**3.3. Genetic basis of resistance to root rot pathogens in selected common bean cultivars**

**Experiment III.** As found in E-II, no clear relationship between root rot severity and seed yield was detected in this experiment (**Figure 2b**; **Table 5**). All 36 genotypes showed grain yields ranged from 550 to 1100 kg h−1. Here, we found that most of germplasm showed a root rot severity ranged from 2 to 5.5 (intermediate), while grain yield ranged from 550 to 1000 hg h−1. Only three cultivars were clearly different from all other cultivars: BAT 477 (that showed the highest seed yields), SEA 20 (that exhibited the lowest root rot severity) and Flor de Mayo 2000 (that showed the highest root rot severity). No differences can be appreciated on days to flowering or days to maturity or seed yield between resistant and susceptible cultivars (**Table 4**). **Rainfed-irrigated experiment.** In Chapingo, germplasm showed later biological cycle and greater seed yields and root rot severity than at Sandovales. In both locations, rainfed conditions reduced seed yields and increased root rot severity (**Table 6**). In this experiment, negative relationship between seed yield and root rot severity was more clear than other experiments and a positive association was found between seed yield and days to flowering and flowering and root rot severity (**Table 5**). The relationship between root rot severity and grain yield exhibited different patterns across locations. In Sandovales, we found a greater variation on root rot severity on the germplasm, while an opposite response was found at Chapingo.

**Figure 2.** Relationship between root rot severity caused by *F. solani* f. sp. *phaseoli* and grain yield in common beans: (a)

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

experiment II and (b) experiment III.

Genetic analysis identified one QTL significantly associated with resistance to FSP in BAT 477 growing under controlled conditions. This QTL explained 2.7% of variation in response to the disease and the marker was found at LG 5 [16].

Results suggest that resistance to root rot pathogens in common beans could be operating as a resistance gene cluster that controls similar strategies to defend roots and stems against root rot fungi. Further research could confirm this suggestion. No clear association between host and fungus genotypes was found; this relation was reported in *M. phaseolina-*common beans [20] in contrast with other biotrophic pathogens of common bean as *Colletotrichum lindemuthianum* [21], where a clear formation of genetic lineages based on geographical origin

Analysis of *Fusarium*-Common Beans Pathosystem in Aguascalientes, Mexico

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

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* 

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

**4.3. Genetics of resistance to** *Fusarium solani* **f. sp.** *phaseoli* **in common bean cv.** 

Genetic analysis identified one QTL significantly associated with resistance to FSP in BAT 477 growing under controlled conditions. This QTL explained 2.7% of variation in response to the disease and the marker was found at LG 5 [16]. Identification of few QTLs with high effects on explanation of phenotypic variation is important and promising to simplify the

was found.

**conditions**

*Macrophomina* [20, 22, 24].

under field conditions than Chapingo.

**BAT 477**

**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.

#### **4. Discussion**

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

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 of *Fusarium* biodiversity.

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]. Results suggest that resistance to root rot pathogens in common beans could be operating as a resistance gene cluster that controls similar strategies to defend roots and stems against root rot fungi. Further research could confirm this suggestion. No clear association between host and fungus genotypes was found; this relation was reported in *M. phaseolina-*common beans [20] in contrast with other biotrophic pathogens of common bean as *Colletotrichum lindemuthianum* [21], where a clear formation of genetic lineages based on geographical origin was found.
