ABSTRACT

In western Canada canola disease surveys were traditionally done in randomly selected fields without regard to cultivar or agronomic factors. From 1996 to 1998, pathologists collaborated to survey about 200 fields per year throughout the major production areas. Standardized sampling and disease rating methods were used. Many agrologists assisted by selecting fields and obtaining data on production practices and yield. Disease data and yield were cross-tabulated with factors such as length of rotation, disease resistance, nitrogen fertility and fungicide applications. Mean incidence of blackleg (Leptosphaeria maculans) was not consistently correlated with yield or resistance level of cultivars. Mean incidence of stem rot (Sclerotinia sclerotiorum) was not related to rotation or application of a fungicide, but increased with higher nitrogen fertility. Severity of pod spot (Alternaria spp.) and incidence in Alberta of brown girdling root rot (Rhizoctonia sp., etc.) were not consistently correlated with rotation length. Incidence of staghead (Albugo candida) on Brassica rapa was lower in long rotations. While disease level and yield could not always be related to recommended control measures, such as using resistant cultivars and crop rotation, diseases influenced agronomic practices. The average level of blackleg resistance of cultivars in the survey increased slightly from 1996 to 1998 and fungicides to control sclerotinia were more commonly used in crops with high nitrogen fertility.

INTRODUCTION

Until 1996 canola disease surveys in western Canada were done in randomly selected fields without regard to factors such as cultivar, rotation or fertilizer input (Kaminski et al.,1996, Platford, 1996).

Moreover, all major canola-growing areas were not surveyed for all diseases annually and sampling methods varied among individuals. Thus, while the data generated may have assisted in determining research priorities, there was no information derived on the influence of agronomic practices on diseases.

Three major changes have occurred in Canadian canola production in the last decade: (a) a large increase in acreage to the recent annual average of about 5 X 10⁶ ha., (b) an increase from a handful of registered cultivars, all blackleg-susceptible, to over 100 cultivars, many blackleg-resistant and some with novel traits, and (c) widespread adoption of reduced tillage and direct seeding. The diversity of cropping practices suggested a need to relate disease survey results to agronomic factors. Consequently, from 1996 to 1998 about 15 pathologists collaborated to gather comprehensive disease data, using standardized sampling methods, as well as background agronomic data on commercial canola in the western provinces.

METHODS

The numbers of fields surveyed in each area were approximately representative of relative acreages planted. Extension agrologists were asked to select clusters of fields in their local areas and obtain background agronomic data from the farmers. Each agrologist was asked to choose six fields in the area, to represent a range of cultivars, fertilizer inputs and other agronomic practices, but not to select only fields belonging to the best growers. The farmer or agrologist completed a questionnaire on agronomic practices and added information on late-season pesticide inputs and yields after harvest.

Diseases were usually assessed in each field shortly before swathing in August by collecting 20 plants at each of five sites at least 20 m from each other. The presence or absence of lesions on each plant was scored to give percent incidence figures for: blackleg (Leptosphaeria maculans); sclerotinia stem rot (Sclerotinia sclerotiorum), foot rot (Rhizoctonia, Fusarium), brown girdling root rot (Rhizoctonia, etc.), aster yellows (phytoplasma) and staghead (Albugo candida). For alternaria pod spot (Alternaria brassicae and A. raphani) percent severity of pod lesions on each plant was assessed (Conn et al., 1990). When alternaria pod spot was present in a field, but at a level estimated at below 1%, severity was recorded as a "trace". Similarly, when the other diseases were observed in a field, but not among the 100-plant sample, incidence was also recorded as a "trace". In calculating means, all traces were counted as 0.1%. Mean disease levels were calculated relative to rotation lengths, disease resistance, and fertilizer inputs.

RESULTS AND DISCUSSION

The total numbers of fields surveyed were: 1996 - 204; 1997 - 253; 1998 - 237. However, there was incomplete or no agronomic data on several fields in each year because of failure of agrologists or growers to comply with requests. The distribution of fields among provinces over the three-year period was approximately:- Alberta and B.C. Peace region 33%; Saskatchewan 46%; Manitoba 21%.

Blackleg

Blackleg has probably been the major disease of canola in western Canada since the early 1980's (Gugel and Petrie, 1992). Mean incidence was higher in 1996 and 1998 than in 1997 (Table 1) and there were substantial differences among regions within years. Although more blackleg occurred in susceptible than in resistant cultivars, the trend to higher disease incidence in the more susceptible cultivars was weak and inconsistent over years or levels of susceptibility (Table 1). Among susceptible cultivars, incidence was consistently higher in Brassica rapa than in B. napus. Mean yields were higher in 1996 than 1997 and 1998. Yield did not generally increase with greater blackleg resistance (Table 1). However, the sample of susceptible B. napus cultivars was small and consisted mainly of inherently higher yielding hybrids, grown in Manitoba, where yields are generally higher than in the other provinces. Mean yields of B. rapa were between 70 and 80% of those of B. napus.

