The fungi Colletotrichum truncatum and Macrophomina phaseolina are among the main pathogens associated with lima bean seeds in Brazil, and they are the causal agents of anthracnose and charcoal rot, respectively. The aim of this study was to verify the seed-seedling transmission of C. truncatum and M. phaseolina in artificially inoculated lima bean seeds. Potato-dextrose-agar (PDA) media with different hydric potentials were obtained after the addition of mannitol. Isolates of C. truncatum and M. phaseolina were peaked to these media. Forty grams of seeds were distributed over the fungal mycelium, remaining there for different exposure periods, as a function of each treatment: T1: 0.0 MPa without inoculum for 48 h; T2: 0.0 MPa for 48 h; T3: -0.2 MPa for 48 h; T4: -0.4 MPa for 48 h; T5: -0.6 MPa for 72 h; T6: -0.8 MPa for 96 h and T7: -1.0 MPa for 120 h. The seeds were incubated at 25 ºC under a 12-hour photoperiod. Germination in a paper roll, emergence of seedlings in sand and seed health in PDA were evaluated. T4 provided the best conditions to temporarily prevent seed germination, promoting a greater incidence of seeds infected by C. truncatum and a greater percentage of diseased plants. M. phaseolina reduced seed germination, independently of the inoculated treatment. Both fungi were transmitted from the seeds to the seedlings of lima bean, causing symptoms in various aerial parts. C. truncatum and M. phaseolina transmission rate was 83.1% and 76.46%, respectively.
Charcoal rot, caused by Macrophomina phaseolina, is an important disease of lima bean (Phaseolus lunatus L.) in the Northeast of Brazil. Considering that there are no reports of resistance to this disease in Brazil, 37 lima bean accessions were evaluated for their resistance reaction and resistance stability against isolates of M. phaseolina in two periods of the year (February-March and July-August 2016), with the aim of selecting genotypes with the potential for use in the management of this disease. Seeds were inoculated at sowing, using rice grains with husk that were colonized by the pathogen. The evaluations of genotypes were carried out at every five days, from the tenth day after sowing, using a score scale and dividing them into six reaction classes. From a population of 37 accessions of P. lunatus, no immunity reaction to M. phaseolina was seen; however, thirteen accessions presented the lowest severity levels and greatest stability in the two seasons: UFPI 892, UFPI 908 and UFPI 905, which were considered resistant to the isolate COUFPI 06. The resistance reaction of accessions considered resistant varied depending on the isolate of M. phaseolina. Accession UFPI 908 expressed the most promising results for resistance and stability of resistance against the isolates COUFPI 06, COUFPI 08 and COUFPI 10. Therefore, accession UFPI 908 constitutes a promising source of resistance to M. phaseolina, making possible its use in breeding programs and in integrated management of charcoal rot.
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