Nitrogen and water management can limit premature ripening of sunflower induced by Phoma macdonaldii

Premature ripening (PR) is one of the most important diseases of sunflower in France since the 90s. Previous results indicated that girdling canker of the stem base, caused by Phoma macdonaldii was its primary cause but elucidation of critical environmental factors involved is crucial for better control of the disease. A field study was conducted in three contrasting cropping seasons (2006–2008) and investigated the effect of N fertilization (0, 75 and 150 kg N ha−1) and water regime (rainfed, irrigated) on two cultivars with artificial inoculation (AI) and natural infection (NI). Disease assessment was recorded weekly to calculate the area under disease progress curve (AUDPC) and the final percentage of PR plants. Data showed that high levels of N fertilization led to significantly (P < 0.05) more PR than non-fertilization. Water deficit conditions were significantly (P < 0.05) involved in disease severity, and AUDPC and PR were increased when dry conditions were associated with high N supply. This was true for two cultivars which differed in their susceptibility to the disease but cv. Heliasol RM was significantly (P < 0.05) more affected than cv. Melody, partially resistant to PR. Despite contrasting weather patterns, these results demonstrated a clear role of crop management and environmental conditions on the incidence and severity of stem base attacks responsible for the PR syndrome. These findings suggest that sunflower crop husbandry should be adapted to minimize premature ripening induced by P. macdonaldii

Low plant density can reduce sunflower premature ripening caused by Phoma macdonaldii

In France, premature ripening (PR) is a widespread damage of a fungal disease of sunflower caused by Phoma macdonaldii. Previous results indicated that girdling canker at the stem base, caused by P. macdonaldii, was its primary cause. Previous studies have reported the influence of nitrogen and water supply on the incidence and severity of PR but an additional study was required to analyze the effect of plant density on the level of attack for a more comprehensive cultural control of PR. In a 2-year field study (2008 and 2009) in Toulouse (France), a susceptible cultivar (cv. Heliasol) artificially inoculated at star bud stage with P. macdonaldii was grown at three plant densities (4, 6.5 and 9 plants m−2) factorially combined with three N fertilization rates (0, 50/75 and 150 kg N ha−1) and two water regimes (irrigated and rainfed). P. macdonaldii symptoms were scored weekly to calculate the area under disease progress curve (AUDPC) and percentage of PR plants. Microclimatic conditions were monitored using thermo-hygrometers within the crop. The fraction of photosynthetically active radiation intercepted by the canopy (fPARi) and leaf area index (LAI) were measured at anthesis. Plant water status during the disease progression was characterized by crop simulation (SUNFLO) and N status at anthesis was assessed from shoot N content (Nm) analysis and N Nutrition Index (NNI) calculation. Increasing plant density resulted in a greater proportion of PR plants, and this proportion increased further when N was applied at 150 kg ha−1, the highest rate, and the crop was not irrigated. Despite differing canopy development, differences in microclimatic conditions between density levels were too small to explain the PR differences. However plant N concentration and diameter at stem base were closely related to PR incidence. Thin plants (grown at high density) with non-limiting N supply were the most susceptible to premature ripening. This study opens new avenues for the control of PR through crop management and emphasizes the key role of plant morphology in the development of the disease. Stem base diameter is a morphological trait that could be manipulated through crop management (plant density, N fertilization) and probably breeding in the future when developing integrated disease management systems in sunflower

Spatial Genetic Structure and Pathogenic Race Composition at the Field Scale in the Sunflower Downy Mildew Pathogen, Plasmopara halstedii

Yield losses in sunflower crops caused by Plasmopara halstedii can be up to 100%, depending on the cultivar susceptibility, environmental conditions, and virulence of the pathogen population. The aim of this study was to investigate the genetic and phenotypic structure of a sunflower downy mildew agent at the field scale. The genetic diversity of 250 P. halstedii isolates collected from one field in southern France was assessed using single-nucleotide polymorphisms (SNPs) and single sequence repeats (SSR). A total of 109 multilocus genotypes (MLG) were identified among the 250 isolates collected in the field. Four genotypes were repeated more than 20 times and spatially spread over the field. Estimates of genetic relationships among P. halstedii isolates using principal component analysis and a Bayesian clustering approach demonstrated that the isolates are grouped into two main genetic clusters. A high level of genetic differentiation among clusters was detected (FST = 0.35), indicating overall limited exchange between them, but our results also suggest that recombination between individuals of these groups is not rare. Genetic clusters were highly related to pathotypes, as previously described for this pathogen species. Eight different races were identified (100, 300, 304, 307, 703, 704, 707, and 714), with race 304 being predominant and present at most of the sites. The co-existence of multiple races at the field level is a new finding that could have important implications for the management of sunflower downy mildew. These data provide the first population-wide picture of the genetic structure of P. halstedii at a fine spatial scale.publishedVersio