In all three years the highest mean incidence of blackleg was in fields where canola had been grown two years previously (Table 2). This is consistent with reports that maximum ascospore release by L. maculans occurs from 2-year old stubble (G.A. Petrie, personal communication). However, blackleg incidence was not consistently lower with longer rotations. A high incidence in 1996 in fields where canola had not been grown for four years might have been due to difficulty in controlling volunteer canola in the other broad-leaved crops grown in these fields in the rotation. Similar trends were not observed in 1997 or 1998.

In general, the data did not show strong effects of rotation length or cultivar resistance on blackleg incidence. This was true even when rotation length was considered within particular blackleg susceptibility levels and vice-versa. However, since the data were not obtained from controlled experiments and other factors influenced disease incidence, the two major control recommendations for blackleg in western Canada (planting resistant cultivars and long crop rotations) should not be abandoned.

Table 1. Blackleg, yield, rotation and cultivar susceptibility, western Canada, 1996-98

Year No. fields surveyed

Resistant Mod. res. Mod. susc. Susc. Susc. Overall B.napus B. napus B. napus B. napus B. rapa

1996 194 1997 245 1998 222 1996 194 1997 245 1998 222 1996 194 1997 245 1998 222

Mean % disease incidence (% fields surveyed) 9 (24.7) 12 (20.6) 18 (20.1) 12 (9.8) 24 (24.7) 16 7 (26.1) 10 (20.4) 11 (33.9) 10 (5.3) 20 (14.2) 9 8 (20.4) 11 (33.3) 15 (40.5) 24 (3.6) 39 ( 6.3) 15 Mean yield in kg/ha 2000 1800 1900 2050 1300 1800 1650 1500 1550 2050 1000 1500 1750 1650 1650 1950 1350 1650 Mean length of rotation in years 3.7 3.9 3.9 3.8. 3.1 3.6 3.4 3.7 3.7 3.4 3.0 3.5 3.7 3.6 3.9 3.5 3.2 3.7

Table 2. Mean percent incidence of blackleg (and percentage of fields surveyed) in relation to length of crop rotation, western Canada, 1996-98

Year	No. fields surveyed	Years since previous canola crop
		1 year	2 years	3 years	4 years	$5 years	Unknown
1996 1997 1998	194 245 227	14 (3.6) 1 (2.0) 13 (4.5)	30 (12.9) 14 (15.5) 21 (5.8)	14 (19.6) 12 (26.9) 17 (28.8)	22 (19.6) 11 (25.7) 16 (33.8)	11 (27.3) 9 (19.2) 10 (19.4)	10 (17.0) 7 (10.6) 6 ( 7.7)

It is interesting to consider whether blackleg influences choice of agronomic practices. From 1996 to 1998 the percentage of blackleg-resistant and blackleg-susceptible cultivars decreased, while moderately resistant and moderately susceptible cultivars increased (Table 1). These changes are more likely due to factors other than blackleg. The large increase in moderately susceptible cultivars was probably because of the rapid adoption of herbicide tolerant cultivars, most of which are in this class; the decline in susceptible B. rapa cultivars was mainly due to their lower yield potential. There was no evidence from this study that growers used shorter rotations with blackleg resistant cultivars (Table 1). The only major difference in mean rotation length relative to blackleg susceptibility was that B. rapa was planted on shorter rotations. Over the three years, there was a decrease in the percentage of crops on 2-year rotations and increases in 3- and 4-year rotations (Table 2). These changes may have been related to concern about blackleg but the decline in >5-year rotations is probably due to an increase in total canola acreage.

Sclerotinia stem rot

All canola cultivars are susceptible to sclerotinia stem rot, but fungicides have been registered to

control this disease in Canada since the early 1980's. The raw data showed substantially higher levels of stem rot in Alberta and Manitoba than in Saskatchewan in 1996 and 1997. However, in 1998 mean levels in Alberta and Saskatchewan were about equal. In crops which were not sprayed with a fungicide, there was no evidence of lower mean disease incidence with longer rotations between canola or between sclerotinia-susceptible crops (mostly pea, lentil, alfalfa and flax) (Table 3). The results were consistent with those of previous studies (Morrall and Dueck, 1983) which suggest that rotation is ineffective in western Canada to control stem rot because of the longevity of sclerotia and wide host range of S. sclerotiorum, and possibly because of its airborne ascospores.

Table 3. Stem rot*, crop rotation and fungicide treatment, western Canada, 1996-98

Year No. non- treated fields	Rotation length Un- 1 year 2 year 3 year 4 year $5 year known	Overall non-treated	Overall treated
1996 165 1997 208 1998 189 1996 165 1997 208 1998 189	Mean % incidence relative to last canola crop 3 13 8 10 7 11 7 7 4 7 5 2 0 1 7 7 9 6 Mean % incidence relative to last susceptible crop 7 11 7 12 5 11 4 6 5 6 8 1 4 8 7 4 7 7	9 5 6 9 5 6	8 5 7 8 5 7

* Data for sclerotinia stem rot on Brassica napus and B. rapa combined

Conventional wisdom is that sclerotinia stem rot is a greater threat in fields managed with a high level

of inputs because dense stands provide a more favorable microclimate for disease development. In 1996 data on nitrogen input to the fields were very limited, but in 1997 and 1998 it was possible to classify most of the fields according to actual N input. In 1997 disease incidence was generally higher in fields with higher N input, but in 1998 this relationship was not evident (Table 4). However, fungicide application to control sclerotinia was more common in fields with higher N input in both years. The percentages of fields in the survey that were sprayed with a sclerotinia control product were 18%, 17% and 20% in 1996, 1997, and 1998 respectively. In some of these fields, disease incidence was still high, suggesting poor timing of the fungicide application and accounting for the similar mean incidence values in treated and non-treated fields (Table 3).

