METHODOLOGICAL STUDY OF

LEPTOSPHAERIA MACULANS ASCOSPORES TRAPPING ON RAPESEED

                                            André Pérès and José Fernandès

                   CETIOM, Centre de Grignon, BP 4, 78850 Thiverval-Grignon, France

                                               e-mail : peres@cetiom.fr

                                                                   ABSTRACT

The efficiency of early fungicidal treatments to control Phoma on rapeseed in autumn was studied by setting up a control-network allowing to record the first releases of Leptosphaeria maculans-ascospores. This network has to be made up of  reliable, and if possible cheap, traps.   Therefore, in a first study, we compared two different types of traps : one is called "passive" and aims at defining the functioning of inoculum on the soil, and the other one is called "dynamic" since it gives wider information on the contaminating potential of the plot and its near environment.    Despite a rather good coherence of obtained results between both systems, it appeared that the "dynamic" type provided more complete biological information. That is the reason why a second study comparing three "dynamic" traps was carried out. It underlined the interest of a device designed by CETIOM and offering the best compromise between reliability and costs.

KEYWORDS : winter rape, Phoma lingam, spores, traps

INTRODUCTION

The large recrudescence of collar necrosis due to Leptosphaeria maculans (sexual form of Phoma lingam) makes up the greatest preoccupation of rapeseed producers, specially in Center and       Western regions of France (Lepage and al., 1994). In these regions, yield losses  were very           important over the last years. The genetic control will offer the cheapest and most efficient          protection with the development of tolerant varieties. In the meantime, the fungicidal control is     necessary and secures a good result if the treatment is applied very near the first ascospore releases (Penaud, 1997). Trapping ascospores is the means to give  treatment advice. The first aim of the          following studies is to compare two systems of ascospore trapping, and then to compare three types of traps.

MATERIALS AND METHODS

Study 1 : Comparison of two trapping systems

Two trapping systems were compared in a rapeseed field during the period September (emergence)-November 1995 :

- a trapping system called "passive" (rudimentary system without active mechanism)

- a trapping system called "dynamic" (more elaborate system : "Burkard" trap with active             mechanism)

The "passive trap" was a railing fixed on a frame (40cm x 40cm) ; two slides coated with ovalbumin are placed on the railing. This trap was placed on the soil just above fifteen rape stubbles naturally infected by phoma on collar ; the distance between slide and stubble was 1.5 cm. This trap   caught only the ascospore releases by underlying stubbles. Now it is used on all the CETIOM experimental sites and on some sites of Crop Protection Agencies.

The "dynamic trap" was an English machine equipped with an air aspiration system and a movable captor element (slide coated with ovalbumin defiling just behind a little aspiration split orientated towards the wind by a weathercock system. It was connected to a 12 volt battery coupled with a    solar panel. It was placed above the ground and it caught ascospores in suspension in the air. Two sites of CETIOM were equipped with the dynamic trap : St. Pathus (Figure 1) and St. Florent       (Figure 2).

In both cases, captor elements were standard slides coated with ovalbumin used in optical micro-  scopy (76mm x 26mm). Every day (or every two days) these slides were changed and ascospores were counted under microscope.

Study 2 : Comparison of three types of dynamic traps

It concerned the comparison of three dynamic traps placed in a winter rapeseed field during the first stages of the crop (locality : St. Pathus) :

- the "Burkard" trap previously described

- a trap called "JFC" and made by CETIOM (same principle as Burkard trap but it analyses 40% of supplementary air)

- an American trap called " Rotorod Sampler" (RTS) : as in the case of preceding types, this trap caught ascospores in the air by the speedy moving of captor elements (Sampling Technologies, 1996).

In the case of Burkard and JFC traps, captor elements were laboratory standard slides (c.f. study 1) and in the case of RTS, captor elements were polyethylen rods coated with silicone grease (32mm x 2mm). Every day, slides and rods were changed and ascospores were counted under microscope.

RESULTS - DISCUSSION

Study 1 : Comparison of two trapping systems

The comparison of ascospore counting between both types of traps allowed to record the            synchronization and the intensity of trapping on the sites of St. Pathus and St. Florent (Figures 1 and 2).

