Variabilité de l’agressivité de Plasmopara halstedii, étude de sa stabilité en test de laboratoire

Evolution of pathogenicity in Plasmopara halstedii was analysed by successive artificial infections of two strains with different levels of aggressiveness on two sunflower genotypes not carrying major gene resistance but which show different levels of partial resistance. Symptoms, in particular of plant size and sporulation density, confirmed the differences between the two lines observed in the field. The parasite populations studied showed considerable adaptation to infection conditions: the percentage of plants showing severe symptoms increased by 5% at each infection cycle, for both pathogen strains and both sunflower genotypes. These first laboratory results need to be confirmed in the field to judge the possible durability of quantitative resistances which could be introduced in sunflower breeding programmes

A new method to obtain monozoosporangial isolates of sunflower downy mildew (Plasmopara halstedii)

A new method to obtain monozoosporangial strains was applied on 9 pathotypes of Plasmopara halstedii (100, 300, 304, 314, 700, 704, 710, 707 and 714), the parasite causing sunflower downy mildew. Single zoosporangia were isolated from the surface of agar medium and placed on leaf disks on solid Knop medium. The best fungal development was obtained with leaf disks taken from the first pair of sunflower leaves when these measured from 5 to 8cm in length. The percentage success rate in obtaining monozoosporangial strains varied from 1.4 to 7.4% according to race. The percentage of disks showing sporulation depends on spore viability, the physiological state of leaves and the receptivity of the sunflower genotype. This method enabled us to confirm the presence of a new downy mildew pathotype in France: race 707

Variation in form and size of Plasmopara halstedii (sunflower downy mildew) zoosporangia

Zoosporangia form and size were studied on a collection of 94 strains of Plasmopara halstedii (sunflower downy mildew). Both oval and round forms were present in all strains analysed. The proportion of two forms varied significantly according to strain and plant age but more especially to host plant genotype. Whatever the strain or host genotype, oval zoosporangia were larger than round ones, but there was no relation between the proportion of the oval form and mean zoosporangia size. There was no relation between zoosporangia form or size and race virulence profiles or aggressiveness criteria, with the possible exception of zoosporangia size and sporulation density. It is concluded that, for this obligate parasite, although form and size of zoosporangia depend on pathogen strain, these characters also vary according to growth conditions of Plasmopara halstedii, in particular to the genotype of the plant host

Development of a PCR test to detect the downy mildew causal agent Plasmopara halstedii in sunflower seeds

Plasmopara halstedii, the causal agent of downy mildew of sunflower, is an obligate parasite but viable sporangia and oospores of the pathogen may be found in a quiescent state in seeds of sunflower and therefore may be transported with sunflower seeds in international commercial exchanges. In order to prevent the spread of this pathogen, especially the introduction of potentially new races, an efficient method to analyse sunflower seed samples is required. In this study, a P. halstedii-specific polymerase chain reaction (PCR) test was developed based on the ribosomal large sub unit (LSU) DNA. The forward (PHAL-F) and reverse (PHAL-R) PCR primers were designed from two polymorphic regions of LSU. After screening 22 isolates of P. halstedii corresponding to different races and countries and 32 other oomycete, deuteromycete and ascomycete isolates, the PHAL-F/R primers amplified only a single PCR band of c. 310 bp from P. halstedii. The PHAL-F/R PCR test could detect as little as 3 pg of P. halstedii genomic DNA per 20 mu L reaction volume and enabled the direct detection of P. halstedii in 35 g sunflower seed samples without the need for any prior biological baiting step. An internal amplification control (IAC) was developed to help discriminate against false negative samples due to the potential presence of inhibitory compounds in DNA extracts. The test was successfully used on samples of naturally contaminated seeds. These new molecular tools should be of great interest for quarantine seed testing purposes

A Method to Measure Aggressiveness of Plasmopara halstedii (Sunflower Downy Mildew)

For the first time, a method was used to measure aggressiveness of two Plasmopara halstedii races (100 and 710), the parasite causing sunflower downy mildew. Two sunflower lines showing different levels of quantitative resistance were used to measure two aggressiveness criteria: latent period and sporulation density. A strain of race 100 had a shorter latent period and greater sporulation density than a strain of race 710. The sunflower inbred line BT, rather susceptible in the field, presented a greater sporulation density and a shorter latent period than another inbred line FU, which shows greater resistance in the field. These results indicated that race 100 was more aggressive than race 710. The behaviour in the field of the two inbred lines was confirmed in the laboratory observations

Molecular variability within Diaporthe/Phomopsis helianthi from France

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Molecular variability of sunflower downy mildew, Plasmopara halstedii, from different continents

Downy mildew of sunflower (Helianthus annuus L.), caused by the pathogen Plasmopara halstedii, is a potentially devastating disease. Seventy-seven isolates of P. halstedii collected in twelve countries from four continents were investigated for RAPD polymorphism with 21 primers. The study led to a binary matrix, which was subjected to various complementary analyses. This is the first report on the international genetic diversity of the pathogen. Similarity indices ranged from 89% to 100%. Neither a consensus unweighted pair group method with arithmetic means (UPGMA) tree constructed after bootstrap resampling of markers nor a principal component analysis based on distance matrix revealed very consistent clusterings of the isolates, and groups did not fit race or geographical origins. Phylogenies were probably obscured by limited diversity. Analysis of molecular variance (AMOVA) and Nei's genetic diversity statistics gave similar conclusions. Most of the genetic diversity was attributable to individual differences. The most differentiated races also had the lowest within-diversity indices, which suggest that they appeared recently with strong bottleneck effects. Our analyses suggest that this pathogen is probably homothallic or has an asexual mode of reproduction and that gene flow among countries can occur through commercial exchanges. Knowledge of the downy mildew populations' structure at the international level will help to devise strategies for controlling this potentially devastating disease

Variation in aggressiveness of Plasmopara halstedii (sunflower downy mildew) (vol 116, pg 247, 2009)

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