Ashy stem blight (ASB) caused by the fungus Macrophomina phaseolina (Tassi) Goidanich is an important disease in the common bean (Phaseolus vulgaris L.) in tropical and subtropical regions in the Americas and elsewhere (Singh and Schwartz, 2010; Ambachew et al., 2021). This plant pathogen affects roots and all aerial plant parts causing damping off, root rots, stem blight, leaf burning, and premature senescence symptoms (Islam et al., 2012; Kaur et al., 2012; Marquez et al., 2021). Yield losses over 60% in susceptible common bean genotypes were reported (Mayek et al., 2003; Kaur et al., 2012; Viteri and Linares, 2022). The fungus is endemic and seed-transmitted, and its sclerotia can survive in the soil up to 15 years (Marquez et al., 2021). Furthermore, different levels of aggressiveness between M. phaseolina isolates were reported (Miklas et al., 1998a; Mayek et al., 2001a; Viteri and Linares, 2017). Disease avoidance mechanisms such as genotypes with upright growth habit may prevent severe ASB infections in the field (Viteri and Linares, 2022). However, the use of germplasm with higher levels of resistance is crucial, since fungicide applications do not provide an adequate control for this pathogen (Singh and Schwartz, 2010; Marquez et al., 2021).

Low to higher levels of ASB resistance have been reported in the common bean and tepary bean (Phaseolus acutifolius A. Gray) (Miklas et al., 1998a; Mayek et al., 2001b; Pastor-Corrales and Abawi, 1988; Viteri and Linares, 2017). Furthermore, the resistance to ASB can be inherited qualitatively or quantitatively depending of the genotypes used, plant organ evaluated, environment, virulence of the isolate, and/or methodology of the screening used (Olaya et al., 1996; Miklas et al., 1998b; Viteri and Linares, 2019). For instance, common bean genotypes A 195, ‘Badillo,’ NY6020-4, PRA154, and PRA155 were reported with partial ASB resistance (Viteri and Linares, 2017). Likewise, BAT breeding lines (BL) (e.g., BAT 85, 332, 477, 1651), black common bean genotypes, ‘Negro Tacaná,’ ‘Negro Perla,’ TARSMST-1, and XAN 176, red-mottled cultivars ‘PC 50’ and ‘San Cristobal 83,’ and landraces from the Dominican Republic were reported to have intermediate and higher levels of resistance (Echavez-Badel and Beaver, 1987; Pastor-Corrales and Abawi, 1988; Viteri and Linares, 2017). Also, tepary bean accessions Mex-114, PI 440806, and PI 321637 were reported with higher levels of ASB resistance (Miklas et al., 1998a). Within the common bean gene pool, five quantitative trait loci (QTL) located on chromosomes Pv04, Pv06, Pv07, and Pv08 and derived from XAN 176 conferred field resistance in the Dorado/XAN 176 common bean population (Miklas et al., 1998b). Likewise, Mayek et al. (2009) and Méndez et al. (2017) identified that two complementary genes and nine QTL on Pv03, Pv05, Pv06, Pv08, Pv09, and Pv10 chromosomes derived from BAT 477 provided field resistance in the UI-114/BAT 477 recombinant inbred line population. Conversely, two complementary genes, namely, Mp-1 and Mp-2, conferred resistance in the A-70/BAT 477 cross in a controlled environment, and these genes were also derived from BAT 477 (Olaya et al., 1996). More recently, Viteri and Linares (2019) reported that two complementary recessive genes and one recessive gene conferred resistance to ASB in the PC 50/’Othello’ and Badillo/PR1144-5 populations, respectively, in the greenhouse by the cut-stem method. These genes were apparently derived from Andean genotypes Badillo and PC 50. In the same study, one dominant gene conferred resistance in the A 195/PC 50 population.

Breeding for ASB resistance has been limited compared to other necrotrophic fungi such as Sclerotinia sclerotirum L. de Bary (the causal agent of white mold) from which diverse germplasms have been released in the last years (Schwartz and Singh, 2013; Singh et al., 2016; Singh et al., 2017). The cream-colored BAT 477 BL and Negro Tacaná and red-mottled San Cristobal 83 cultivars were the only genotypes used to develop the common bean germplasms with resistance to soilborne fungi, including M. phaseolina, and drought and heat tolerance. For example, SEA 3 and SEA 5 BL were developed from the multiple parent crosses BAT 477/San Cristobal 83//BAT 93/‘Jalo EEP 558’ and BAT 477/San Cristobal 83//‘Guanajuato 31’/‘Rio Tibagi,’ respectively (Singh et al., 2001). Likewise, Porch et al. (2012) developed the TARS-MST1 black common bean with ASB resistance from the single cross Negro Tacaná/VAX 6. However, the development of a new germplasm combining genotypes from the Andean (e.g., A 195, PC 50, PRA154, and PRA155) and Middle American races of the common bean is necessary to increase the levels of ASB resistance. The objectives of this research were to 1) develop BL with improved resistance to ASB and 2) compare the response of the newly developed BL with known sources of resistance.

