Similar Aggressiveness of Phenotypically and Genotypically Distinct Isolates of Sclerotinia sclerotiorum.

Understanding how Sclerotinia sclerotiorum aggressiveness varies among isolates may be useful for breeding programs aimed at developing common bean cultivars resistant to white mold. The aggressiveness of 20 S. sclerotiorum isolates collected in common bean fields from four Brazilian states was tested against two common bean genotypes (Pérola and A195) using two inoculation methods. The isolates were characterized using 10 microsatellite (SSR) loci, mycelial compatibility groups (MCGs), partial sequences of the oxaloacetate acetylhydrolase (OAH) gene, and morphological traits. Twenty SSR and seven OAH haplotypes, 10 MCGs, and high variability in colony morphology were found. One isolate was more aggressive when inoculated on plants of the genotype A195, but all other isolates had similar aggressiveness. Aggressiveness was not related with MCGs, SSR, OAH haplotypes, mycelial pigmentation, growth rate, or sclerotia production.

Potencial de óleos essenciais de plantas medicinais no controle de fitopatógenos / Potential of essential oils from medicinal plants to control plant pathogens

Além do valor como recurso terapêutico, plantas medicinais também possuem potencial para serem utilizadas como fonte de princípios ativos contra fitopatógenos. O objetivo deste trabalho foi avaliar o efeito de óleos essenciais das espécies medicinais Baccharis dracunculifolia (alecrim-do-campo), Schinus terebinthifolius (aroeirinha) e Porophyllum ruderale (arnica-brasileira) sobre o crescimento dos fungos fitopatogênicos Fusarium oxysporum f. sp. phaseoli (Fop), F. solani f. sp. phaseoli (Fsp), Sclerotinia sclerotiorum (Ss), S. minor (Sm), Rhizoctonia solani (Rs), Sclerotium rolfsii (Sr) e Macrophomina phaseolina (Mp). Avaliou-se em placas de Petri o crescimento radial desses fungos em meio batata-dextrose-ágar (BDA) com cinco concentrações (0, 250, 500, 1000 e 3000 mg L-1) dos óleos essenciais. Discos de micélio (5 mm de diâmetro) de cada fungo em crescimento foram transferidos para placas de Petri que foram mantidas a 23°C no escuro por 48 horas. O óleo essencial de alecrim-do-campo foi o mais eficiente na redução do crescimento micelial de todos os fungos, com inibição completa quando se utilizou a concentração de 3000 mg L-1. A redução de crescimento variou de 29% (Fs) a 80% (Rs) a 250 mg L-1 do óleo essencial de alecrim-do-campo; a 500 mg L-1, variou de 29% (Fs) a 98% (Sr); e a 1000 mg L-1, de 41% (Fs) a 100% (Sr). A redução do crescimento dos fungos pelo óleo de aroeirinha na concentração de 3000 mg L-1 variou de 27% (Fsp) a 74% (Rs). Nessa concentração, o óleo de arnica-brasileira reduziu o crecimento micelial de Ss em 72%, o de Rs em 80% e o de Mp em 82%, sem efeitos significativos sobre o crescimento micelial de Fsp e Fop. Conclui-se que os óleos essenciais de alecrim-do-campo, aroeirinha e arnica-brasileira possuem potencial para o controle dos fungos fitopatogênicos estudados, com destaque para o óleo de alecrim-do-campo.

