Wheat blast is known for developing itself more intensely under relatively high temperature conditions but many aspects related to its epidemiology remain unknown. The objective of this research was to evaluate the sporulative capacity of Pyricularia oryzae Triticum (Pot), the causal agent of wheat blast, in tissues of wheat plants under different temperatures degrees. Wheat plants of the cultivar Anahuac 75, susceptible to blast, were inoculated in the stage of flowering with conidial suspensions (105 conidia/mL) of the Pot isolates Py 12.1.209 and Py 12.1.132. Seven days after the inoculation, plants were cut in the following segments: leaves, stems and rachis (with blast severity ranging from 40 to 60%). Groups of each one of the three plant segments with the lesions were disposed in Petri-dish moist chambers, that were submitted to six different temperature treatments (18, 21, 24, 27, 30 and 33 °C). The most appropriate model that related the conidia production with temperature was identified in the evaluations conducted with stems. The established equations allowed identifying that the highest production of conidia of Pot occurs between 24 and 27 °C.

The first report of wheat blast in the world was in Brazil, in 1986. Since then, a great effort has been made towards the development of wheat cultivars resistant to this disease, which is caused by the fungus Pyricularia oryzae Triticum (PoT). The objective of this research was to (i) evaluate the resistance of wheat genotypes to blast and (ii) verify the correlation between disease severity on wheat spikes and sporulation rate of PoT on spike rachises. Plants of 40 cultivars grown in pots, at the flowering stage (stage 65 on the Zadoks scale), were inoculated with a suspension of conidia of a PoT isolate representative of the main variant of the fungus reported in Brazil. Severity of blast on the spikes at 5 and 7 days after inoculation (dai) and the rate of sporulation of the fungus on the rachis (conidia per g of rachis) were evaluated. Eighty percent of the cultivars that were classified in the group with the lowest sporulation rate were also classified in the group with the highest resistance at 7 dai. However, the correlation coefficients of the analysis established between the cultivar severity at 5 and 7 dai averages and the PoT sporulation rate averages were not significant (r=0.2464 and r=0.2047, respectively). Results obtained represent the updated characterization to blast of wheat cultivars in Brazil and constitute an important exploratory framework for the evaluation of the reaction of wheat genotypes based on the sporulation rate of PoT on their tissues.

Wheat head blast (WHB), caused by the fungus Pyricularia oryzae Triticum pathotype, is one of the most damaging diseases of wheat. In Brazil, the disease has limited the expansion of wheat in the tropical regions of the Cerrado biome. In this study, wheat yield (Y) and visual estimates of WHB incidence (I) and severity (S) (assessed 25 to 30 days after flowering) were measured in a single location in the Cerrado (Minas Gerais state) where field experiments were conducted during seven sequential years (2013 to 2019) using three cultivars and 12 planting dates each year. Pearson correlation (r) analysis, used to study the association between each of the three disease variables (I, DHS = diseased-head severity and S) and Y, under a meta-analytic framework, showed the strongest association between incidence and yield (r I-Y = -0.92) and severity and yield (r S-Y = -0.90). The functional relationship between WHB intensity (I or S) and Y was studied based on the fit of a random coefficients model to I or S and the log-transformed Y data. The population average estimated intercept and slope for the I or S and log-Y relationships were 7.798 log units (2,435.7 kg/ha) and -0.020 for the I-Y and 7.721 log units (2,373.4 kg/ha) and -0.030 for the S-Y relationship. The relative yield loss when I or S reaches 10% was predicted at 18.4% and 26.1% yield loss, respectively. This study provides a basis for understanding and predicting yield losses due to wheat blast based on the visual assessment of two disease measures at a single time during the epidemics.

