Advances in Phoma macdonaldii (Leptosphaeria lindquistii) epidemiology

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إنجليزي. Phoma macdonaldiiBoerema (teleomorph Leptosphaeria lindquistii)is the causal agent of the black stem disease of sunflower. This disease, which appeared in the early 1990s, is currently present in the entire French sunflower cropping area. Little research on the infection stage of the disease has been carried out, and the biological cycle of P. macdonaldiiis not completelyknown. Sunflower infected residues are the source of primary inoculum. Short rotations (mainly two year sunflower-wheat in south-western France) and simplified soil tillage of wheat crops increasedinoculum production by leaving infected residues on the soil surface. At the end of winter, perithecia ofL. lindquistiiand pycnidia of P. macdonaldiiappear on infected residues. In the springtime, these fructifications release some ascospores and conidiawhich can contaminate surrounding sunflower fields. These contaminations would be mainly due to ascospores. The aims of this study were toquantifyprimary inoculum production and to characterize the effects of environmental conditions on the release of L. lindquistiiascospores. In 2011, at the end of winter, sunflower residues were taken from twenty fields in south-western France. These fields were chosen to represent a wide range of environment and tillagepractices. Primary inoculum production was characterized by measuring the number of perithecia persurface unit of soil. Perithecial maturation was studied during springtime onresidues oftwo fields among thetwenty. In addition, the number of ascospores per perithecium was estimated with a haemocytometer.The influence of climate on ascospore releasehasbeen studied to help predict periods of contaminations. Burkard® volumetric airsamplers were used to measure atmospheric concentration in L. lindquistiiascospores inINRAfieldtrials(Auzeville, France), from 2008 to 2011. Infected stubble was displayed around the volumetric air samplers in order to do ease the measurement of the dynamic of primary inoculum production duringthe sunflower development cycle(from late spring to mid-august). Airborne particles were captured on bovine serum albumin or vaseline-coated cellophane tape wrapped around the sampler drum. The tapes wereobserved undera light microscope(x 400) to estimate the daily atmospheric concentration in ascospores. The obtained dynamics were plotted with respect to climatic data. Sunflower residues were observed in each of the 20fieldswhich were all contaminated by P. macdonaldiithe previous year. The density of residues left at soil surface ranged 8 to 20 residues per square meter, with a mean value of 16 residues per square meter. The density of L. lindquistiiperithecia ranged 33100 to 270000 m-2, with a mean value of 81200perithecia m-2. Perithecial maturation had differentdynamics on the two fieldsobserved. The perithecial maturationrate was 28% on March 30th, and 80% on June 22ndfor the first field whereas it was 76% on April 6th, and 100% on June 29thfor the second field. The number of ascospores per perithecium ranged from 0 to 6000, with a mean value of 1676. A total of 367 days havebeen observed during the 4 years of spore trapping. The daily atmospheric concentration in L. lindquistiiascospores ranged from 0 to 176 per cubic meter. Spore emissions were observed for80% of rainy days and for 70% of dry days. However, significant emissions were observed in 50% of rainy days whereas they were observed in only 28% of dry days. Spore discharges were thus generally triggered by rain but not necessarily. In fact, they occurred for a wide range of daily precipitations: from 0 to 39mmday-1. This study isthe first one to quantifyL.lindquistiiprimary inoculum. New experiments will be carried out to confirm these results.The main conclusions are that ascospore discharges can occur during the entire sunflower cycleand that tillage can be an efficient lever to control black stem epidemics. In addition, a high variability of emissions was observed as a function of weather variables. These results will be used to developa forecast model of ascospore release to determine ascospore discharge periods and to help design control strategies against P. macdonaldiion sunflower.