LEPTOSPHAERIA MACULANS : EFFECT OF TEMPERATURE, RAINFALL AND  HUMIDITY ON THE FORMATION OF PSEUDOTHECIA

 

André Pérès, Blandine Poisson, Virginie Le Sourne and Catherine Maisonneuve

 

                    CETIOM, Centre de Grignon, BP n°4, Thiverval-Grignon, France

                                                      email- : peres@cetiom.fr

 

 

                                                        ABSTRACT

 

The aims of two studies carried out in 1997 and 1998 were to observe the effect of temperature, rainfalls and relative humidity on the formation and  differentiation of Leptosphaeria maculans pseudothecia. The data were involved in the development of a forecasting model pseudothecia maturation. The formation of pseudothecia was followed on rape stubble naturally infected by phoma (studies in incubators).

The first experiment (1997) studied the effect of 3 temperature levels associated with 4 rainfall levels. The results showed that a mean temperature of 14°C and a high frequency of rainfalls (2,5mm every 3-4 days) were  the best conditions for the first generation of pseudothecia. In the second experiment (1998), we  studied the effect of different temperatures varying between 10-14-16-18 and 20°C,  and the effect  of relative humidity (without rain). The results confirmed the favourable effect of temperatures close to 14°C and showed  the necessity of saturating relative humidity (100%) for lack of rain.

These results confirming the epidemiological observations in the field  will be integrated  to develop a forecasting model of pseudothecia maturation.

 

KEYWORDS : rapeseed, phoma lingam, climate, pseudothecia, stubbles

 

INTRODUCTION

 

The climatic and biological observations made in the field show that, in general the first releases of ascospores of Leptosphaeria maculans strongly depend on the climatic conditions (Brunin B., 1970 - Mac Gee D.C.,1977 Gabrielson R.L., 1983 - Pérès and al, 1997). Indeed, the first captures of ascospores (control network), then the first leaf spots are generally observed after the return of the rainfalls and after the decreasing of the temperatures of the end of summer (Figure 1). In view of these observations, one can consequently think up to forecast the risk of contamination thanks to a climatological system. To adjust this latter, a precise study on the effect of temperature, rain and humidity on the formation of pseudothecia which can produce  ascospores (mature pseudothecia) is necessary. The present paper deals with the results of two studies which compare the effect of different climatic scenarios on the appearance of the first mature pseudothecia.

 

Figure 1 : Climatical data and ascospores captures in the field (St. Pathus : autumn 1995)

 

 

 

 

 

MATERIALS AND METHODS

 

Two studies made in 1997 and in 1998 allow to follow the formation of pseudothecia on the stubbles of rapeseed affected by L. maculans and coming from the previous crop. These stubbles are placed in incubators with an adjustable temperature. In these latters, the level of relative humidity can be regulated by a damp and more or less ventilated micro-enclosure (80-100% of r.h.).

 

Study 1 (Table 1)

 

Its goal is to study the effect of twelve climatic scenarios combining three levels of temperature (7°C - 14°C - 20°C) and four frequencies of rain (without rain - a rain every 3 days - a rain every 4 days - a rain every 8 days) on the maturation of pseudothecia. Each rain of 2.5mm is produced by a laboratory micro-sprayer. The surrounding humidity is kept at 80%. This experiment which studies the twelve climatic modalities starts on the September 9th and lasts 30 days (September 9th takes place during the period of the seedling of winter rape in France).

 

Study 2 (Table 2)

 

It aims at precizing the effect of temperature on the maturation of pseudothecia on a scale of  10°C-14°C-16°C-18°C-20°C without rain but with the level of relative humidity kept high (95-100% of RH). This experiment which consequently studies five climatic modalities starts on the August 23th and lasts 34 days (August 23th corresponds to the very beginning of the period of the winter oilseed rape seedling in France).

In these two studies, two stubbles a modality are observed. These stubbles are examined every three days. As soon as the first pseudothecia begin developing, they are taken, opened and examined through a microscop to establish their stage of maturation (two pseudothecia a stubble). A pseudothecia is considered to be "mature" when it contains some ascus with four ascospores or more. These two studies only with the first generation of pseudothecia.

 

RESULTS

The important and expected result is the time period of appearance of pseudothecia for each climatic modality which is experimented.

Study 1 (Table 1)

14°C proves to be the most favourable temperature for the maturation of pseudothecia : the three modalities with rain allow to reach the stage of "mature pseudothecia" in a time period from 13 to 23 days (modalities n°6, 7 and 8). The higher the rain frequency is, the faster the maturation of pseudothecia is.

On the contrary, 7°C and 20°C  are not really favourable temperatures for the maturation of pseudothecia. Four modalities with rain out of the six which are experimented do not allow to reach the stage of "mature pseudothecia" (modalities n° 2, 4, 10 and 11). Only two modalities out of the six allow the maturation of pseudothecia but fairly late and with a high rain frequency (modalities n° 3 and 12). In most of the cases, the whole quantity of rain to reach the stage of "mature pseudothecia" is after all quite low (from 7.5 to 12.5 mm). Finally, the maturation of pseudothecia cannot take place without rain and with 80% hygrometry.