A model-based approach to assist variety assessment in sunflower crop

Poster presented for International Crop Modeling Symposium (ICROPM2016), 15-17 March 2016, Berlin, German

The main dynamics of sunflower research presented at the 19th International Sunflower Conference in 2016

The International Sunflower Conference takes place every four years under the auspices of the ISA, International Sunflower Association. Before the 20th Conference in June 2020, a review of the proceedings of the Edirne conference, in June 2016, offers a vision of the crop and research dynamics in the world, as well as international cooperation programmes in progress or to be developed in the period 2016–2020. They are essential for the future of this crop for which demands tend to diversify. Sustained efforts have led to significant progress in genomics. The challenges posed by pests and by climatic constraints remain significant. The interest in gathering knowledge on crop models incorporating varietal parameters appears clearly, both to establish long-term adaptation strategies and to make better use of current resources

Phoma du tournesol et dessèchement précoce : point sur les avancées de la recherche

Phoma macdonaldii is the causal agent of black stem disease of sunflower (Helianthus annuus L). In France, the incidence and severity of the disease increased dramatically in the early 90s and the entire sunflower cropping area is now affected. Premature ripening appeared in 1994: it is at the present time one of the most widespread and detrimental diseases of sunflower. Recent investigations confirmed that stem base girdling canker due to P. macdonaldii is the primary cause of this syndrome. Crop management and varietal susceptibility can influence significantly the incidence and severity of P. macdonaldii attacks and premature ripening. Partial resistance and presence of tissue-specific resistance genes have also been demonstrated

Quelles perspectives pour la lutte génétique vis-à-vis du mildiou du tournesol ?

Downy mildew of sunflower exists as physiological races which have changed in France following introductions or selection pressure applied by large-scale use of varieties with single major genes giving resistance to the disease. The duration of efficiency of single or pyramided resistance genes, their alternation over different years or use of mixtures of several isogenic forms of a hybrid with different resistance genes, were studied under six irrigated 60m2 net cages which made it possible to obtain disease attack whatever the weather conditions. Studies have been made over 4 years of continuous cropping. The susceptible form of the hybrid showed from 37 to 75% attack, always by race 710, present at the beginning of the study. Monoculture of the two forms of the hybrid each carrying one efficient resistance gene showed no significant attack in years 1 and 2, but then 4-5% attack in year 3 and 15-21% attack in year 4, with the appearance of races new to the trial plots but which have appeared elsewhere in France where varieties with similar resistances are grown. In contrast, there were no significant attacks on the form of the hybrid carrying both efficient resistance genes. In the cage in which different forms of the hybrid were alternated, after 75% attack on the susceptible form, cropping of single resistance genes and then of the pyramided form, always gave less than 2% attack. The cage in which a mixture (25% of each hybrid form) was grown always showed one quarter of the attack in the cage containing the susceptible hybrid, and very little change in virulence. Thus, pyramiding, alternation or mixtures of a few major genes all appear to increase the duration of effective control of downy mildew. Their possible use on a large scale, the difficulties that would be encountered by both breeders and farmers and combination with other possible methods to limit downy mildew attacks are discussed

Evaluation of inoculation methods to reproduce sunflower premature ripening caused by phoma macdonaldii

Three inoculation methods were evaluated for effectiveness to cause sunflower premature ripening (PR). Evaluations were conducted on a sunflower (Helianthus annuus) cultivar susceptible to PR in replicated, multilocation experiments under greenhouse conditions. Plants were inoculated with Phoma macdonaldii, either with mycelium, conidia, or infected residues at the stem base or with buried residues. Disease severity (DS) was measured by percent girdling necrosis at the stem base and percent final PR; the infection spread was assessed using the area under the disease progress curve (AUDPC). Inoculation with mycelia or 1 × 106 spores/ml caused significantly more DS and PR than lower spore concentrations or infected residues (P < 0.05). Amending soil with residues induced root necrosis but no PR. P. macdonaldii was mainly isolated at the stem base and above but rarely on root systems. Microscopic evaluations showed that hyphae colonized mainly the cortex and vascular stem tissues. The overall results demonstrated a clear role of aerial infection in PR compared with soilborne inoculum, and that inoculation at the stem base with a spore suspension could be used for screening genotypes for resistance to PR