LEPTOSPHAERIA MACULANS (PHOMA LINGAM) : FIRST RESULTS OF DEVELOPMENT OF A  FORECASTING CLIMATOLOGICAL SYSTEM

 

Christophe Bernard,  Catherine Maisonneuve, Blandine Poisson, André Peres, Annette Penaud, Etienne Pilorgé and Benjamine Van de Putte

 

CETIOM, BP n° 4, 78850 Thiverval-Grignon   France

e.mail : bernard@cetiom.fr

 

ABSTRACT

 

The purpose of  our studies carried out since 1996 was to develop  a climatological model to forecast the occurrence of "mature pseudothecia apt to release ascospores".

A large gathering of weather data (temperatures, rainfalls and humidities) from six regions and under different climates, and biological data (stages of pseudothecia maturation on infected stubble) allowed us to  develop  this model.

After three years of studies (1996, 1997 and 1998),  it was shown that  mean temperatures,  frequent rainfalls and high humidity levels  had a favourable effect on the maturation of Leptosphaeria maculans pseudothecia in autumn (main  component analysis). Thus, two preliminary models  were developed. Now, complementary work is in progress to obtain a final equation, which will be checked in the field.

 

KEYWORDS : phoma, pseudothecia, risk, climatic parameters

 

 

INTRODUCTION

 

            Despite the use of tolerant varieties and the practice of an agronomical fight, phoma can bring about important damages. This is why the fungicidal protection, provided it is well-thought-out (Penaud,1996), can be considered.

 

            Reaching the reasoning of fungicidal applications supposes to anticipate the infection, so to know in the most precise way the moment when the contamination event takes place : the ejection of ascospores. This affection supposes that the pseudothecia have reached the stage of maturation. Thus, one has been able to establish that the release of ascospores was maximal when more are less half of the pseudothecia were mature (Maisonneuve, 1998). beginning from this report, the CETIOM has tried to characterize the climatic elements leading to the maturation of pseudothecia, this process, when it is finished, could be integrated to the adjustment of a forecasting climatological system.

 

            The necessity to do frequent microscopic observations, in order to follow at the best the stage of maturation of the pseudothecia, is a restraint which makes the acquisition of a sufficient volume of data difficult.

 

 

 

MATERIALS AND METHODS

 

 

Contaminating material

 

            During the harvest, the straws of diseased rapeseed are taken then put near a weather station which will record all the climatic parameters to the study.

 

 

Observations

 

            Regularly, the experimenter watches out for the appearance of touches on the diseased straws. Then, he observes those laters under a binocular magnifying glass in order to make sure that they are really pseudothecia and that they belong effectively to the Leposphaeria maculans family.

 

            Once the pseudothecia really characterised, the experimenter does a microscopic observation of their contents. This observation will enable him to judge the stage of maturation of the pseudothecia by writing down their percentage of presence in each of the following classes (table 1) :

 

Table 1 : Classification of maturation asci and ascospores of pseudothecia

 

CLASS
PSEUDOTHECIA
ASCI
ASCOSPORES
A

DIFFERENCIATED NO MATURE
Undifferneciated
Undifferenciated

B
DIFFERENCIATED NO MATURE

Differenciated

Undifferenciated

C

DIFFERENCIATED NO MATURE

Differenciated
Differenciated :

< 8 spores / ascus

< 4 cells / spore

D

DIFFERENCIATED MATURE

Differenciated
Differenciated :

8 spores / ascus

³ 4 cells / spore

 

 

Method

 

            One gets interested in the evolution of the rate of mature pseudothecia, so in the increase of the percentage of pseudothecia in the D class. Thus, the periods of studied references will be the ones which separate the two samples with an increase of the D class.

 

            Using the daily weather data of these periods, one tries to characterize the evolution of the maturation of pseudothecia according to the following climatic parameters :

 

* Sum of the daily average temperature (Temp)

* Average pluviometry (Pluie)

* Number of days of rain superior to 1 mm (JPluie)

* Number of hours of an hygrometry superior to 90 percent (Hygro)

* Average to the daily and average temperatures on the period (Temperature)

* Average pluviometry on the period (pluiemoy)

* Number of hours of an hygrometry superior to 90 % average on the period (Hygromoy)

 

RESULTS

 

Beginning of the maturation

 

            If the study is about the phase of increase of the rate of pseudothecia in the D class, one must be interested in the phase dividing the appearance of the first differenciated pseudothecia (A class) and the appearance of the first mature pseudothecia (D class) too. The knowledge of this period is important inasmuch as the triggering off of the studied system depends precisely on the appearance of the first pseudothecia in the D class (A in the schedule) to the climatic date of the phases of the increase of the rate of pseudothecia in the D class (D in the schedule).

