Changes in virulence of Plasmopara halstedii populations under different major gene (Pl) management strategies were studied over 5 years continuous cropping of one sunflower hybrid under netting cages. Strategies were monoculture of forms of the hybrid with 1 gene or with combinations of 2 genes, alternation of different genes, and mixtures of several different forms of the hybrid. Monoculture with single resistance genes led to loss of efficient resistance after 3 years, with high levels of disease and increased variability of the pathogen, whatever the Pl gene used. Combinations of genes, alternation and mixtures gave longer term control of downy mildew. In particular, combinations of resistance genes coming from both female and male parents of the hybrid (such that even impurities had a resistance gene) gave the best control and least variation in pathogen virulence. Results are discussed with the object of durable control of downy mildew by all methods available.

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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

AbstractmThe beneficial effect of arbuscular mycorrhizal (AM) fungi on plant growth is well known, but the physiological processes involved are still discussed. The purpose of this study was to determine if Glomus mosseae affects the growth of Hevea brasiliensis seedlings and, if it is the case, if it could be associated with variations in leaf CO2 and H2O gas exchange. H. brasiliensis rubber trees were grown for 9 months in a medium containing either propagules of G. mosseae or sterilized inoculum. Plant size, root collar diameter and leaf area, as well as net CO2 assimilation, stomatal conductance (gs) and water-use efficiency of photosynthesis were evaluated during the first 5 stages of growth. At stage 2, a growth depression occurred in the mycorrhizal seedlings coincident with the first AM infections. Then, at stage 5, Glomus mosseae-inoculated plants had moderate colonization (47% of root length) and were taller than control plants with a larger root collar diameter and an enhanced leaf organogenesis. This enhanced growth was accompanied by increased photosynthesis, transpiration, and stomatal conductance. After 9 months, dry weights of shoots and roots of inoculated plants were greater than those of controls by 27 and 17%, respectively.

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.