On both sites, the trapping synchronization between both traps was relatively good during the       period October 15 to end of trapping. Nevertheless, the passive trap did not allow us  to record the first ascospore releases during the period  September 15- October 15 unlike the "dynamic" trap (figures 1 and 2). In fact, the latter was able to make a  larger trapping (ascospores in the air from the field or from neighbouring fields) whereas the passive trap  only gave information  about ascospores released by underlying stubbles.

This deficiency of passive trap about the first ascospore releases (often very little releases) was      also observed  in the two other sites where the leaf spots were observed before trapping.

The intensity of trapping was different between traps and localities. In St. Pathus, generally, the passive trap gave the strongest intensity, which   reached 70,000 ascospores by slide (figure 1). The following explanation may be given : the passive trap analyses a concentrated "cloud" of                ascospores, unlike the dynamic trap which catches ascospores "diluted" in the air. In St. Florent, this tendency was less clear,  and the maximum of trapping was 1,600 ascospores by  slide (figure 2).

We could  observe a good relation between rainy sequencies and ascospore release sequencies.

In conclusion, the synchronization of ascospore trapping was relatively good between both types of trapping when the inoculum was entirely functional (from mid/end October). Nevertheless, the    differences observed in the intensity of releases (particularly in St. Pathus) or about the ability to     account for first ascospore releases (September 15-October 15) were revealing of the basal          differences of both systems (in fact, they gave two types of information). The dynamic trap gave  wider information on the contaminating risk potential by a large aerian trapping. On the other hand, the passive trap gave  more restrictive information (only  information about functioning of inoculum on the soil with a risk of error because of the limited representativity of infected stubbles).

Considering these results, the dynamic trap was the best system because it accounted for the wider epidemiological reality. Consequently, in a second time, it was necessary to carry out a study about       different types of dynamic traps (study 2).

Study 2 : Comparison of three types of dynamic traps

On the whole, the synchronization of trapping was satisfactory between the three traps. Concerning the number of ascospores caught by a slide (or a rod), the most efficient trap was the JFC, then the Burkard and lastly the RTS. In comparison with the Burkard, the superiority of the JFC was due to the larger quantity of air analysed (+40%). Moreover, both  equipments allowed to count ascospores easily (slides without dust and water dropslet). On the other hand, the counting of ascospores on the rods of the RTS was often more difficult when the weather was foggy and windy. The rods of RTS was clogged up by dust and were droplets (because they stir air by their rotary moving).

In conclusion, JFC and Burkard traps based on the same  principle gave approximately the same    results concerning the reliability and performance to a less degree (the JFC catching more asco-     spores than the Burkard). The RTS with its different principle was less reliable under particular   conditions (fog, wind and strong rains). CETIOM is currently using the JFC trap in the trapping network in France because of its reliability and  cost (cheaper than the Burkard).

CONCLUSION

The whole methodological study underlined the interest of dynamic trapping which gave  wider biological information. Particularly, it underlined the interest of JFC machine because of its three    qualities : performance, reliability and cheaper costs.

REFERENCES

LEPAGE R., PENAUD A., REGNAULT Y., PERES A. and PINOCHET X., 1994. Phoma du colza - Numéro Spécial Oléoscope - P.5.

PENAUD A., 1997. A la recherche du bon indicateur pour traiter. Oléoscope n°35. p.16-17

SAMPLING TECHNOLOGIES, 1996. Operating Instructions for the Rotorod Sampler p.1-15

Figure 1 - Comparison between passive and dynamic trapping at Saint-Pathus in 1995 (results expressed in numbers of trapped spores on a sensor blade)

Figure 2 -  Comparison between passive and dynamic trapping at Saint-Florent in 1995      (results expressed in numbers of trapped spores on a sensor blade)

                  sp : first leave spots

Figure 3 - Comparison between spore captures with three types of dynamic traps (Burkard - JFC - RTS)

  (results expressed in number of ascospores trapped on a slide or rod)