ABSTRACT

Powdery mildew caused by Erysiphe cruciferarum is a common disease of oilseed rape crops grown in the South of France. Field experiments were carried out to assess the yield losses and to evaluate the efficacy of fungicides. During the 1989-1993 period and in 1998, yield losses were quantified using an effective chemical control and an untreated check in 20 field trials. Powdery mildew has resulted in crop losses estimated to 0.5 T per hectare. Several fungicides were evaluated, the treatment flusilazole + carbendazim (200 + 100 g a.i./ha) was the most effective one. These results are considered in developing spraying recommendations for farmers.

INTRODUCTION

Over several years, powdery mildew caused by the obligate fungus Erysiphe cruciferarum was regularly observed on winter oilseed rape grown in the South of France (Penaud, 1991).

Disease symptoms appear in autumn on leaf surface as small, white powdery spots which as they increase in size coalesce to cover the whole leaf surface. In spring the powdery mildew covers first the lower leaves and gradually it develops on stems and on pods. Severely diseased pods produce small seeds.

The development of the disease is influenced by weather factors such as temperature and rain which directly affect growth of the fungus on leaf or pod surface.

All the cultivars are currently susceptible to powdery mildew infection. There is no information on the effect of the disease on yield but some increases in seed production were reported from plots where powdery mildew was controlled by fungicides (unpublished data). So studies were undertaken to assess the yield losses caused by E. cruciferarum and to find out effective fungicides for controlling the disease.

MATERIALS AND METHODS

Assessment of yield losses caused by E. cruciferarum :

Two series of ten field trials have been done. The first one carried out over a 4-year period between 1989 and 1993 compared untreated control and the most effective chemical control applied against powdery mildew. In the second one in 1998, a healthy check obtained using 3 or 4 applications of flusilazole + carbendazim (200 + 100 g a.i./ha) was compared to untreated control. Yields were expressed at 91% dry matter.

Efficacy of fungicides :

In 1998 experiments were carried out in four locations : Béziers(34), Portiragnes(34), Juzes (31) and Chambon (17). Fungicides were applied at the first visible symptoms at the flowering stage. Because of a high disease pressure, a second spray was applied 2 to 4 weeks later in three locations.

Treatments were applied in 200 to 250 litres of water / ha using a hand held sprayer (Prescispray ).

The fungicides used were : flusilazole 250 g/l + carbendazim 125 g/l (Punch CS 0.8 l/ha , Du Pont De Nemours), difenoconazole 62.5 g/l + carbendazim 125 g/l (Eria 2 l/ha, Novartis), iprodione 175 g/l + carbendazim 87.5 g/l (Calidan 3 l/ha, Philagro), carbendazim 500 g/l (Bavistine FL 1 l/ha, BASF). Untreated control and healthy check were also included. The experimental design was a randomised complete block with four replicates.

Assessments were made by visual score of powdery mildew severity on pods of a sample of 5 to 20 plants per plot using the following scale :

0 = healthy

1 = occurence of some white spots

3 = superficial white powdery layer

5 = white powdery layer covering the whole pods

7 = black lesions under the white powdery layer

9 = gray to black colour on the whole pods

The datascore were analyzed using a nonparametric method based on taking the ranks of the score and analyzing these ranks instead of the original values.

RESULTS

Effects of powdery mildew on yields :

Between 1989 to 1993, the yield response to fungicide treatment compared to untreated control varied from 3 to 9.4 q/ha. So, the mean yield loss was estimated to 5 q/ha (Fig 1).

Figure 1 : Differences of yield (q/ha) between untreated control and the best powdery mildew control in the period 1989-1993 (10 field trials).

In 1998 differences between untreated control and healthy plots were found in the range 0 to 12.7 q/ha. 80 % of experiements showed at least 3 q/ha of yield loss, the mean being 5.6 q/ha (Fig 2).

Both series of trials have provided similar results. To some extend, powdery mildew on oilseed rape was able to cause noticeable yield losses. The overall yield loss reached approximatively 5.5 q/ha.

Figure 2 : Differences of yield (q/ha) between untreated control and the best powdery mildew control in 1998 (10 field experiments).

Efficacy of fungicides :

The final assessments on pods were carried out 2 to 4 weeks before harvesting (Table 2). In the four trials, untreated control was severely infected by E. cruciferarum as indicated by a score greater than 5.

All fungicides provided substantial disease control when compared with untreated plots. However, flusilazole + carbendazim (200 + 100 g a.i./ha) has been regularly observed as the most effective one against powdery mildew. Difenoconazole + carbendazim and iprodione + carbendazim have also given good control but both appeared slightly less effective than flusilazole + carbendazim (200 + 100 g a.i./ha). At last, carbendazim used alone showed large variability in efficacy.

