Biochemical responses of common bean to white mold potentiated by phosphites.

Considering that the mechanisms for phosphite-afforded disease control remain elusive, this study investigated whether zinc (Zn) and copper (Cu) phosphites could possible potentiate common bean resistance to white mold, caused by Sclerotinia sclerotiorum, through the stimulation of biochemical defence responses. Lesion area and disease severity were decreased by phosphites spray, but Zn phosphite outcompeted Cu phosphite. Histopathological observations revealed fewer fungal hyphae and less collapse of the mesophyll cells in the Zn and Cu phosphite-sprayed plants compared to water-sprayed ones. The S. sclerotiorum-triggered accumulation of reactive oxygen species, oxalic acid (a fungal secreted toxin) and malondialdehyde (an indicator of cellular damage) were constrained as a result of Zn and Cu phosphites spray. Activities of antioxidant enzymes (superoxide dismutase, peroxidase, ascorbate peroxidase and glutathione-S-transferase at 12 h after inoculation (hai) and catalase at 60 and 84 hai) were higher for Zn and Cu phosphites-sprayed plants than for water-sprayed ones. Activities of defence-related enzymes chitinase (CHI) at 12 hai, ß-1,3-glucanase (GLU) and polyphenoloxidase (PPO) were higher at 12-84 hai for Zn, and Cu phosphites sprayed plants, phenylalanine ammonia-lyase at 36-84 hai for the Zn phosphite sprayed ones, CHI at 12-36 hai, GLU at 12-60 hai, PPO at 36 hai and PAL and lipoxygenase at 12 hai for the Cu phosphite sprayed ones upon inoculation with S. sclerotiorum relative to their water-sprayed counterparts. Concentrations of total soluble phenols and lignin-thioglycolic acid derivatives were not affected by Cu phosphite spray on infected plants but were higher and lower, respectively, for Zn phosphite sprayed plants at 60 hai compared to water-sprayed ones. Taken together, the findings from the present study shed light on the biochemical defence mechanisms involved in the Zn and Cu phosphites-mediated suppression of white mold in common bean.

Oxalic acid-mediated biochemical and physiological changes in the common bean-Sclerotinia sclerotiorum interaction.

The success of Sclerotinia sclerotiorum infection relies mainly on the production of the non-host selective toxin named oxalic acid (OA). This toxin is known to play multiple roles in a host infected by the fungus, but its effect on photosynthesis and the antioxidant system of common bean plants remain elusive. Therefore, we performed detailed analysis of leaf gas exchange, chlorophyll a fluorescence, activities of antioxidant enzymes, concentrations of reactive oxygen species and photosynthetic pigments to investigate the OA's role during the S. sclerotiorum pathogenesis. To achieve this goal, common bean plants were sprayed with water or with oxalic acid (referred to as -OA and +OA plants, respectively) and either non-challenged or challenged with a wild-type (WT) or an OA-defective mutant (A4) of S. sclerotiorum. Irrespective of OA spray, the WT isolate was more aggressive than the A4 isolate and spraying OA increased OA concentration in the leaflets as well as the aggressiveness of both isolates. Biochemical limitations were behind S. sclerotiorum-induced photosynthetic impairments notably for the +OA plants inoculated with the WT isolate. Inoculated plants were not able to fully capture and exploit the collected energy due to the degradation of photosynthetic pigments. Photoinhibition of photosynthesis and photochemical dysfunctions were potentiated by OA. Higher activities of superoxide dismutase, peroxidase and ascorbate peroxidase besides reductions on catalase activity were noticed for plants inoculated with the WT isolate. OA was able to counteract most of the increases in the activities of antioxidant enzymes thereby increasing the generation of superoxide and hydrogen peroxide and the concurrent damage to the membranes of host cells as evidenced by the high malondialdehyde concentration. In conclusion, OA was found to enhance biochemical limitations to photosynthesis, photochemical dysfunctions and oxidative stress in the leaflets of common bean plants infected by S. sclerotiorum.

