La résistance systémique induite en cultures fruitières et en grandes cultures

Context

The concept of sustainable agriculture leads to more interest in alternative means of pest and diseases control. However, pesticides use remains very intensive in agriculture. Over the period 1998-2000, the total amounts of active substances applied on average per annum and per hectare are estimated at 35 kg in apple culture, 29 kg on potatoes, 4,7 kg on sugar beets and 3,5 kg on cereals. In the wet areas, the fungicides represent the great majority of the products used in pome fruit culture and potatoes. In professional apple culture, fungicides sprayings account for on average 74% of the treatments carried out. In the other cultures, if the situation is not also alarming, diseases control concerns very important surfaces cultivated in the Walloon area. Plant disease control by the use of chemicals is actually the only economical solution, but encounters nevertheless several difficulties, particularly for the environment, by the risks of residues and the risks of the appearance of resistant pathogenic agents. In addition, within the framework of the Plant Protection Product Directive (91/414/EEC), the process of revision has already withdrawn half of the active substances from the market. Through the creation of commercial varieties less sensitive to the diseases, the use of decision support systems and the improvement of the spraying techniques, one complementary approach to reduce input rate for disease control consists in inducing a resistance in the crop plants thanks to the use of “elicitor” agents. Those are able to trigger the own plant defence system by activating various biochemical mechanisms which are normally latent. Thus it does not act directly against the pathogen but acts through the use of the natural defence reactions of the plant. This principle is often defined under the terms “induced systemic resistance” (ISR) and “systemic acquired resistance” (SAR).

Objectives

To study the effectiveness of several agents known as inductors of resistance to certain plants but the actions of which were never identified in the case of apple scab. New compounds will also be studied. They are minerals, vegetable extracts, micro-organisms, oligosaccharides resulting from seaweed and chemical molecules as control. The cultivar kinetics of the defence mechanisms induced by elicitors will also be studied. The combination of these factors could contribute to reduce the fungicidal treatments. Other work will aim at better understanding the biochemical mechanisms which are set up in the plants following their contact with elicitors. We hope to develop a relatively simple method to highlight the multiple actions of the induced resistance.

Description of tasks

The realization of several tests on apple seedlings in controlled conditions must lead to the selection of new inducers that act against scab. The most effective elicitors are tested in vitro to check if they have a direct action on the pathogenic agent. Other tests are carried out on grafted cultivars to study the interaction between the genetic resistance of the varieties and the effect of the elicitors. Various analysis methods are applied on leaves treated with elicitors in order to characterize the RSI such as measurements of enzymatic activities, the detection of the phenolic compounds and other biochemical analyses. An experimental orchard of 20 ares was planted in Gembloux at spring 2005, according to a split-plot design including five replications and with the possibility of testing five spraying schemes and a control. Each spraying scheme is a combination of a foliar elicitor application with a ground agent application. The orchard is constituted of three apple cultivars presenting variable tolerances for scab: `Reinette de Waleffe', old rather resistant variety, `Pinova', slightly tolerant variety and the Belgian standard ‘Jonagold', very sensitive. These varieties are grafted on the M9 rootstock. The main cryptogamic diseases and pests, as well as the evaluation of the quality of the fruits will be carried out according to the various spraying schemes applied. The first goal of this trial is to validate in real conditions the effectiveness tests of the elicitors already studied in controlled conditions against scab. A second goal is to evaluate the effect of these treatments on the incidence of the other apple diseases such as powdery mildew, canker and postharvest diseases. A tunnel sprayer is used for the application of the various spraying schemes to avoid the spindrift on the contiguous rows of apple trees. Later, work will be extended to field crops like potato and cereals.

Expected results

From the practical point of view, many factors must still be defined: the optimal amounts of active substances to use as well as the moments of their application in relationship with the phenologic stage of the plant, or the duration of the ISR effects; also to better define the place of these elicitors as disease control agents. Initially, they could be used as substitution agents of the chemicals and thus play a role within the framework of a strategy of delayed action of the appearance of pathogenic agents resistant to fungicides. It is also possible to use them in synergy with conventional fungicides, which would possibly make it possible to reduce the amounts of fungicides used. Finally, combined with the use of varieties not very sensitive to the diseases, they could substantially reduce the use of traditional fungicides. The potential results of this work are interesting both for the conventional grower as for those who work following Integrated Production Methods or organic agriculture.