ABSTRACT

Blackleg disease caused by Leptosphaeria maculans can be controlled using resistant cultivars and chemical control. To apply fungicide only when necessary, a method involving epidemic and agronomic risks has been developped. Its application requires two steps. First, the risk of contamination by ascospores can be forecast using weather or biological indicators such as 7 days of rainfall after sowing, 50 percent of mature pseudothecia and a threshold of 20 trapped ascospores per day. Then, a set of four criteria, such as the cultivar susceptibility, the type of soil, the growth stage and the vigour of the crop, will be studied before deciding to apply a fungicide for disease control.

This method has been evaluated for two years under field conditions in a network of treated and untreated plots. 65 trials have been carried out in various oilseed cropping areas. In each case, disease severity and yield were measured.

The results show that the three indicators are rather similar to predict the first risk of contamination. Due to an important development of the crop before the risk of contamination occurred, the recommendation of a treatment was not frequent over the last two years. Consequently, data concerning the decision of treatment failed. Under these conditions, a complete evaluation of the decision method is not possible yet. Nevertheless the study will go on for another year. After a rainy autumn, we expect a high pressure of the disease, which should allow use to evaluate all the conditions of the decision schedule.

INTRODUCTION

Blackleg, caused by Leptosphaeria maculans, can cause important damages despite the use of tolerant varieties and the setting up of prophylactic measures. In these conditions, the chemical control can then out necessary but to be economically justified, it must be well thought-out. This is in this perspective that, for two years, the CETIOM has set up a network of trials aiming at testing a schedule of decision. This schedule is a tool susceptible to help the farmer in a real time to decide the necessity of a treatment against phoma. In order to do it, four variables are taken in consideration :

• The cultivar susceptibility (Pierre & Regnault, 1982).

• The type of soil

• The stage of the rapeseed.

• The aspect of the rapeseed.

These variables are placed in a schedule in which they interact to lead to a decision. It is this schedule which is called the "schedule of decision". This present study aims at validating this schedule in the conditions of the agricultural practices. Its application starts at the preliminary announcement of a top treatment given by a climatic or a biological indicator.

MATERIALS AND METHODS

Device

The experimental device is a system of lines without repetition whose facility of implementation and of following allows to increase the number of testing. This allows to cape with situations as varied as possible. It is composed of an untreated check and of three plots, each getting Eria at 2l/ha (ai : 62.5 g/l of difenoconazole + 125 g/l of carbendazime) applied to a data given by an indicator (Pilogré & Penaud, 1982). The used indicators are :

* After seven days of rain drawn simultaneously (rain superior to 1 mm) = Top Rain.

* At the moment when 50 percent of the pseudothecia are mature (for only the second testing year) = Top Pseudothecia

* When the first ascospores are trapped (Le Page & Penaud, 1995) = Top Ascospores.

At the moment of the treatment, for each device, the stage and the aspect of the observed rapeseed in the plots are placed in the schedule. Then, this one says whether one must spray or not. The tested schedule is the following :

Decision Rule

NOTATIONS

The notations deal with the necrosis on collar three weeks before the harvest using the index G2 (Pierre and al, 1982), note whose value varies from 0 to 9.

This note is more reliable them the ones based on the leaf spots whose link with the yield is not obvious. The yield can not be considered to be a criterion of reliable notation inasmuch as the visual aspect of the rapeseed intervenes in the schedule to characterise the situations when one must treat it. So, in the cases of the schedule's advice treat it, one can not know whether a low yield of the control lines is due to phoma or simply to the naturally lower potential of a weak rapeseed. Thus, one takes the risk to advantage the schedule when considering the phoma as the only responsible for low yield.

RESULTS

Comparing the results of control plots to the treated ones, one check if the advice of the schedule was judicious, the comparison criterion being the G2 note. This comparison is realised by means of a test of Student putting together the devices according to the recommendations of the schedule.

Overall behaviour

Out of 147 recommendations collected during two years, for 112 pieces of advice of no-treatment given by the schedule there are only 36 situations in which the treatment was adviced. Comparing the untreated control plots between themselves, one see that a significiant difference exists between the note of the situations with the advice of treatment and the note of situations without recommendations. This, the schedule seems to allow to distinguish the risky situations (table one).

Table 1 : Comparison of control parcels according to the recommendations

Behaviour according to the indicators of the fungicidal treatment

If one compares the periods of intervention apart from the schedule, the difference of G2 note between treated and untreated is more or less constant, whatever the indicator of treatment used (figure 1). If one compare the recommendations of the schedule apart from the periods of interventions, one notices that the rule allows to evaluate a risky situation, the G2 notes in the case in which the treatment is adviced being invariably stronger. Lastly, if one compares the recommendations of the schedule according to the intervention periods - Then, one can only compare the top ascospores to the top rain, the top pseudothecia only tested since 1998 has not given any recommendations of treatment -, one realises a better answer of the schedule to the top rain, the difference of G2 note between "treated" and "without advice of intervention" being higher in the case of the top ascospores (for 0.82 there is 0.62).

Figure 1 : Answer of the tops to the recommendations

Practical interest

One considers that the threshold of harmfulness is reached with a G2 note superior to 4. Consequently if the note is superior to 4 in the plots, the schedule will have been right to advice the treatment , and inversely for a G2 note inferior to 4. On this base, it is possible to compare two different strategies : the use of the schedule and the absence of intervention.

In the context of the two years 1997 and 1998 when the pressure of the phoma was really weak, the systematic absence of treatment seems to be the best decision : for 67 % of good results, there are 60 % with the use of the schedule.

CONCLUSION

The schedule does not seems to bring a real advantage in the situation when it could be tested, this is to say in the years of a weak pressure of phoma (as suggested by a G2 note average of 3.2 in the control plots). Consequently, it remains to test the schedule in a year of strong pressure to really judge its behaviour. It is on this view that the experimentation has been renewed in 1999.

REFERENCES

1. PIERRE J.G, REGNAULT Y –1982- Contribution à la mise au point d’une méthode de plein champ destinée à mesurer la sensibilité des variétés de colza au phoma – Informations Techniques CETIOM,1982, n° 81, p.03-18.

2. PILORGE E., PENAUD A. –1997- Lutte contre le phoma du colza à l’aide d’indicateurs. – Cinquième conférence internationale sur les maladies des plantes, vol.2, p.747-752.

3. LE PAGE R., PENAUD A. –1995- Phoma du colza, tout se joue avec le premier pic d’ascospores – Oléoscope,n°28, p.25-27.