In addition to their value as therapeutic resources, medicinal plants also have the potential to be used as a source of alternative compounds against plant pathogens. The objective of this study was to evaluate the effect of essential oils extracted from the medicinal species Baccharis dracunculifolia, Schinus terebinthifolius and Porophyllum ruderale on the growth of the fungal plant pathogens Fusarium oxysporum f. sp. phaseoli (Fop), F. solani f. sp. phaseoli (Fsp), Sclerotinia sclerotiorum (Ss), S. minor (Sm), Rhizoctonia solani (Rs), Sclerotium rolfsii (Sr) and Macrophomina phaseolina (Mp). The radial mycelial growth of the fungi was evaluated on potato dextrose agar (PDA) in Petri dishes with five concentrations (0, 250, 500, 1000 and 3000 μL L-1) of the essential oils. Mycelial discs (5 mm diameter) of the growing colonies of each fungus were transferred to Petri dishes, which were maintained at 23 °C in the dark for 48 hours. The essential oil of B. dracunculifolia was the most effective oil in the reduction of the mycelial growth of all fungi. It completely inhibited their growth at 3000 mg L-1. At 250 mg L-1, the growth reduction caused by the oil of B. dracunculifolia varied from 29% (Fs) to 80% (Rs); at 500 mg L-1, it varied from 29% (Fs) to 98% (Sr); and at 1000 mg L-1, it varied from 41% (Fs) to 100% (Sr). The reduction of the mycelial growth caused by the oil of S. terebinthifolius at 3000 mg L-1 varied from 27% (Fsp) to 74% (Rs). At this concentration, the oil of P. ruderale reduced the mycelial growth of Ss by 72%, of Rs by 80% and of Mp by 82%, without significant effects on the mycelial growth of Fsp and Fop. We conclude that the essential oils of B. dracunculifolia, S. terebinthifolius and P. ruderale have the potential to be used to control the plant pathogens tested, especially the oil of B. dracunculifolia.

Adaptation and Resistance to Diseases in Brazil of Putative Sources of Common Bean Resistance to White Mold.

Common bean breeding programs for white mold (WM) resistance are in their initial stages in Brazil. Sources of partial resistance to WM are available abroad but their performance in Brazil is unknown. In two greenhouse (straw test) and three field experiments conducted in three districts in the state of Minas Gerais, Brazil, we evaluated a total of 23 lines with putative WM resistance with the objective to select lines with resistance to WM and other diseases associated with high yield potential. Two field-resistant local lines, two susceptible local cultivars, and two susceptible international lines were also included in the study. In the greenhouse, Cornell 605, A 195, and G122 were among the lines with the highest partial resistance to WM. In the field, these three lines were highly resistant to WM and had intermediate resistance or were resistant to anthracnose, angular leaf spot, rust, and Fusarium wilt. Cornell 605 and A 195 had high-yield potential but G122 yielded 47% less than the local lines under WM pressure. Our results suggest that Cornell 605 and A 195 are the most useful sources of resistance to WM for use in common bean breeding programs in Brazil.

Fungicide Sensitivity of Sclerotinia sclerotiorum: A Thorough Assessment Using Discriminatory Dose, EC50, High-Resolution Melting analysis, and Description of New Point Mutation Associated with Thiophanate-Methyl Resistance.

Thiophanate-methyl (TM), fluazinam, and procymidone are fungicides extensively used for white mold control of common bean in Brazil. We assessed the sensitivity of Brazilian isolates of Sclerotinia sclerotiorum to these three fungicides using discriminatory doses and concentration that results in 50% mycelial growth inhibition (EC50) values. In total, 282 isolates from the most important production areas were screened and none was resistant to fluazinam or procymidone. The EC50 values varied from 0.003 to 0.007 and from 0.11 to 0.72 µg/ml for fluazinam and procymidone, respectively. One isolate was resistant to TM. The EC50 of the TM-resistant isolate was greater than 100 µg/ml, whereas the EC50 of the sensitive isolates varied from 0.38 to 2.23 µg/ml. The TM-resistant isolate had a L240F mutation in the ß-tubulin gene. This is the first report of mutation at codon 240 causing resistance to a benzimidazole fungicide in S. sclerotiorum. The high-resolution melting analysis allowed the distinction of TM-sensitive and -resistant isolates by specific melting peaks and curves. The TM-resistant isolate had mycelial growth, sclerotia production, and aggressiveness comparable with that of the sensitive isolates, indicating that this genotype will likely compete well against sensitive isolates in the field. This study demonstrates that resistance to TM, fluazinam, and procymidone is nonexistent or rare. Resistance management practices should be implemented, however, to delay the spread of TM-resistant genotypes.