Françoise Montfort

The effect of soil moisture content on the sorption of five sterol biosynthesis inhibiting fungicides as a function of their physicochemical properties

We investigated the sorption of five widely used sterol biosynthesis inhibitor fungicides (SBIs: flusilazole, propiconazole, epoxiconazole, fenpropimorph and prochloraz) on a loam soil to assess availability of the SBI residues that are usually left in soil after crop treatments. We focused particularly on the soil moisture content effect, which is poorly documented and is difficult to investigate under realistic conditions. SBI sorption was determined (using diuron as a reference) at two soil moisture contents (26.1% and 46.6% w/w) over a period of 3 weeks using a direct soil solution sampling method. After 24 h of contact, <1% of each applied fungicide was recovered in the soil solution. Despite their low availability in the liquid phase, long-term sorption was observed for all the compounds, reducing concentrations in the soil solution and doubling the value of the partition coefficient. Significant effects of soil moisture on long-term sorption were observed, depending on the properties of the chemicals and the sorption mechanisms. Wershaws humus model (humic substances have a membrane-like structure) was adapted to fit our observations. Low soil moisture content is assumed to modify the structure of humic substances and to generate hydrophobic surfaces, which favour sorption of hydrophobic fungicides (flusilazole, propiconazole and epoxiconazole). This effect is likely to decrease with the increase in the hydrophobic character of non-ionic pesticides. It becomes adverse for the more hydrophilic compounds (diuron), which are more sorbed at high soil moisture content due to their higher affinity for hydrophilic regions of humus and to diffusion. Soil moisture effects are more complex when compounds are likely to be protonated in soil. Weakly basic compounds (prochloraz) may partition rapidly into the liquid-like interior of humus at low soil moisture content but increased diffusion at high soil moisture content may cause additional sorption by ion exchange at colloid surfaces. Strongly basic compounds (fenpropimorph) may essentially adsorb due to ionic interactions with colloids, and their sorption is enhanced at high soil moisture content due to diffusion. Consequences for environmental fate and biological activity of pesticides are briefly discussed. © 2000 Society of Chemical Industry

Effects of Two Triazole Seed Treatments, Triticonazole and Triadimenol, on Growth and Development of Wheat

The triazole fungicides, triadimenol and triticonazole, applied as seed treatments at various rates on spring wheat, affected plant growth, shoot development, and root axis production. The main growth regulatory effects were reductions in lengths of the coleoptile, the first leaf and the subcrown internode. A marked effect on tiller appearance occurred with high rates of triadimenol. Modification of root-system development included a reduction in the number of seminal roots, increased outgrowth of roots associated with the coleoptilar node, and a reduction of roots at the first foliar node (correlated with reduced production of Tiller 1). Seed treatment effects on both shoot and root development illustrated the close relationship between these plant structures. Both fungicides induced both beneficial and deleterious effects on wheat growth and development, but the magnitude of deleterious effects was less with triticonazole than with triadimenol.

Injury profile SIMulator, a Qualitative aggregative modelling framework to predict injury profile as a function of cropping practices, and abiotic and biotic environment. II. Proof of concept: design of IPSIM-wheat-eyespot.

IPSIM (Injury Profile SIMulator) is a generic modelling framework presented in a companion paper. It aims at predicting a crop injury profile as a function of cropping practices and abiotic and biotic environment. IPSIMs modelling approach consists of designing a model with an aggregative hierarchical tree of attributes. In order to provide a proof of concept, a model, named IPSIM-Wheat-Eyespot, has been developed with the software DEXi according to the conceptual framework of IPSIM to represent final incidence of eyespot on wheat. This paper briefly presents the pathosystem, the method used to develop IPSIM-Wheat-Eyespot using IPSIMs modelling framework, simulation examples, an evaluation of the predictive quality of the model with a large dataset (526 observed site-years) and a discussion on the benefits and limitations of the approach. IPSIM-Wheat-Eyespot proved to successfully represent the annual variability of the disease, as well as the effects of cropping practices (Efficiency = 0.51, Root Mean Square Error of Prediction = 24%; bias = 5.0%). IPSIM-Wheat-Eyespot does not aim to precisely predict the incidence of eyespot on wheat. It rather aims to rank cropping systems with regard to the risk of eyespot on wheat in a given production situation through ex ante evaluations. IPSIM-Wheat-Eyespot can also help perform diagnoses of commercial fields. Its structure is simple and permits to combine available knowledge in the scientific literature (data, models) and expertise. IPSIM-Wheat-Eyespot is now available to help design cropping systems with a low risk of eyespot on wheat in a wide range of production situations, and can help perform diagnoses of commercial fields. In addition, it provides a proof of concept with regard to the modelling approach of IPSIM. IPSIM-Wheat-Eyespot will be a sub-model of IPSIM-Wheat, a model that will predict injury profile on wheat as a function of cropping practices and the production situation.

Interplay between parasitism and host ontogenic resistance in the epidemiology of the soil-borne plant pathogen Rhizoctonia solani.

Spread of soil-borne fungal plant pathogens is mainly driven by the amount of resources the pathogen is able to capture and exploit should it behave either as a saprotroph or a parasite. Despite their importance in understanding the fungal spread in agricultural ecosystems, experimental data related to exploitation of infected host plants by the pathogen remain scarce. Using Rhizoctonia solani / Raphanus sativus as a model pathosystem, we have obtained evidence on the link between ontogenic resistance of a tuberizing host and (i) its susceptibility to the pathogen and (ii) after infection, the ability of the fungus to spread in soil. Based on a highly replicable experimental system, we first show that infection success strongly depends on the host phenological stage. The nature of the disease symptoms abruptly changes depending on whether infection occurred before or after host tuberization, switching from damping-off to necrosis respectively. Our investigations also demonstrate that fungal spread in soil still depends on the host phenological stage at the moment of infection. High, medium, or low spread occurred when infection was respectively before, during, or after the tuberization process. Implications for crop protection are discussed.

Main Lessons Drawn from the Analysis of the Literature

Planting and destruction are the two main phases in cover crop management techniques. Their dates and implementation conditions play a key role in terms of the various functions targeted for these crops (nitrate-trapping, erosion prevention, weed control, effect on next crop, etc.). The analysis of the literature therefore focused on cover crop planting and destruction techniques. The aim was to identify the different methods and, if possible, their effectiveness with respect to the success of sowing and destruction. The issue of the implementation of these techniques by farmers was then examined by analysing the constraints in terms of organization of labour and implementation costs.