Experiments	Chambon (17)	Juzes (31)	Béziers (34)	Portiragnes (34)	Mean rank
Treatments (date)	21/04 stage G3 05/06 stage G4	06/04 stage F1 14/05 stage G3	26/03 stage F1	26/03 stage F1 02/04 stage G2
Healthy check	6.3 (1.5)	1.9 (1)	3.7 (1)	5.5 (3)	1
flusilazole + carbendazim	5.7 (1.5)	1.3 (2)	3.9 (2)	5.0 (1)	2
difenoconazole + carbendazim	6.6 (3)	4.2 (4)	5.5 (3)	5.8 (4)	3
carbendazim	6.7 (5)	5.4 (5)	5.5 (5)	5.0 (2)	5
iprodione + carbendazim	6.5 (4)	3.6 (3)	6 (4)	6.3 (5)	4
untreated control	6.9 (6)	6.4 (6)	8.3 (6)	6.5 (6)	6

Table 2 : Effects of fungicides for controlling powdery mildew on pods in 1998. (...) mean rank of the fungicide in each experiment.

Fungicides treatments have resulted in marked increase in seed yield (Table 3) and because of a better disease control flusilazole + carbendazim (200 + 100 g a.i./ha) has produced the better yield response, 5 q/ha in average compared with untreated control.

	Chambon (17)	Juzes (31)	Béziers (34)	Portiragnes (34)
Treatments	3 treatments on healthy check	4 treatments on healthy check	3 treatments on healthy check	3 treatments on healthy check
Healthy check	35.4 a	37.4 a	18.6 a	24.8
flusilazole + carbendazim	35.7 a	33.3 ab	17.4 ab	25.4
difenoconazole + carbendazim	32.0 b	30.8 abc	16.8 ab	23.9
carbendazim	30.1 bc	30.8 abc	16.1 ab	23.2
iprodione + carbendazime	31.2 bc	28.3 bc	17.8 ab	23.0
untreated control	28.8 c	24.7 c	13.9 b	21.6
CV Pr>F	4.5 0.0001	10.9 0.003	10.4 0.016	9.1 0.23

Table 3 : Effects of fungicide treatments on the yield of oilseed rape in 1998.

DISCUSSION - CONCLUSION

The studies reported in the paper have showed that powdery mildew on oilseed rape can cause yield losses in the range 10-30%. This could mean a loss up to 12 q/ha potential yield in some instances, underlining the need for effective control measures.

Triazole fungicides revealed an interesting efficacity for controlling powdery mildew but somewhat variable according to the active ingredient. Flusilazole has been regularly found the most effective against E. cruciferarum. These data have partially supported a registration file and since February 1999, the commercial product Punch CS (flusilazole 250 g/l + carbendazim 125 g/l) at the rate of 0.8 l/ha was registrated in France.

In many cases for a satisfactory disease control, two applications are necessary, the first one when powdery mildew is occuring, mainly at the flowering stage, and the other one 2-4 weeks later to achieve control on pods. There is also a need to take account of sclerotinia stem rot which may infect the plants at the same period. Consequently, strategies have been investigated to control both diseases and to limit expensive fungicide applications (Penaud, 1998). The control measures which are recommended are as follows :

i/ at the flowering stage, to spray an effective fungicide against both E. Cruciferarum and Sclerotinia sclerotiorum when powdery mildew is developping but if powdery mildew is not yet visible, to apply a very cheap carbendazim treatment for controlling sclerotina stem rot. Nevertheless, in the areas where sclerotinia stem rot fails to cause damage and where powdery mildew is prevalent, a fungicide such as flusilazole plus carbendazim applied even when no symptoms of E. Cruciferarum have been observed may save potential yield.

ii/ at the pod stage, to apply a second spray to provide a lasting control on pods or to spray a single fungicide such as flusilazole plus carbendazim only when late powdery mildew symptoms appear.

Further work is needed to evaluate these recommendations and to improve stragegies control which should be profitable for farmers.

ACKNOWLEGEMENTS

The author gratefully acknowleges the excellent technical assistance of the CETIOM teams of Béziers, En Crambade and Surgères.

REFERENCES

1. Penaud A. 1998. Oïdium du colza, la protection du colza est maintenant possible. Oléoscope 50 : 36-38.

2. Penaud A. 1991. Les nouvelles maladies du colza. Phytoma 427 : 15-18.

Trials	period 1989 - 1993	Field experiments	1998
C89LO11O	4.8	C98OID31	12.7
C89LO11V	3.2	C98LFO31	11.6
C89LO31G	4.5	C98LFO79	8.2
C89LO31V	6	C98OID17	6.6
C90OID34	6	C98OID34B	4.7
C91LO31C	3	FR-X17	3.8
C91LO31G	3	FR-R11	3.4
C91OIP34	9.4	C98OID34P	3.2
C92LFO34	4	C98LFO69	1.7
C93OID34	7.7	FR-S82	-0.2
Mean	5.16		5.57