Wheat head blast (WHB), caused by the fungus Pyricularia oryzae Triticum pathotype, is one of the most damaging diseases of wheat. In Brazil, the disease has limited the expansion of wheat in the tropical regions of the Cerrado biome. In this study, wheat yield (Y) and visual estimates of WHB incidence (I) and severity (S) (assessed 25 to 30 days after flowering) were measured in a single location in the Cerrado (Minas Gerais state) where field experiments were conducted during seven sequential years (2013 to 2019) using three cultivars and 12 planting dates each year. Pearson correlation (r) analysis, used to study the association between each of the three disease variables (I, DHS = diseased-head severity and S) and Y, under a meta-analytic framework, showed the strongest association between incidence and yield (rI-Y = -0.92) and severity and yield (rS-Y = -0.90). The functional relationship between WHB intensity (I or S) and Y was studied based on the fit of a random coefficients model to I or S and the log-transformed Y data. The population average estimated intercept and slope for the I or S and log-Y relationships were 7.798 log units (2,435.7 kg/ha) and -0.020 for the I-Y and 7.721 log units (2,373.4 kg/ha) and -0.030 for the S-Y relationship. The relative yield loss when I or S reaches 10% was predicted at 18.4% and 26.1% yield loss, respectively. This study provides a basis for understanding and predicting yield losses due to wheat blast based on the visual assessment of two disease measures at a single time during the epidemics.

Fungicides play an important role in wheat blast management in Brazil, but the variable and inconsistent responses to some active ingredients have been associated with fungicide resistance, among other factors. To determine whether the reported patterns of fungicide resistance are present in Pyricularia oryzae populations from wheat and signal grass grown in Minas Gerais state, Brazil, we characterized the sensitivity of 64 fungal isolates to seven fungicides belonging to DMI (tebuconazole and epoxiconazole), QoI (azoxystrobin and pyraclostrobin and SDHI (bixafen, fluxapyroxad and benzovindiflupyr) groups. We further assessed if the differential sensitivity of selected isolates affected disease control using commercial doses of the fungicides applied protectively (prior to inoculation) on the heads of potted wheat plants. Despite founding relatively low levels of in vitro sensitivity to some of the fungicides (e.g. azoxystrobin, fluxapyroxad and bixafen), control efficacies (percent reductions in head blast severity and sporulation on the rachis) were generally not affected by the sensitivity of the strain to all fungicides but azoxystrobin. The levels of head blast control for most fungicides (> 70%) in the greenhouse were similar or higher than previously reported in the field. The molecular data for a set of isolates representing different fungicide sensitivity levels were generally not predictive of the phenotype. The wheat-infecting isolates were less sensitive than signal grass-infecting isolates to all fungicides including azoxystrobin. Our findings suggest that at least one active ingredient, within each of the three chemical groups (e.g. tebuconazole, pyraclostrobin and benzovindiflupyr), is capable of successfully controlling the disease.

Fungicides play an important role in wheat blast management in Brazil. To determine

The relationship between wheat head blast incidence (I; proportion of diseased heads in a sample) and severity (S; the average diseased area in a sample of heads) was studied with the aim of determining whether severity could be reliably predicted from incidence. Data were collected from two main sources: a) the EPAMIG (non-treated) field experiments spanning seven years (2013-2019 with three cultivars) designed to evaluate the effect of planting dates on wheat blast (no fungicide use); and b) the UFTs (fungicide-treated) field experiments conducted by researchers of the network of uniform fungicide trials conducted during nine years (2012-2020) across 10 locations (totaling 14 cultivars). An ordinary linear regression model fitted to the complementary log-log transformation of the data provided a good fit to the data based on the square correlation of the predicted Ŝ, and the observed S (R2): The R2 was 0.95 for the pooled (all years) non-treated dataset and 0.88 for the pooled fungicide-treated dataset. A covariance analysis indicated that year, but not cultivar, significantly affected the parameters of the I-S relationship in the non-treated dataset. We conclude that wheat head blast S can be predicted from I but more reliably in non-treated epidemics given the larger influence of chemical treatments on the relationship than cultivar or year, a surrogate for varying weather.

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