Miller da Silva Lehner

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 β-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.

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.

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.

Interactions between foliar diseases: concepts and epidemiological approaches

This review deals with the phenomenon of plant disease interactions. The epidemiological implications of foliar diseases occurring simultaneously on the same crop are important because the establishment of disease management strategies depends on the knowledge of disease interactions. We discuss some concepts and the terminology related to the interaction studies and present related examples with special emphasis on interacting wheat diseases.

Are Sclerotinia sclerotiorum populations from the tropics more variable than those from subtropical and temperate zones

The genetic variability of Sclerotinia sclerotiorum populations has been investigated in many countries and, consequently, under different ecological conditions. The first studies were conducted to gather information about the genetic structure of populations sampled in temperate climate areas. Based on DNA fingerprinting analysis of restriction fragment length polymorphism (RFLP) data and mycelial compatibility groups, most studies revealed a clonal population structure. Later, using DNA sequences, populations from subtropical regions of North America were reported to be more variable than those from temperate regions. In the past few years, new analyses in temperate, sub-tropical and tropical areas were conducted using microsatellite (SSR) markers. Most studies using SSR reported both high variability and evidence of outcrossing in S. sclerotiorum populations, but evidence of clonality has also been found. Some authors argue that populations from subtropical and tropical areas are more diverse. We argue that the reports of high variability of S. sclerotiorum in tropical or subtropical regions may be due to an increase in the number of studies performed with SSR markers in these regions and in some cases due to inappropriate interpretations of the results.

Limitations in controlling white mold on common beans with Trichoderma spp. at the fall-winter season

We studied the effectiveness of application of Trichoderma spp. in controlling white mold on common beans at the fall-winter crop in the Zona da Mata region of the State of Minas Gerais, Brazil. There was no effect of the antagonist in reducing the disease severity, which could be explained by the low temperatures and the high inoculum pressure in the field. We concluded that Trichoderma applications are not recommended for control of white mold on common beans at the fall-winter season in regions with average temperature bellow 20 °C, since this condition favor more the pathogen than the antagonist.

Trichoderma spp. decrease Fusarium root rot in common bean

The effectiveness of six Trichoderma-based commercial products (TCP) in controlling Fusarium root rot (FRR) in common bean was assessed under field conditions. Three TCP, used for seed treatment or applied in the furrow, increased seedling emergence as much as the fungicide fludioxonil. FRR incidence was not affected, but all TCP and fludioxonil reduced the disease severity, compared to control. Application of Trichoderma-based products was as effective as that of fludioxonil in FRR management.

Susceptibility of leguminous green manure species to Rhizoctonia solani and Sclerotium rolfsii

Estudou-se a suscetibilidade de leguminosas utilizadas como adubos verdes em campos cultivados com feijao a podridao-radicular (Rhizoctonia solani) e a podridao-do-colo (Sclerotium rolfsii). Crotalaria breviflora, Canavalia ensiformis, Cajanus cajan, Dolichos lablab, Stizolobium cinereum, S. aterrimum, e as cultivares de feijao Perola, Valente e Carnaval foram semeadas em solo infestado por R. solani AG-4 ou S. rolfsii em casa de vegetacao. A emergencia de D. lablab em solo infestado por R. solani foi reduzida a 62%. C. breviflora, C. ensiformis e Valente apresentaram a menor severidade de podridao-radicular. O fungo S. rolfsii reduziu drasticamente a emergencia de todas as especies; nenhuma plântula de C. cajan e S. cinereum emergiu. Todas as especies apresentaram alta severidade de podridao-do-colo. As especies de leguminosas testadas nao sao apropriadas para serem utilizadas como adubo verde em areas de cultivo de feijao com alta populacao de R. solani e S. rolfsii.

Genetic structure and mating type analysis of the Pyricularia oryzae population causing widespread epidemics in southern Brazil

Blast is a major disease of rice in Brazil, the largest rice-producing country outside Asia. This study aimed to assess the genetic structure and mating-type frequency in a contemporary Pyricularia oryzae population, which caused widespread epidemics during the 2012/13 season in the Brazilian lowland subtropical region. Symptomatic leaves and panicles were sampled at flooded rice fields in the states of Rio Grande do Sul (RS, 34 fields) and Santa Catarina (SC, 21 fields). The polymorphism at ten simple sequence repeats (SSR or microsatellite) loci and the presence of MAT1-1 or MAT1-2 idiomorphs were assessed in a population comprised of 187 isolates. Only the MAT1-2 idiomorph was found and 162 genotypes were identified by the SSR analysis. A discriminant analysis of principal components (DAPC) of SSR data resolved four genetic groups, which were strongly associated with the cultivar of origin of the isolates. There was high level of genotypic diversity and moderate level of gene diversity regardless whether isolates were grouped in subpopulations based on geographic region, cultivar host or cultivar within region. While regional subpopulations were weakly differentiated, high genetic differentiation was found among subpopulations comprised of isolates from different cultivars. The data suggest that the rice blast pathogen population in southern Brazil is comprised of clonal lineages that are adapting to specific cultivar hosts. Farmers should avoid the use of susceptible cultivars over large areas and breeders should focus at enlarging the genetic basis of new cultivars.

A aplicação foliar de molibdênio na fase de enchimento de vagens do feijão-comum pode reduzir a qualidade da semente

Aiming to improve the technology of producing seed with high Mo content, the effects of 600 g ha-1 of foliar-applied Mo on common bean seed quality were evaluated. In the field, this Mo rate was applied at the V4 growth stage (third trifoliolate leave) or divided into two or four rates applied at V4, R6 (flowering), R7 and/or R8 (pod filling). Mo (300 g ha-1) applied at R6 did not affect seed quality relative to control (90 g ha-1 of Mo applied at V4). In the three treatments in which 200, 255 or 300 g ha-1 of Mo were applied at R7, only in one (255 g ha-1) Mo decreased seed germination and vigor relative to control. When 200, 255 or 300 g ha-1 of Mo were applied at R8 (five treatments), in general, Mo decreased seed germination and, in two cases, also seed vigor relative to control. It was concluded that 255 g ha-1 of Mo applied at R7 and, especially, doses of 200 g ha-1 of Mo or higher applied at R8 can reduce common bean seed quality.