**4.2. Reactions of common bean germplasm to** *F. solani* **f. sp.** *phaseoli* **under field conditions**

**4. Discussion**

of *Fusarium* biodiversity.

**4.1. Variability of** *Fusarium solani* **f. sp.** *phaseoli* **from Aguascalientes, México**

112 Fusarium - Plant Diseases, Pathogen Diversity, Genetic Diversity, Resistance and Molecular Markers

A high morphologic, pathogenic and genetic variability was found in FSP isolates from Aguascalientes, despite the identical host (common beans) and geographical origin. In addition, no relationship among morphology, pathogenicity and genotype was found. Our results indicated the high values of genetic variability in the species due to the presence of heterokaryosis and parasexualism as genetic exchange mechanisms between vegetative compatible isolates. Single members of the same vegetative compatibility group (VCGs) are genetically similar and they are related on basis of genetic lineages [17]. The characterization of VCGs on *Fusarium* isolates from Aguascalientes and other regions of México could clarify the association among *Fusarium* populations and genetic lineages. This research confirmed the diverse and heterogeneous nature on the genus. Host specialization could be useful to establish artificial taxonomic divisions and to perform pathogenic groups and *formae speciales*. However, the host plays an important biological role in selection pressure to the fungus. In addition, the genetic exchange between isolates is supported by the development of VCGs or other strategies for DNA transmission. The evolution of pathogenicity and VCGs contribute to increase in molecular variability. Further research that includes traditional and molecular methodologies will improve the knowledge and understanding

**Figure 3.** Relationship between root rot severity caused by *F. solani* f. sp. *phaseoli* and grain yield in 49 common bean genotypes under two soil moisture regimes: (a) Sandovales, Aguascalientes and (b) Chapingo, State of México, México.

The most of common bean cultivars were susceptible to most of FSP isolates, and all isolates were pathogenic to common beans. This result is opposite to Cramer et al. [18]. Most of the resistant germplasm belonged to Mesoamerica of Jalisco genetic races, while susceptible cultivars are classified as Durango race. High frequencies of resistance to other root rot pathogen (*Macrophomina phaseolina*) of common beans were found in Mesoamerican beans [19]. A high variation on reactions to FSP was found in both locations and no immunity was found, while no immunity to root rot pathogens in common bean germplasm was detected previously [8, 22, 23] in Pabellón de Arteaga, Aguascalientes and Chapingo, State of México. No clear association between root rot severity and seed yield or phenology was found in all experiments. However, results indicated that resistance to FSP was more frequent on black beans, while susceptibility was common on pinto beans, which has been found in previous works [20, 22]. Results suggest that black beans from Mesoamerican race could provide resistance to FSP in México. Under rainfed conditions, genotypes as BAT 477 and SEA 20 stood out for their high seed yields and resistance to root rot pathogens. BAT 477 has showed a consistent resistance to root rot pathogens such as *Fusarium*, *Rhizoctonia and Macrophomina* [20, 22, 24].

In both locations, rainfed conditions reduced seed yields and increased root rot severity in common bean germplasm. Navarrete-Maya et al. [23] reported a positive relationship between rain precipitation and *Fusarium* severity in Chapingo. We suggest that low water availability increased physiological stress in the host. Therefore, host defense mechanisms are not efficient to arrest fungal infection or for slow pathogenesis. The relationship between root rot severity and grain yield exhibited different patterns, since a broad variation on root rot severity on the germplasm (Sandovales) or an opposite response (Chapingo). Opposite patterns in grain yields were found, the highest seed yields were found in Chapingo and the lowest in Sandovales. Our data suggested that climate and fungi conditions of Sandovales are more appropriate for common bean germplasm screening for resistance to root rot pathogens under field conditions than Chapingo.
