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Effect of chemicals, nitrogen, time of sowing and panicle brown spot epidemics on rice grain discolouration in Italy
No full textRice grain discolouration has a complex etiology and cannot be diagnosed prior to harvest. In Italy, the disease was thought to be associated with fungal infections, however several studies contradicted this hypothesis. We report the distribution of grain discolouration, the most susceptible Italian rice varieties, and the effects of epidemics of brown spot, induced by Cochliobolus miyabeanus, and abiotic factors, such as incidence of grains with open glumes, chemical treatments, nitrogen fertilization, and time of sowing, on disease incidence over three years. Among 39 heavily diseased rice lots, the early cvs Selenio and Loto were the most susceptible. We did not find any significant correlation between incidence of grain discolouration and incidence of brown spot. In contrast, incidence of discoloured grains was weakly correlated with incidence of grains with open glumes (r = 0.521 P = 0.001), while incidence of brown spot was better correlated with incidence of grains with sealed glumes (r = 0.570, P < 0.001) than to incidence of grains with open glumes (r = 0.355, P = 0.026). On susceptible cv. Selenio, percent of grains with open glumes varied among years, but increased significantly from milk dough stage to fully ripe maturation of kernels (P < 0.001). The highest percent of grains with open glumes was found for late-sown rice. However, late sowing reduced significantly (P ≤ 0.001) the incidence of grain discolouration, whereas chemical and fungicide treatments had no significant effect. We speculate that grain discolouration may have a bacterial rather than a fungal aetiology
Impact of tricyclazole and azoxystrobin on growth, sporulation and secondary infection of the rice blast fungus, Magnaporthe oryzae
Full text linkBACKGROUND: Rice blast, caused by Magnaporthe oryzae B Couch sp. nov., is one of the most destructive rice diseases worldwide causing substantial yield losses every year. In Italy, its management is based mainly on the use of two fungicides, azoxystrobin and tricyclazole, that restrain the disease progress. The aim of this study was to investigate and compare the inhibitory effects of the two fungicides on the growth, sporulation and secondary infection of M. oryzae. RESULTS: Magnaporthe oryzae mycelium growth was inhibited at low concentrations of azoxystrobin and relatively high concentrations of tricyclazole, while sporulation was more sensitive to both fungicides and was affected at similarly low doses. Furthermore, infection efficiency of conidia obtained from mycelia exposed to tricyclazole was affected to a higher extent than those produced on azoxystrobin-amended media, even though germination of such conidia were reduced after azoxystrobin treatment. CONCLUSIONS: This study presents for the first time detailed azoxystrobin and tricyclazole growth-response curves of M. oryzae mycelium growth and sporulation. Furthermore, high efficacy of tricyclazole towards inhibition of sporulation and secondary infection indicate an additional possible mode of action of this fungicide that is different from inhibition of melanin biosynthesis
Lack of correlation between incidence of rice grain discolouration and panicle brown spot
No full textGrain discolouration of rice is a serious diseases intropical countries, caused both by fungal pathogens like Cochliobolus miyabeanus, Magnaporthe grisea and Sarocladium spp., and by pathogenic and saprophytic bacteria. In Italy, the disease was reported occasionally in the past, mainly as a consequence of severe epidemics of brown spot and blast. However, incidence of discoloured grains has recently increased, in spite of improved fungal disease control. Failure of fungicides to reduce incidence of grain discoloration suggests that pathogens other than fungi may cause the disease. We identified the most sensitive rice varieties grown in Italy and searched for factors increasing disease incidence. Sensitivity of rice varieties was assessed randomly, collecting samples all over the rice-cultivated area. Forty-six percent of the samples (N=39) were of cv Selenio, 18% of cv Loto and 8% of cv Balilla, which resulted the most sensitive varieties with an average discoloured grains incidence of 13.7%, 14.6% and 8.7%, respectively. Results of two-year field trials, carried out in Rosasco (Province of Pavia) on cv Selenio (subsp. Japonica), showed that the percentage of discoloured grains decreased by delaying the date of sowing. Disease incidence was not affected by increasing the level of nitrogen or by fungicide treatments and it was not correlated to panicle brown spot incidence and severity. We are currently studying the microflora associated with discoloured rice grains in order to identify the casual agent of the disease
Diversity of Pseudomonas and Pantoea species isolated from discoloured rice grains in Italy
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