Alexandra Schoeny

Influence of Take-All Epidemics on Winter Wheat Yield Formation and Yield Loss

ABSTRACT The effects of take-all epidemics on winter wheat yield formation were determined, and disease-yield relationships were established to assess the agronomic efficacy and economic benefits of control methods. Epidemics were generated in naturally infested fields by varying cropping season, crop order in the rotation, and experimental fungicide seed treatment. Disease incidence and severity were assessed from tillering to flowering. Yield components were measured at harvest. Models simulating the formation of the yield components in the absence of limiting factors were used to estimate the losses caused by take-all. Losses were predicted by the disease level at a specific time or the area under the disease progress curve, reflecting accumulation during a specific period. Losses of grain number per square meter and 1,000-grain weight were linked to cumulative disease incidence between the beginning of stem elongation and flowering, and disease incidence at midstem elongation, respectively. Yield losses were accounted for by both cumulative disease incidence between sowing and flowering, and disease incidence at midstem elongation. Results confirm the importance of nitrogen fertilization in reducing the impact of take-all on wheat.

Modeling of Take-All Epidemics to Evaluate the Efficacy of a New Seed-Treatment Fungicide on Wheat

ABSTRACT Take-all, caused by Gaeumannomyces graminis var. tritici, is a damaging disease of wheat that remains difficult to control. The efficacy of an experimental fungicide, applied as a seed treatment, was evaluated in five naturally infested field experiments conducted during three cropping seasons. Plants were sampled and assessed for take-all incidence and severity at different growth stages. Nonlinear models expressing disease variables as a function of degree-days were fitted to the observed data. The incidence equation involved two parameters reflecting the importance of primary and secondary infection cycles. The earliness of infection was identified as an important variable to interpret the effects of the fungicide. In an early epidemic, the fungicide significantly reduced take-all incidence during all or most of the cropping season, whereas in late epidemics, it provided only moderate reductions of incidence. The seed treatment reduced incidence by delaying the primary infection cycle. The fungicide significantly reduced severity during the whole epidemic. It appeared more efficient in limiting root-to-root spread than in slowing down the extension of necrosis on diseased roots.

Improvement and validation of a pea crop growth model to simulate the growth of cultivars infected with Ascochyta blight (Mycosphaerella pinodes)

A model simulating the growth of a pea crop infected with Ascochyta blight was improved and validated using 6 spring pea cultivars, all equally susceptible to Ascochyta blight, but differing in architectural features (stem height, branching ability, standing ability). This model takes into account the spatial distribution of the disease, including the contribution of each layer of the canopy to the radiation interception efficiency (RIE) and the radiation use efficiency (RUE) of the crop. The decreasing contribution of each layer due to the disease was estimated by the relationship between the photosynthesis of a layer and its disease score. The effect of disease on photosynthesis was assessed in controlled conditions as a means of evaluating the effect of disease on each cultivar. All cultivars were affected equally. In field conditions, cultivars with different canopy architectures displayed differences in the profile of disease on leaves. Cultivar Aladin reached higher disease levels at the top of the plant. Epidemics affected crop growth, and the cultivars tested differed in the magnitude of the decrease in growth. Observed and simulated data were compared. The disease-coupled crop growth model gave satisfactory predictions of crop growth for the six cultivars tested.

Effect and underlying mechanisms of pea-cereal intercropping on the epidemic development of ascochyta blight

Field experiments were conducted in western France for two consecutive years to investigate the effect of pea-cereal intercropping on ascochyta blight, a major constraint of field pea production world-wide. Disease pressure was variable in the experiments. Intercropping had almost no effect on disease development on stipules regardless of disease pressure. In contrast, disease severity on pods and stems was substantially reduced in the pea-cereal intercrop compared to the pea monocrop when the epidemic was moderate to severe. Therefore, a pea-cereal intercrop could potentially limit direct yield loss and reduce the quantity of primary inoculum available for subsequent pea crops. Disease reduction was partially explained by a modification of the microclimate within the intercrop canopy, in particular, a reduction in leaf wetness duration during and after flowering. The effect of intercropping on splash dispersal of conidia was investigated under controlled conditions using a rainfall simulator. Total dispersal was reduced by 39 to 78% in pea-wheat canopies compared to pea canopies. These reductions were explained by a reduction in host plant density and a barrier or relay effect of the non-host plants.

Vat, an Amazing Gene Conferring Resistance to Aphids and Viruses They Carry: From Molecular Structure to Field Effects

We review half a century of research on Cucumis melo resistance to Aphis gossypii from molecular to field levels. The Vat gene is unique in conferring resistance to both A. gossypii and the viruses it transmits. This double phenotype is aphid clone-dependent and has been observed in 25 melon accessions, mostly from Asia. It is controlled by a cluster of genes including CC-NLR, which has been characterized in detail. Copy-number polymorphisms (for the whole gene and for a domain that stands out in the LLR region) and single-nucleotide polymorphisms have been identified in the Vat cluster. The role of these polymorphisms in plant/aphid interactions remains unclear. The Vat gene structure suggests a functioning with separate recognition and response phases. During the recognition phase, the VAT protein is thought to interact (likely indirectly) with an aphid effector introduced during cell puncture by the aphid. A few hours later, several miRNAs are upregulated in Vat plants. Peroxidase activity increases, and callose and lignin are deposited in the walls of the cells adjacent to the stylet path, disturbing aphid behavior. In aphids feeding on Vat plants, Piwi-interacting RNA-like sequences are abundant and the levels of other miRNAs are modified. At the plant level, resistance to aphids is quantitative (aphids escape the plant and display low rates of reproduction). Resistance to viruses is qualitative and local. Durability of NLR genes is highly variable. A. gossypii clones are adapted to Vat resistance, either by introducing a new effector that interferes with the deployment of plant defenses, or by adapting to the defenses it triggered. Viruses transmitted in a non-persistent manner cannot adapt to Vat resistance. At population level, Vat reduces aphid density and genetic diversity. The durability of Vat resistance to A. gossypii populations depends strongly on the agro-ecosystem, including, in particular, the presence of other cucurbit crops serving as alternative hosts for adapted clones in fall and winter. At the crop level, Vat resistance decreases the intensity of virus epidemics when A. gossypii is the main aphid vector in the crop environment.

Molecular and biological characterization of two potyviruses infecting lettuce in southeastern France

Several potyviruses affect lettuce (Lactuca sativa) and chicory (Cichorium spp.) crops worldwide and are important constraints for production because of the direct losses that they induce and/or because of their seed transmission. Here, the molecular and biological properties are described of two potyviruses that were recently isolated from lettuce plants showing mosaic or strong necrotic symptoms in an experimental field in southeastern France. The first potyvirus belongs to the species Endive necrotic mosaic virus and is present in a large number of wild plant species, especially Tragopogon pratensis. It is unable to infect lettuce cultivars with a resistance to Turnip mosaic virus that is present in many European cultivars and probably conferred by the Tu gene. The second potyvirus belongs to the tentative species lettuce Italian necrotic virus and was not observed in wild plants. It infected all tested lettuce cultivars. Wild accessions of Lactuca serriola, Lactuca saligna, Lactuca virosa and Lactuca perennis were identified as resistant to one or the other potyvirus and could be used for resistance breeding in lettuce. No resistance against these two potyviruses was observed in the tested Cichorium endivia cultivars. In contrast, all tested Cichorium intybus cultivars or accessions were resistant.