Populations of 90 Uromyces appendiculatus isolates were collected from throughout the Americas and evaluated for virulence on 19 standard bean rust differentials, and also on 12 landraces of Phaseolus vulgaris from South and Central America. The landrace differentials represented geographical centers of bean domestication. Three groups were observed. Two groups were isolates from centers of bean domestication and a third heterogeneous group comprised isolates from countries in South and Central America. Molecular analysis using random amplified polymorphic DNA-polymerase chain reaction (RAPD-PCR) was also conducted on these isolates. Cluster analysis of the molecular profiles showed three groups that corresponded to those obtained by virulence tests. These results show a clear differentiation of the pathogen population along similar lines as its host and suggest parallel evolution in the bean rust pathosystem.
Loss of varietal diversity is a worldwide challenge to crop species at risk for genetic erosion, while the loss of biological resources may hinder future breeding objectives. Loss of varieties has been mostly investigated in traditional agricultural systems where variety numbers are dramatically high, or for most economically important crop species for which comparison between pre-intensive and modern agriculture was possible. Varietal dynamics, i.e., turnover, or gains and losses of varieties by farmers, is nevertheless more rarely studied and while we currently have good estimates of genetic or varietal diversity for most crop species, we have less information as to how on farm agro-diversity changes and what cause its dynamics. We therefore investigated varietal dynamics in the agricultural yam system in the Caribbean island of Guadeloupe. We interviewed producers about varieties they cultivated in the past compared to their current varieties, in addition to characterizing yam cropping characteristics and both farm level and producers socio-economic features. We then used regression tree analyses to investigate the components of yam agro-diversity, varietal dynamics and impact of anthracnose on varieties. Our data demonstrated that no dramatic loss of varieties occurred within the last decades. Cultivation changes mostly affected widespread cultivars while frequency of uncommon varieties stayed relatively stable. Varietal dynamics nevertheless followed sub-regional patterns, and socio-economic influences such as producer age or farm crop diversity. Recurrent anthracnose epidemics since the 1970s did not alter varietal dynamics strongly, but sometimes translated into transition from Dioscorea alata to less susceptible species or into a decrease of yam cultivation. Factors affecting changes in agro-diversity were not relating to agronomy in our study, and surprisingly there were different processes delineating short term from long term varietal dynamics, independently of disease risk. Our results highlighted the importance of understanding varietal dynamics, an often overlooked component of agriculture sustainability, in addition to evolutionary forces shaping agro-diversity and genetic diversity distribution within crops. It is also crucial to understand how processes involved do scale up worldwide and for different crop species, so as not to mislead on-farm conservation efforts and efficacy of agro-diversity preservation.
Cassava (Manihot esculenta Crantz) is an increasingly important tropical root crop. It is however affected by an endemic disease in the Caribbean called Super-elongation caused by the fungus Sphaceloma manihoticola. Traditional methods of detection and laboratory confirmation of S. manihoticola infection depend on the observation of seasonally variable stem elongation symptoms, and confirmatory fungal growth of cultures that can take up to four weeks, due to the slow-growing nature of the pathogen. The elongation symptoms observed are due to the production of large amounts of gibberellin GA 4 by the pathogen. In this study, specific PCR primers were developed to target the Smp450-2 gene (Gen Bank Acession AM 886290), in the gibberellin biosynthesis gene cluster of S. manihoticola. Shadehouse plants were inoculated with a suspension of S. manihoticola and DNA extracted over a 28-day period, followed by PCR amplification using the Smp450-2 primers. Detection of S. manihoticola in infected samples was possible 7 days post-infection in at least one local cassava variety, whereas visible disease symptoms appeared after 21 days. This PCR based method was also able to detect the pathogen in leaves and cankers of diseased field tissues, but not in disease-free material from either the laboratory or cassava fields. A new approach for the detection of S. manihoticola causing Super-elongation disease in cassava is therefore presented, and its potential use as a diagnostic tool for early detection of this disease is explored.
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