Agnes Calonnec

Effects of induced resistance on infection efficiency and sporulation of Puccinia striiformis on seedlings in varietal mixtures and on field epidemics in pure stands

An inducer race ofPuccinia striiformis inoculated two days before a challenger race on wheat seedlings of cv. Clement, reduced infection efficiency by 44% and lesion expansion by 7.7%; there was no effect on sporulation rate. In field epidemics, induced resistance restricted the disease intensity on cvs. Clement and Austerlitz in pure stands by 44% and 57% respectively. In the glasshouse at the seedling stage, disease intensity in a varietal mixture was reduced by 82% compared to the susceptible pure stands; one third of the total disease reduction was attributable to induced resistance.

Switching from a mechanistic model to a continuous model to study at different scales the effect of vine growth on the dynamic of a powdery mildew epidemic

BACKGROUND AND AIMS Epidemiological simulation models coupling plant growth with the dispersal and disease dynamics of an airborne plant pathogen were devised for a better understanding of host-pathogen dynamic interactions and of the capacity of grapevine development to modify the progress of powdery mildew epidemics. METHODS The first model is a complex discrete mechanistic model (M-model) that explicitly incorporates the dynamics of host growth and the development and dispersion of the pathogen at the vine stock scale. The second model is a simpler ordinary differential equations (ODEs) compartmental SEIRT model (C-model) handling host growth (foliar surface) and the ontogenic resistance of the leaves. With the M-model various levels of vine development are simulated under three contrasting climatic scenarios and the relationship between host and disease variables are examined at key periods in the epidemic process. The ability of the C-model to retrieve the main dynamics of the disease for a range of vine growth given by the M-model is investigated. KEY RESULTS The M-model strengthens experimental results observed regarding the effect of the rate of leaf emergence and of the number of leaves at flowering on the severity of the disease. However, it also underlines strong variations of the dynamics of disease depending on the vigour and indirectly on the climatic scenarios. The C-model could be calibrated by using the M-model provided that different parameters before and after shoot topping and for various vigour levels and inoculation time are used. Biologically relevant estimations of the parameters that could be used for its extension to the vineyard scale are obtained. CONCLUSIONS The M-model is able to generate a wide range of growth scenarios with a strong impact on disease evolution. The C-model is a promising tool to be used at a larger scale.

Modeling of the Invasion of a fungal Disease over a vineyard

The spatiotemporal spreading of a fungal disease over a vineyard is investigated using a susceptible-exposed-infected-removed (SEIR)-type model coupled with a set of partial differential equations describing the dispersal of the spores. The model takes into account both short and long range dispersal of spores and growth of the foliar surface. Results of numerical simulations are presented. A mathematical result for the asymptotic behavior of the solutions is given as well.

Chromosomal location of genes for resistance to Puccinia striiformis in the wheat line TP1295 selected from the cross of Soissonais-Desprez with Lemhi

Crosses of a wheat line TP1295 with the cultivar Chinese Spring monosomic series were used to locate, on chromosome 1D, a major gene for resistance to isolate WYR 85-22 of race 6E0 of Puccinia striiformis. The gene is designated as Yr25 and is probably present in several of the cultivars currently widely used for differentiating races of this pathogen. The expression of the gene was modified by the environment and by at least one minor gene which may be located on chromosome 6A. In F2 and F3 generations from a cross between TP1295 and euploid Chinese Spring, a wide range of variation in infection type (IT) was observed. This precluded the classification of the plants as either resistant or susceptible, so they were assigned to 6 classes and analyzed by factorial correspondence analysis and non-hierarchical classification. When all F3 plants in a family were fully resistant, like TP1295 itself (IT ;), both Yr25 and the modifying gene were assumed to be present and homozygous. In environments favourable to expression of the gene, families thought to carry Yr25 alone had a distribution of ITs from fully resistant (IT ;) to intermediate (IT 2, rarely 3 or 3+). This F3 analysis indicated that use of IT data alone, in the monosomic analysis, would not reveal the chromosomal location of the genes and that chromosome counting of numerous plants was necessary. As well as indicating the chromosomes carrying the genes for resistance to isolate WYR 85-22, the data showed that plants monosomic for chromosomes 5B and 5D were more resistant than the corresponding disomics, indicating that these chromosomes promote susceptibility and supporting other evidence of the effects of these chromosomes on yellow rust resistance.