Physiological changes promoted by a strobilurin fungicide in the rice-Bipolaris oryzae interaction.

Strobilurins are among the most important fungicides that are used for plant disease control worldwide. In addition to their fungicide effect, strobilurins can also improve crop physiology. Nonetheless, the impact of azoxystrobin (Az), the main marketed strobilurin, on rice physiology is still unknown. Detailed gas exchange measurements and chlorophyll a fluorescence analysis were used to examine the Az effects on the photosynthetic performance of rice plants (cultivar Metica-1) either challenged or not with Bipolaris oryzae, the causal agent of brown spot. Az impaired carbon (C) fixation in the non-inoculated plants in a manner that was not related to photochemical or biochemical limitations, but rather to decreased stomatal conductance that limited the CO2 influx into the mesophyll cells. The photosynthesis of rice plants that were not sprayed with Az dramatically decreased upon B. oryzae infection, which was chiefly governed by photochemical and biochemical limitations. The energy surplus that was caused by limited C fixation in the rice plants that were treated with Az and inoculated with B. oryzae was thermally and effectively dissipated until 72h after inoculation. In Az absence, however, this mechanism was not sufficient to prevent chronic photoinhibition to photosynthesis. The inoculated plants were not able to fully capture and exploit the collected light energy, but these constraints were greatly limited in the presence of Az. In conclusion, Az impaired the photosynthetic performance of non-infected plants by diffusive constraints, but prevented, to a greater extent, the damage to the photosynthetic apparatus during the infection process of B. oryzae.

Ação de acibenzolar-s-methyl isolado e em combinação com fungicidas no manejo de doenças na cultura do feijoeiro

ABSTRACT The bean crop is affected by several diseases, most notably anthracnose (Colletotrichum lindemuthianum) and bacterial common blight (Xanthomonas axonopodis pv. phaseoli). Thus, there is a need to develop crop management techniques aimed at their control. The present study was carried out with the objective to evaluate the effect of acibenzolar-S-methyl (ASM) alone and with fungicidal mixture on the control of these diseases in the bean crop. The treatments were ASM applied at 7, 14 or 21 DAE (days after emergence), 2 applications of ASM (7 DAE + 14 DAE or 14 DAE + 21 DAE) and 3 applications done at 7, 14 and 21 DAE. These treatments were combined with three chemical control programs: tin triphenyl hydroxide; tin triphenyl hydroxide + azoxystrobin; and azoxystrobin + difenoconazole. One application of ASM made at 7 DAE was sufficient to reduce anthracnose and bacterial common blight by 60 and 38%, respectively, increasing grain yield by 17%. However, the best results were observed when ASM and fungicides were used together. The data suggest the potential for using ASM in a program for the control of bean diseases.

RESUMO A cultura do feijoeiro é afetada por diversas doenças destacando-se a antracnose, causada por Colletotrichum lindemuthianum, e o crestamento bacteriano comum (CBC), causado por Xanthomonas axonopodis pv. phaseoli. O objetivo deste trabalho foi o de avaliar o efeito de acibenzolar-S-methyl (ASM) isolado e em mistura com fungicidas no controle dessas enfermidades e na produtividade do feijoeiro. Foram utilizados os tratamentos com ASM isolado aplicados aos 7, 14 ou 21 DAE (dias após emergência), ASM com duas aplicações, aos 7 e 14 DAE, aos 14 e 21 DAE; 7, 14 e 21 DAE. Estes tratamentos foram combinados a três programas de controle químico com fungicidas, sendo trifenil hidróxido de estanho, uma mistura de trifenil hidróxido de estanho + azoxystrobin e azoxystrobin + difenoconazole. Uma aplicação de ASM realizada aos 7 DAE foi suficiente para promover redução de 60 e 38% na severidade de antracnose e de CBC, respectivamente, bem como incremento de 17% na produtividade da cultura. No entanto, as melhores respostas foram obtidas quando indutor e fungicidas foram combinados. Os dados sugerem o potencial de utilização do ASM em programas de controle de doenças na cultura do feijoeiro.