Phaeoisariopsis griseola (Sacc.) Ferr., the agent of angular leaf spot disease of common bean, is a highly variable pathogen for which resistance gene diversification is required. This study analysed genetic resistance to this disease within genotypes of three Phaseolus species. Twenty‐nine genotypes of Phaseolus vulgaris, Phaseolus coccineus and Phaseolus polyanthus were inoculated with 54 isolates of Phaeoisariopsis griseola. The genetic resistance was estimated according to the symptom intensity observed for each plant genotype‐pathogen isolate combination. Globally, genotypes of the common bean secondary gene pool were resistant to a higher number of isolates than common bean varieties. Interactions between plant genotypes and pathogen isolates suggested vertical resistance genes within P. vulgaris, as well as within P. coccineus and P. polyanthus. The ‘NI666’accession (P. coccineus) showed resistance to all the fungal isolates inoculated while the variety ‘Aroana’(P. vulgaris) was susceptible to most of the isolates. Interspecific hybridization between these two genotypes gave F1 hybrid plants which showed resistance to angular leaf spot disease.
Fifty four isolates of Phaeoisariopsis griseola, the agent of common bean angular leaf spot disease from the Great Lakes Region of Africa, were characterised according to their virulence behaviour and their molecular patterns. Virulence properties were revealed through the inoculation of 29 genotypes of Phaseolus vulgaris, Phaseolus coccineus and Phaseolus polyanthus. Differences in reaction types revealed high variability among these isolates. Most of them, even when collected within the same location, showed differences in their respective reactions on many plant genotypes. For molecular typing, RAPD amplifications were performed for each isolate using five random primers. Isolates with different patterns were collected within one region. Simultaneously, similar molecular patterns were found in isolates collected at different sites. However, the average of molecular similarity, based on the percentages of shared bands for each isolates pair, was higher among isolates collected within one site. No direct correlation between molecular pattern and pathotype was observed.
Samples of cassava leaves exhibiting severe symptoms of cassava mosaic disease (CMD) were collected with the PhytoPASS kit in fields surrounding the city of Bujumbura (Burundi). These materials were then sent to Belgium for polymerase chain reaction determination of the CMD begomoviruses inducing the observed symptoms. Different pairs of specific primers were used to amplify DNA sequences specific to African cassava mosaic virus (ACMV), East African cassava mosaic virus (EACMV), East African cassava mosaic Cameroon virus (EACMCV), East African cassava mosaic Malawi virus (EACMMV), East African cassava mosaic Zanzibar virus (EACMZV), the Uganda variant of East African cassava mosaic virus (EACMV‐UG) and South African cassava mosaic virus (SACMV). It was revealed that mixed infections were prevailing in the analyzed materials. Most of the samples submitted to this analysis were found to be co‐infected by three different begomoviruses (ACMV + EACMV + EACMV‐UG). The so revealed mixed infections could explain the high severity of CMD symptoms noticed on cassava in the region of Bujumbura while the diversity within the CMD causal agents illustrates the importance to take this parameter into consideration for a successful use of plant genetic resistance to control the disease.
A field experiment was designed to compare control with tissue culturederived plants of rice, and to study the effects of natural selection pressure applied to the first generation (RO) of in vitro plants on the performance of their progeny. Control-and RO in vitro plants of four rice varieties from middle altitude in Burundi, i.e. 'Facagro 57', 'Facagro 76', 'Kirundo 3' and 'Kirundo 9', were cultivated at four diflferent altitudes (800, 1380, 1650 and 1900 m). Several parameters concerning tillering capacity, plant height development and seed production were measured. The means for most measured parameters were higher in control plants than in the in vitro plants, while the higher variation coefficients and most extreme values were usually found in the in vitro plants. For each variety, the 20 control plants and 20 in vitro plants having the highest production were selected at each altitude of 1380 and 1650 m, and their progenies were cultivated on the same site at 1580 m. Among the plants arising from 1650 m, most maximum values, higher variation coefficients and, in some cases, higher means were found in the in vitro plants. In contrast, among the plants arising from 1380 m, higher means most usually corresponded to control plants, while the tendency for maximum values and variation coefficients were unclear. These results indicate the positive effect of natural selection pressure applied in the first generation to plants rising from tissue culture. They also demonstrate that considerable variation may be generated in vitro. The interest for plant breeding purposes is discussed.
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