot of Minneola tangelo (EPPO Alert List) and rough lemon (C. jambhiri) was studied in Florida. 4 Alternaria sub-populations were collected from 2 rough lemon groves and 2 Minneola tangelo groves from central Florida. Pathogenicity tests showed a high level of host specificity. The majority of Alternaria isolates from rough lemon were pathogenic to rough lemon and not to Minneola tangelo (with a few exceptions). But 44 % of isolates were non-pathogenic. Isolates from Minneola tangelo were only pathogenic to this host (never on rough lemon), and only very few (3 %) were non pathogenic. The genetic structure of the sub-populations was assessed by using RAPD analysis. Results showed a high level of genetic variation between sub-populations which could be partly related to host plants. Within each sub-populations, 2 or 3 distinct clusters of isolates could be distinguished. The authors have tried to compare their results with the new taxonomy proposed by Simmons (see EPPO RS/063). Interestingly, Alternaria limoniasperae (species most commonly found associated with rough lemon disease) and A. tangelonis (most commonly found in Minneola brown spot) correspond to 2 different clusters. A. citrimacularis (rough lemon) and A. citriarbusti (brown spot) are in one cluster containing isolates from rough lemon and Minneola tangelo. It could be said that the morphotaxonomy broadly corresponds to genetic data. However, some points of conflict exist. It has been found that the RAPD genotype of A. limoniasperae is identical to two isolates of A. alternata also included in this study (from Arachis hypogea and Datura metel). The same is observed for A. citriarbusti and two isolates described as A. citrimacularis. In addition, it was also found that an isolate of ;A. longipes (from tobacco) was very similar to A. tangelonis. It is concluded that more work is still needed on the taxonomy of Alternaria species found on citrus. However, these studies also support the view that citrus brown spot of Minneola tangelo is caused by several species of Alternaria.
Peever, T.L.; Canihos, Y.; Olsen, L.; Ibañez, A.; Liu, Y.C.; Timmer, L.W. (1999) Population genetic structure and host specificity of Alternaria spp. causing brown spot of Minneola tangelo and rough lemon in Florida.
Phytopathology, 89(10), 851-860.