Efficacy of fungicides with various modes of action in controlling the early stages of an Erysiphe necator-induced epidemic.

BACKGROUND Limiting the use of fungicides is due to become an important issue in managing Erysiphe necator (Schwein) Burrill infections in vineyards. The authors determined how three fungicides currently used by vine growers could be managed to control the early stages of an E. necator-induced epidemic. RESULTS Leaf-disc bioassays and field experiments suggested that the protectant quinoxyfen induced minor disruption in E. necator development, but compounds with protectant and curative properties (tebuconazole and trifloxystrobin) caused significant, although different, disruption during E. necator-induced epidemics. Bioassays showed that each of the antifungals were most effective at different stages of fungal development, tebuconazole before sporulation and trifloxystrobin after sporulation of the colonies. Results from the bioassay also highlighted likely occurrences in the field, where several stages of fungal development are encountered simultaneously. CONCLUSION The present findings were complementary: leaf-disc tests showed when the fungicides were most effective at inhibiting E. necator infection cycles; the field trial provided results in terms of incidence and severity of disease on bunches without reference to the pathogenic cycle development. A protection strategy combining the different types of fungicide under study is suggested.

Parallel simulation of the propagation of powdery mildew in a vineyard

This paper describes a parallel simulator for the propagation of a parasite in a vineyard. The model considers the structure, the growth and the susceptibility of the plant which play a major role in the development of the fungus and the spread of epidemic. Two spatial scales are distinguished for the dispersal of the parasite. We use both a realistic discrete model for the local dispersal, and a stochastic model for the long-range dispersal that averages the displacement of spores. An algorithmic description of the parallel simulator is given and real life numerical experiments on IBM SP3 are provided, that use up to 128 processors.

Evaluation of a decision support strategy for the control of powdery mildew, Erysiphe necator (Schw.) Burr., in grapevine in the central region of Chile

BACKGROUND The primary strategy to control powdery mildew in Chilean vineyards involves periodic fungicide spraying, which may lead to many environmental and human health risks. This study aimed to implement and evaluate the effectiveness and economic feasibility of a novel decision support strategy (DSS) to limit the number of treatments against this pathogen. An experiment was conducted between the 2010 and 2013 seasons in two irrigated vine fields, one containing a cultivar of Cabernet Sauvignon (CS) and the other a cultivar of Chardonnay (CH). RESULTS The results showed that the DSS effectively controlled powdery mildew in CS and CH vine fields, as evidenced by a disease severity lower than 3%, which was lower than that observed in untreated vines (approximately 10 and 40% for CS and CH respectively). The DS strategy required the application of only 2-3 fungicide treatments per season in key vine phenological stages, and the cost fluctuated between

Modeling the spread of a pathogen over a spatially heterogeneous growing crop

US 322 and 415 ha-1 , which was 40-60% cheaper than the traditional strategy employed by vine growers. CONCLUSION The decision support strategy evaluated in this trial allows a good control of powdery mildew for various types of epidemic with an early and late initiation.

Effect of Crop Growth and Susceptibility on the Dynamics of a Plant Disease Epidemic: Powdery Mildew of Grapevine

The spread of a pathogen within an anthropized crop plot depends on many factors acting at contrasted spatio-temporal scales. This is of paramount importance for vine and powdery mildew, one of its airborne pathogen. We aimed at developing a coupled PDEs-ODEs model for plant-pathogen interactions at the plot scale level in order to assess the effects of various host heterogeneities on the epidemic spread.

A host-pathogen simulation model: powdery mildew of grapevine

For a better understanding of host / pathogen dynamical interactions and of the capacity of host development to modify disease progress, we devised epidemiological simulation models coupling plant growth with the dispersal and disease dynamics of an airborne plant pathogen. Our first model is a complex discrete deterministic model that explicitly incorporates the dynamics of host growth (e.g., distance between organs and their susceptibility) and the development and dispersion of the pathogen. The second one is a much simpler compartmental SEIR-like model. It takes into account the host growth with the foliar surface as unit of host growth and the ontogenetic resistance of the leaves. We compare the output of both models when applied to the specific system formed by the powdery mildew, Erysiphe necator, of grapevine.