 

Table 2 : Comparison of climatic data between A and B periods

 

 

BEZIER

NANCY

DIJON

ST-FLORENT

ST-PATHUS

SURGERES

 

Temp

Pluie

H %

Temp

Pluie

H %

Temp

Pluie

H %

Temp

Pluie

H %

Temp

Pluie

H %

Temp

Pluie

H %

 

A

9.27

1.99

76.63

14.64

1.80

75.69

18.34

3.04

86.34

16.67

1.65

74.32

20.05

8.63

72.50

15.66

1.98

82.67

B

10.27

1.98

74.16

9.35

1.69

79.35

12.29

1.52

82.60

12.06

1.16

83.09

13.48

1.89

80.63

11.99

2.41

92.86

P > | T |

0.18

0.99

0.41

0.001

0.89

0.32

0.000

0.07

0.77

0.000

0.47

0.000

0.000

0.26

0.03

0.000

0.60

0.000

 

A = Phases from the appearance of the first differenciated pseudothecia the appearance of the first mature pseudothecia.

B = Phase of the increase of the rate of mature pseudothecia.

 

            Béziers has a few references, since no pseudothecia could be observed in the autumn 1998, on in the winter 1998/1999, moreover the period of appearance of the first differenciated pseudothecia is different from the one of the other places. Consequently, one can consider Béziers from the five other places.

           

            If one observes the meteorological parameters, one notices that the phase of appearance of the first mature pseudothecia (in the D class) is different from the phases of later maturation with a higher average temperature and also with a lower average hygrometry (Table 2).

 

 

Maturation of the pseudothecia

 

            The two axis of the principal component analysis (fig.1) do not allow to find a preferential vector to a more or less high increase to the rate of mature pseudothecia.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


           

 

 

 

The lowering multiple with a variable explained D = percentage of mature pseudothecia reveals as the best explanatory variables the factors HYGRO and TEMPMOY, those which give the following equation :

 

 

D = 0.158 x HYGRO + 0.849 x TEMPMOY

D = Percentage of mature pseudothecia.

HYGRO = Number of average and daily hours of an hygrometry superior to 90 percent.

TEMPMOY = Average of temperatures.

 

 

 

Speed of growth of the pseudothecia

 

            Characterizing the speed of growth of the pseudothecia could allow to discover an acceleration of their maturation, an acceleration which is susceptible to reach faster the rate of  fifty percent of mature pseudothecia. In this study, the phases of growth are divided into :

 

* High growth  = superior to 12 %.

* Medium growth  = between 8 an 12 %.

* Low growth  = Below 8 %.

 

            One searches to establish in the whole climatic variables can characterize these phases. According to the climatic variables, a discriminant analysis establishes which speed of growth has the most numerous chances to be reached (high, medium or low). Then, it is sufficient to compare the speed of the expected growth (expected class on fig.2) to the one which are really observed (observed class) in order o judge if the climatic variables are able to characterize the speed of the maturation of pseudothecia.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Only 40 % of the phases of low growth , 48 % of the phases of medium growth and 40 % of the phases of high growth are properly tacken into account. The climatic variables do not seem to permit the evaluation of the more are less important speed of the maturation of pseudothecia.

 

DISCUSSION

 

            Under its actual form, three brakes are opposed to the practical use of this system :

 

* The fact that the study is about the differenciated pseudothecia supposes to use only this system in a real presence of pseudothecia.

* The phases of stagnation and of decline of decline of the rate of mature pseudothecia are not taken into account, but they often come in between several phases of growth.

* The system still takes precision and deserves to be reinforced and sharpened by the taking into account of new references.

 

 REFERENCES

 

Penaud A.,1996. A la recherche d’un bon indicateur pour traiter. Oléoscope 35: 16-17.

 

Maisonneuve C., 1998. Leptosphaeria maculans, mise au point d’un modèle de prévision du risque d’attaque pour une culture de colza. Dossier Technique Cetiom (unpublished).