Response of Common and Tepary Bean Genotypes to Three Macrophomina phaseolina Isolates Inoculated at Different Growth Stages

Ashy stem blight caused by the fungus Macrophomina phaseolina (Tassi) Goidanich affects Phaseolus spp. in all growth stages. The identification of genotypes with durable resistance is crucial for disease management. The objectives of this research were to determine the following: the aggressiveness of three fungal isolates; the response of common ( P. vulgaris L.) and tepary bean ( P. acutifolius A. Gray) genotypes at vegetative and midflowering stages; and the presence or absence of SMe1Em5.110 and SS18 molecular markers. Seventeen genotypes were inoculated with the PRI21, PRI22, and PRI23M isolates at the V5 and R2 stages in the greenhouse in Lajas and Isabela. The disease severity was evaluated at 42 days postinoculation. The isolate PRI23M was more aggressive (mean score of 6.2 at V5 and 6.8 at R2) than PRI21 (5.4 and 5.3) in Isabela, whereas there were no significant differences between PRI21 (5.5 and 4.2) and PRI22 (5.0 and 4.3) in Lajas. Tepary bean PI 462025 was intermediate (3.6 to 6.5) to the three isolates, whereas BAT477 and SEA 5 common beans were intermediate to PRI21 and PRI22 and susceptible (scores >6.5) to PRI23M at both growth stages. Likewise, ‘Bella’, ‘Beníquez’, and ‘Verano’ were susceptible to PRI21 and PRI23M at V5 and R2 stages in Isabela. Neither of these genotypes amplified for the two molecular markers. In contrast, NY6020-4 with the SS18 marker was intermediate to PRI21 and PRI22 at the R2 stage in Isabela and Lajas. Moreover, UPR-Mp-22 with the SMe1Em5.110 and SS18 markers was resistant (<3.6) to PRI21 at the V5 stage in both locations and at R2 in Isabela. Two inoculations of M. phaseolina isolates are recommended to identify genotypes with resistance throughout the growing season.