 

Table 1 :  Modalities and results  / Study1

N° mod

Temp.

tested

Frequency              of rainfalls

Days / 1srt mature pseudothecia*

Number of

rainfalls*

Total of

rainfall*

Som.

t°*

1

7°C

0

x

 

 

 

2

7°C

2,5mm / 8 days

x

 

 

 

3

7°C

2,5mm / 4 days

23 days**

5

12.5mm

161°C

4

7°C

2,5mm / 3 days

x

 

 

 

5

14°C

0

x

 

 

 

6

14°C

2,5mm / 8 days

23 days**

3

7.5mm

322°C

7

14°C

2,5mm / 4 days

16 days**

4

10mm

224°C

8

14°C

2,5mm / 3 days

13 days**

5

12.5mm

182°C

9

20°C

0

x

 

 

 

10

20°C

2,5mm / 8 days

x

 

 

 

11

20°C

2,5mm / 4 days

x

 

 

 

12

20°C

2,5mm / 3 days

30 days**

10

25mm

600°C

 

* : From the introduction of the stubbles into incubators up to the appearance of the first mature pseudothecia

** : time period for the appearance of the first mature pseudothecia

x : no mature pseudothecia

 

 

Study 2 (Table 2)

Let's recall that this study is done with no rain but with a high relative humidity  (95-100% of RH).

The medium temperatures of 14°C, 16°C and 18°C allow the fastest maturation of pseudothecia, in relatively the same time periods, which are respectively of 14 days, 17 days and 21 days. With a temperature of 20°C, the time period is clearly longer (34 days). With the lowest temperature of 10°C, the stage of "mature pseudothecia" is never reached.

 

Table 2 : Modalities and results / Study 2

N° mod

Temperatures.

tested

Days / 1srt mature pseudothecia*

Som.

t°*

1

10°C

x

 

2

14°C

14 days**

196°C

3

16°C

17 days**

272°C

4

18°C

21 days**

378°C

5

20°C

34 days**

680°C

 

* : From the introduction of the stubbles into incubators up to the appearance of the first mature pseudothecia

** : time period for the appearance of the first mature pseudothecia

 

CONCLUSION - DISCUSSION

 

These two studies show that the constant temperature of 14°C is, among the tested temperatures, the most favourable to a quick maturation of pseudothecia. However, the second study indicates that with a little higher constant temperatures of 16°C/18°C, the maturation is still possible but a little longer. Out of the range 14°C-18°C, the tested temperatures which are lower or higher are not very favourable to the maturation of the pseudothecia

 (no maturation or late maturation). The results confirm the previous and more fundamental studies done on the biology of L. maculans (Lacoste, 1965).

The importance of rain is proved too. Indeed, with the most favourable temperature of 14°C, the higher the rain frequency is, the faster the maturation of pseudothecia is. To ensure the maturation, the rain frequency seems finally to be more important than the pluviometrical range.

The necessity of a very high relative humidity, near the saturation, is emphasized when there is no rain in the second study. A saturating relative humidity is probably enough to keep the infected stubbles damp and thus allow the biological evolution of the pathogen.

It is necessary to underline that these obtained results are first useful for the adjustment of a forecasting climatological system (in particular integration of the addition of climatological data : amount of temperature, amount of days of rain, amount of range of rain, etc...). These results also allow to determine the threshold of the parameters which give information on the climatological conditions favourable to the maturation of pseudothecia. They bring the confirmation of the favourable effect of frequent rain and of the decreasing of temperatures around 15°C generally observed in natural conditions in open field.

 

 

REFERENCES

 

Brunin B., 1970 - La nécrose du collet de colza - Rôle étiologique des ascospores de Leptosphaeria maculans (Desm.) Ces. & de Not. - Etude histologique de l'infection - Thèse de Doctorat - Faculté des Sciences de l'Université de Lille

Gabrielson R.L., 1983 - Blackleg disease of Crucifers caused by Leptosphaeria maculans (Phoma lingam) and its control - Seed Sci. & Technol., 11, 749-780

Lacoste L., 1965 - Biologie naturelle et culturale du genre Leptosphaeria Cesati et de Notaris - Determinisme de la reproduction sexuelle - Thèse de Doctotat ès Sciences - Toulouse

Mc Gee D.C., 1977 - Black Leg (Leptosphaeria maculans Desm. Ces. et de Not.) of Rapeseed in Victoria : Sources of Infection and Relationships between Inoculum, Environmental Factors and Disease Severity - Aust. J. Agric. Res., 28, 53-62

Pérès A., and Poisson B.,  1997 - Phoma du colza - Avancées en épidémiologie - CETIOM Oléoscope n°40 - p 37-40