| dc.description.abstract | Black dot is a blemish disease of potato caused by the fungus Colletotrichum coccodes
(Wallr.) Hughes. with worldwide distribution. It occurs in Africa, Asia, Australia, Europe,
North America, South America, and Central America. C. coccodes has no known sexual
cycle, but genetic exchange is possible through vegetative conjugation. The determination
of VCG among fungal isolates is usually based on the complementation tests with nitrate
nonutilizing (nit) mutants which enables researchers to compare strains of pathogens. Eight
VCGs have been identified for the Europe/Israel population, seven for the North American
population, and six for the Australian population. Variation in aggressiveness,
morphological traits, and physiological traits have been detected among the different C.
coccodes VCGs. AFLP markers have been used also to study relationships within and
among North American VCGs. With this method, five VCGs were separated from C.
coccodes isolates, coinciding almost completely with the six VCGs of nit mutants.
Additionally, a relationship of specific AFLP bands to corresponding VCGs was reported
using AFLP analysis. The objectives of this study were: to develop VCG-specific sequence
characterized amplified regions (SCAR) markers, to study the population biology of C.
coccodes of North America using the AFLP method, and to study genetic diversity of the
global populations of C. coccodes. A total of 88 isolates representing the different C.
coccodes VCGs were analyzed via the AFLP method to generate SCAR markers. A total of
47 primers were designed and evaluated in PCR reactions. Only one primer, AGb6F/R,
with 156 bp amplification PCR product was found to be specific for NA-VCG6 and NA V CG 7 of C. coccodes. For the second objective, 210 loci were generated and used to
cluster the isolates into their NA-VCGs and to test the genetic structure of the North
American population of C. coccodes. C. coccodes isolates recovered from potato plants
were assigned to fourNA-VCGs: NA-VCGl, NA-VCG2, NA-VCG4/5, and NA-VCG6/7.
No isolates tested belonged to NA-VCG3. NA-VCG2 was the dominant group in the
population (n=238) and was the most frequently detected NA-VCG among states, fields,
farms, and plants. However, in several instances there was more than one NA-VCG
recovered from the same plant, field, farm, and state, indicating variability within the C.
coccodes population in United States. Genetic differentiation among the nine states was
0.331, and overall gene flow for the 366 isolates was (Nm =1.01). For the genetic diversity
of the global C. coccodes population, 855 isolates were studied. Three primer pairs were
used and generated 210 loci. Based on this study, the population of C. coccodes exists as
one large population with four main groups (NA-VCGl/3; NA-VCG2; NA-VCG4/5; and
NA-VCG6/7). NA-VCG5 was the most common VCG globally followed by NA-VCG2.
Among the five regions studied, there was relatively low gene diversity (0.222). The
overall gene flow (Nm) was 1.25, meaning that one or more individuals are exchanged
among the five regions each generation and that the populations will gradually become
similar. Most of the variation among the five geographic regions originated from within
population differentiation. Among VCGs of C. coccodes, there was high VCG
differentiation (GsT=0.463), meaning there is a differentiation among the different C.
coccodes VCGs and a population structure exists. AFLP analysis proved to be valuable in
differentiating and studying the global population of C. coccodes. | en_US |