Mean squares for the genotype and the interaction genotype × location were significant (P ≤ 0.05) for the ASB severity in the greenhouse and the field. Conversely, there were no significant differences (P ≥ 0.05) for location for the two environments ( Table 1 ). Variances for the data analyzed from the two locations were not homogeneous in the greenhouse (χ² = 16.16; P ≤ 0.0001) and the field (χ² = 15.31; P ≤ 0.0001), and therefore, the data for the ASB severity were presented separately in each location.

Resistant genotypes (scores ≤3) were not identified in this study. In fact, the disease range in each genotype varied from resistant to susceptible plants ( Table 2 ). Nonetheless, the UPR-Mp-22, UPR-Mp-34, PRA154, and PRA155 genotypes had >35% of resistant plants to PRI21 M. phaseolina isolate in the greenhouse in Isabela and Lajas ( Table 2 ). In contrast, UPR-Mp-22 reached close to 30% of resistant plants in the field in both locations, while UPR-Mp-42 and PRA155 had >55% of resistant plants in the field evaluation in Lajas ( Table 2 ).

With regard to the ASB severity, UPR-Mp-42 and UPR-Mp-48 had an intermediate response (4–6) compared to white common bean genotypes Bella, NY6020-4, and Verano and black bean TARS-MST1 that were susceptible (scores ≥7) in all environments. XAN 176 had an intermediate response in the evaluations conducted in Isabela; however, it was susceptible in Lajas ( Table 3 ). The overall mean scores of the BL UPR-Mp-22 and Andean genotypes A 195, PRA154, and PRA155 were intermediate in all environments. In contrast, UPR-Mp-34 had significantly lower scores than BAT 477 that had a susceptible reaction in the greenhouse in Isabela and in the field in Lajas and SEA 5 that was susceptible in all environments ( Table 3 ).

Genetics and breeding for resistance to ASB disease have been conducted for more than 20 years in the United States and worldwide (Olaya et al., 1996; Miklas et al., 1998b; Mayek et al., 2001b; Méndez et al., 2017; Pastor-Corrales and Abawi, 1988; Viteri and Linares, 2019). However, common bean BL and cultivars with enhanced levels of resistance are not yet available. One of the reasons may be because the breeding against ASB has been focused only on Middle American beans that possess tolerance to heat and drought (Urrea et al., 2009; Beebe et al., 2013; Ambachew et al., 2021) and the severity of M. phaseolina increases in these environments (Frahm et al., 2004; Mayek et al., 2002). In fact, BAT 477 and Negro Tacaná were used as donor parents to improve the resistance to M. phaseolina and tolerance to the aforementioned abiotic stresses in the SEA 5 and TARS-MST1 BL, respectively (Singh et al., 2001; Porch et al., 2012). Unfortunately, these two genotypes were susceptible to ASB in this research and a previous study using mechanical inoculations (Viteri et al., 2019). Thus, QTL associated with tolerance to heat and drought may not necessarily provide or have a major effect against M. phaseolina. Further research is necessary to tag and identify new resistant QTL in Andean genotypes as the new QTL on Pv07 and Pv09 chromosomes derived from PRA154 and PC 50, respectively (Viteri, non-published information). Furthermore, the presence of avoidance mechanisms in some genotypes (e.g., TARS-MST1, Bella, and Verano) (Viteri and Linares, 2022) and the uneven dispersion of M. phaseolina inoculum in the field (Echavez-Badel and Beaver, 1987) may cause a lower ASB severity or an escape under natural infections and, thus, a misidentification of resistant genotypes.

