Sexual reproduction of Zymoseptoria tritici on durum wheat in Tunisia revealed by presence of airborne inoculum, fruiting bodies and high levels of genetic diversity

Septoria tritici blotch (STB) caused by the heterothallic ascomycete Zymoseptoria tritici is currently one of the most devastating diseases of wheat worldwide. The extent of sexual reproduction of this pathogen is well documented on bread wheat, but not on durum wheat. The objective of the present study was to quantify the occurrence of Z. tritici sexual reproduction on durum wheat in the Tunisian environment. The assessment was undertaken using a triple approach combining fruiting body assessment, ascospore trapping and population genetic analyses. The results highlighted the formation of pseudothecia on leaves and stubble from the autumn until the end of the growing season. Likewise, qPCR monitoring highlighted a constant release of Z. tritici airborne inoculum during the wheat-growing season, with a peak of production at the end of the season. Genetic investigations using microsatellites revealed high levels of gene and genotypic diversities, an equal distribution of mating types, and a lack of genetic clustering within and between growing seasons. Taken together, these findings indicate that Z. tritici undergoes sexual reproduction on durum wheat in Tunisia at least to the same extent than on bread wheat in Western Europe, and that the dry and warm climate does not affect the mating process of the fungus. Frequent occurrence of sexual reproduction is a valuable knowledge to take into account in STB control strategies on durum wheat.

FUSARIUM HEAD BLIGHT SYMPTOM DISCRIMINATION: A USEFUL TOOL IN THE FIELD EVALUATION OF FUNGICIDE TREATMENTS

Fusarium head blight (FHB) is one of the major diseases affecting wheat. It is caused by a complex of fungal species, resulting in yield losses and health problems due to mycotoxin production. The presence of multiple fungal species on wheat ears, with varying responses to active fungicide ingredients used in the field, makes the disease difficult to manage. In order to evaluate the efficacy of the timing of applications (at GS 39, GS 61 and GS 39+61) of a prothioconazole + fluoxastrobin-based fungicide, a 2-year field trial was conducted in Belgium. In both years, applications at GS 61 and GS 39+61 resulted in a significant reduction in symptom severity on ears and in deoxynivalenol (DON) content compared with the untreated control in 2013. In 2012, when Microdochium spp. were the most prevalent species, the treatment at GS 39 significantly reduced ear symptoms. Fusarium graminearum was predominant in the second year (2013) and caused significant DON accumulation in the grain after a single foliar spraying. The two genera were characterized by distinct types of symptoms: grouped bleached spikelet's for F. graminearum and isolated bleached spikelet's for Microdochium spp. This difference enabled the significant effect of the double treatments on symptoms caused by Microdochium spp. to be determined in the second year. This effect, which was also visible on leaf symptoms, suggests that Microdochium spp. epidemics in wheat might be polycyclic. Discrimination between symptoms caused by F. graminearum and Microdochium spp. could be a useful tool to study FHB management using fungicide treatments.

Spore traps network: a new tool for predicting epidemics of wheat yellow rust

A network of Burkard 7-day spore-recording traps was set up in the Walloon Region in Belgium to monitor the airborne inoculum of wheat pathogens. The relationship between the airborne inoculum of Puccinia striiformis f.sp. tritici, the causal agent of stripe rust, and the disease incidence on plants in untreated plots located near each spore traps was studied during the 2008-2009 season. The presence of airborne inoculum was tested in four locations on tapes collected from the Burkard spore traps from 1 April to 14 June 2009. Total DNA from each fragment of spore trap tape corresponding to 1 day sampling was extracted. P. striiformis f.sp. tritici was quantified by real-time polymerase chain reaction (PCR) assay using specific primers and SYBRGreen. The airborne inoculum of P. striiformis was first detected between 7 and 13 April 2009, depending on the location in the Walloon Region. The first symptoms of stripe rust were observed in the fields between 15 May and 2 June 2009. The onset of the disease symptoms was always preceded by a higher peak of airborne inoculum about 15 days earlier. When P. striiformis f.sp. tritici was detected, the daily quantities of spores, collected from a volume of air of 14.4 m3, fluctuated between 0.23 and 154.66. This study shows that spore traps coupled with real-time PCR could be used to assess the airborne inoculum of P. striiformis in order to understand and predict stripe rust outbreaks.

Distribution of airborne Mycosphaerella graminicola inoculum at the field scale

A network of 10 Burkard 7-day spore-recording traps was set up in the Walloon region in Belgium to monitor the airborne inoculum of wheat pathogens. Three spore traps were used to analyse the distribution of Mycosphaerella graminicola inoculum at the field scale, at 1 m above ground level. Two traps were set up in a wheat field 100 m apart. The third trap was placed 70 m away in a sugar beet field adjacent to the wheat field. Total DNA from each fragment of spore trap tape corresponding to 1 day sampling was extracted and the quantity of M. graminicola was assessed using real-time polymerase chain reaction (PCR) assay. The experiment was conducted from July to October 2009. Positive detections were obtained for between 33 and 36 days, depending on the spore traps. When detected, the daily quantities of cDNA, collected from a volume of 14.4 m3, fluctuated between 4.84E+00 and 6.10E+03. Correlation coefficients higher than 0,82 and no significant differences were observed between the quantities of M. graminicola collected by the three spore traps, indicating that, at 1 m above ground level, the distribution of inoculum can be considered as homogenous at the tested field scale. This study confirms that spore traps coupled with real-time PCR could be used to assess the airborne inoculum of M. graminicola and to understand the development of the disease at this scale.