Carlos Eduardo Aucique-Pérez

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.

Wheat Blast: Past, Present, and Future

The devastating wheat blast disease first emerged in Brazil in 1985. The disease was restricted to South America until 2016, when a series of grain imports from Brazil led to a wheat blast outbreak in Bangladesh. Wheat blast is caused by Pyricularia graminis-tritici ( Pygt), a species genetically distinct from the Pyricularia oryzae species that causes rice blast. Pygt has high genetic and phenotypic diversity and a broad host range that enables it to move back and forth between wheat and other grass hosts. Recombination is thought to occur mainly on the other grass hosts, giving rise to the highly diverse Pygt population observed in wheat fields. This review brings together past and current knowledge about the history, etiology, epidemiology, physiology, and genetics of wheat blast and discusses the future need for integrated management strategies. The most urgent current need is to strengthen quarantine and biosafety regulations to avoid additional spread of the pathogen to disease-free countries. International breeding efforts will be needed to develop wheat varieties with more durable resistance.

Photosynthetic response of two mango cultivars submitted to salt stress and infected with Ceratocystis fimbriata

This study aimed to investigate the alterations on the photosynthetic performance of mango plants from cultivars Tommy Atkins and Uba when exposed to salt stresss and infected with Ceratocystis fimbriata . Plants from these two cutivars were grown in plastic pots receiving nutrient solution with 0 and 90 mM NaCl for 50 days. At 42 days after fungal inoculation, the leaf gas exchange parameters net CO 2 assimilation rate [ A ], stomatal conductance to water vapor [ g s ], internal CO 2 concentration [ C i ], and transpiration rate [ E ] as well as the lesion length, the upward and the downward relative lesion length and the radial fungal colonization were evaluated. Based on the disease variables evaluated, plants from cultivar Uba were more resistant to infection by C. fimbriata in comparison to plants from cultivar Tommy Atkins. Lower values of A were obtained for plants from cultivar Tommy Atkins submitted to salt stress and infeted with C. fimbriata resulting, therefore, in reduced values of g s and E . In general, plants from cultivar Tommy Atkins were more affected at the photosynthetic level in comparison to plants from cultivar Uba under salt stress and infected with C. fimbriata . Under salt stress, stomatal closure reduced the C i values especially on plants from cultivar Tommy Atkins. Plants from cultivar Tommy Atkins were more susceptible to infection by C. fimbriata even when exposed to salt stress.

Pyricularia oryzae -wheat interaction: physiological changes and disease management using mineral nutrition and fungicides

This review aims to summarize our major findings on the host physiology affected by wheat blast caused by Pyricularia oryzae and disease management with emphasis on mineral nutrition and fungicides. Infected plants show reduced values of the leaf gas exchange parameters net carbon assimilation rate, stomatal conductance, and transpiration rate and greater values of internal CO2 concentration. Indeed, the photosynthetic machinery is damaged as suggested by reductions in the maximum quantum quenching, photochemical quenching coefficient and electron transport rate. A decrease in the concentration of photosynthetic pigments also occurs. Wheat resistance to blast is intrinsically associated with an increase in the production of reactive oxygen species (ROS), which favors host defense mechanisms against P. oryzae infection. In fact, a more efficient antioxidative system that removes the excess of ROS generated during the infection process of P. oryzae prevents the cellular damage caused by the fungus. As to mineral nutrition, plants supplied with high silicon and low magnesium rate exhibited reduced concentrations of ROS and a more efficient antioxidant system, thus preserving the photosynthetic performance. The expression levels of the defense-related genes pathogenesis-related 1, chitinase, peroxidase and phenylalanine ammonia-lyase were from 2- to 3-fold higher in silicon-amended plants, which showed leaf and rachis lesser colonized by the fungus. Wheat cultivars able to activate defense mechanisms against P. oryzae infection, thus possessing a more efficient antioxidant system, are recommended. However, fungicides applied during flowering time, in addition to host resistance, are necessary to achieve better control of head blast and reduce yield loss under conditions favorable for the disease.