Our emphasis in this study was to obtain an enhanced ASB resistance in BL developed from multiple parent crosses between the common bean races combining the pedigree and gamete selection techniques (Singh, 1994). In fact, our results and previous studies (Viteri and Linares, 2017; Viteri and Linares, 2019) showed that the Andean beans had higher levels of resistance than Middle American genotypes. Furthermore, the improved resistance in the new BL (mostly derived from the Andean race) was significant compared with their counterparts of the same market classes. For instance, the genetic gain for ASB resistance of the new UPR-Mp-34 BL (A 195/PC 50//SEA 5) varied from 27% to 33% compared to BAT 477 and SEA 5, respectively, in the field, while it was close to 42% in the greenhouse. Likewise, UPR-Mp-42 reached a genetic gain for resistance to M. phaseolina close to 23% compared with XAN 176 in both environments, while these values increased to 32% and 38% compared to TARS-MST1 in the greenhouse and the field, respectively. Within the white beans, the most important market class in Puerto Rico (Beaver et al., 2020), UPR-Mp-48 (BAT 477/NY6020-4//PRA154), had a genetic gain for resistance to this fungus between 42% and 50% in the greenhouse and 32% in the field than cultivars Bella and Verano. To the best of our knowledge, UPR-Mp-48 is the first white bean germplasm with intermediate resistance to ASB and determinate growth habit Type I (Singh, 1982). Both of these traits should be introgressed into Bella cultivar that possesses indeterminate growth habit Type II and resistance to the Bean common mosaic virus, Bean common mosaic necrosis virus, Bean golden yellow mosaic virus, common bacterial blight [caused by Xanthomonas campestris pv. phaseoli Smith (Dye)], and web blight [caused by the fungus Thanatephorus cucumeris (Frank). Donk (anamorph: Rhizoctonia solani Kühn)] (Beaver et al., 2018). With this, new cultivars for mechanical harvesting and with resistance to multiple diseases can be developed. Likewise, UPR-Mp-42 may be used to incorporate genes/QTL for ASB resistance and determinate growth habit Type I (Singh, 1982) into susceptible black bean genotypes (e.g., ‘Zorro’; Viteri and Linares, 2017) or TARS-MST1 that also possesses partial resistance to common bacterial blight (Kelly et al., 2009; Porch et al., 2012).

With regard to the crosses between Andean common beans, there was not a significant genetic gain for resistance of UPR-Mp-22 (A 195/PC 50//PRA155) compared to the reaction of its parents and PRA154 separately; in general, an intermediate response was observed. However, the developed UPR-Mp-22 belongs to the cranberry type, considered an important commercial market class in Canada and the United States (Miklas and Riley, 2012). Thus, this BL may be used to improve susceptible cultivars in this market class. Moreover, a new set of crosses between UPR-Mp-22 and the Andean beans Badillo, PRA154, and VA 19 reported with partial resistance to ASB (Viteri and Linares, 2017; Viteri and Linares, 2019; Viteri et al., 2019) may increase the levels of ASB resistance. Likewise, multiple crosses with the UPR-Mp BL would be recommended to incorporate more desirable resistant genes/QTL for the development of new germplasms in the near future. Singh et al. (2017) incorporated higher levels of resistance (scores <3) to white mold in PRA155 by the use of six parents from Andean and Middle American beans (50% of genetic contribution) crossed with A 195 (50% of the remaining contribution).

Since the levels of pathogenicity between M. phaseolina isolates can vary between countries/regions (Marquez et al., 2021) and even in the same country [e.g., Mexico (Mayek et al., 2001a) and Puerto Rico (Miklas et al., 1998a; Viteri and Linares, 2017)], future studies should be focused on the screening of the UPR-Mp BL against multiple isolates. This would help to identify if these BL have any specific or broad spectrum of durable partial resistance. For example, Singh et al. (2014) and Viteri et al. (2015) identified that the common bean BL SE152-6, SE153-7, and SE155-9 were resistant during the entire reproductive stage to ARS12D, CO463, ND710, and NY133 S. sclerotiorum isolates from Argentina and the United States using the same methodology reported in this study. Furthermore, the expression profiling of resistant genes/QTL at different times after the inoculation of M. phaseolina isolates should be an important topic to explore in future research.

In summary, the UPR-Mp BL had an intermediate response to ASB in the field and the greenhouse. These BL may be used to improve black, cranberry, and white common bean germplasms (which are in the 10 market classes that dominate the common bean production in the United States; Miklas and Riley, 2012) and/or other market classes.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

DV: Supervision, conceptualization, data collecting, methodology, investigation, project administration, review and editing, validation, writing-original draft. AL: Data collecting, methodology, supervision, resources, review and editing. All authors contributed to the article and approved the submitted version.

We thank the USDA-NIFA Regional Project S-009 (Award number: 1017544) for supporting the funds of this research. The authors also thank Luis Cabán, Zoralys Miranda, Arturo Luciano, and Roberto Vázquez for their support in the field and greenhouse activities.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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