Table 4. Mean % incidence of sclerotinia stem rot (% inc.) in non-fungicide treated crops, frequency of fungicide use and actual N input, western Canada, 1997-98

	1997		1998
Actual N input (kg/ha)	Non-fungicide treated crops B. napus B. rapa % of* Mean % of Mean crops % inc. crops % inc.	% of fungicide -treated crops	Non-fungicide treated crops B. napus B. rapa % of Mean % of Mean crops % inc. crops % inc.	% of fungicide -treated crops
0-22.5 22.6-45.0 45.1-67.5 67.6-90.090.1-112.4 $112.5 Unknown	6.0 0.3 1.2 0.3 3.2 0.1 2.8 3.1 13.2 5.7 2.8 10.8 26.0 3.0 2.8 9.0 12.8 6.3 2.0 22.8 3.6 14.4 0 - 4.0 4.3 2.0 8.0	0.8 0.4 2.4 5.2 6.0 2.8 0	8.8 4.4 2.3 0.8 7.8 9.1 0.5 0 10.6 2.4 0.9 0 22.6 8.8 0.5 0 10.1 9.0 0.5 0 4.1 9.4 0.9 1.0 12.9 3.6 0.9 0	0 1.8 2.8 4.1 6.0 5.5 0.9

* Percentages refer to the percentages of total crops surveyed in the year.

Other diseases

Alternaria pod spot is considered to be more prevalent on B. rapa than on B. napus. However, this

may partly relate to environmental conditions, as B. rapa is grown mainly in northern Alberta. This regionalization was accentuated from 1996 to 1998. In the survey, pod spot was more severe on B.rapa than on B. napus and, with a few exceptions, severity was very low on B. napus (Table 5), due to dry weather after flowering. There was no relationship of pod spot to rotation in 1996 and 1998, and only a weak trend of lower severity with longer rotations in 1997. These results probably reflect the highly airborne nature of Alternaria conidia and the fact that, even with long rotations, canola fields are often adjacent to 1-year old canola residues.

Table 5. Mean percent severity of alternaria pod spot (and percentage of fields surveyed) in relation to length of crop rotation, western Canada, 1996-98

Canola Year species	Years since previous canola crop 1 year 2 years 3 years 4 years $5 years Unknown	Overall
1996 B. napus 1996 B. rapa 1997 B. napus 1997 B. rapa 1998 B. napus 1998 B. rapa	24.8 (0.6) 0.2 (10.4) 0.5 (22.1) 0.9 (18.8) 0.6 (29.9) 0.9 (18.2) 4.6 (12.5) 3.1 (20.7) 3.5 (10.4) 2.3 (18.8) 3.0 (18.8) 5.9 (18.8) 6.7 (1.4) 2.0 (13.6) 0.8 (26.2) 0.9 (27.5) 1.5 (20.1) 2.6 (11.2) 6.7 (5.6) 4.6 (25.0) 3.0 (33.4) 4.3 (11.1) 3.1 (13.9) 2.0 (11.1) 0.3 (3.7) 0.3 (5.6) 0.7 (30.4) 0.5 (31.3) 0.6 (20.1) 3.8 (8.9) 0.5 (14.2) 0 (7.2) 0.1 (14.2) 0.8 (57.2) (0) 0 (7.2)	0.8 (100) 3.7 (100) 1.4 (100) 3.6 (100) 1.4 (100) 0.6 (100)

Two races of A. candida cause staghead on B. rapa in western Canada and cultivars vary in resistance to these races. Based on small samples of fields in each rotation category, the survey suggested that staghead incidence was similar, except on rotations of 4 or more years. Incidence was highest in cultivars susceptible to Race 7. Brown girdling root rot, a disease of complex etiology found mainly in northern Alberta, occurred at high levels there, especially in 1996 and 1997, but incidence was unrelated to rotation. Foot rot and aster yellows were too sporadic in the fields surveyed to investigate their possible relationships with agronomic factors.

CONCLUSIONS

The survey showed fewer relationships than expected between disease levels and recommended agronomic practices for control, such as crop rotation and using resistant cultivars. This may be because of the sample size and the wide range of weather in three years over a large geographic area. However, diseases probably influenced management practices. The average level of blackleg resistance of cultivars in the survey increased slightly from 1996 to 1998 and fungicides to control sclerotinia were more commonly used in crops with high N fertility.

ACKNOWLEDGEMENTS

We thank over 100 people (pathologists, agrologists, farmers, and sponsors) who made this study possible.

REFERENCES

1. Conn K.L., Tewari J.P. and Awasthi R.R. 1990. A disease assessment key for Alternaria blackspot in rapeseed and mustard. Canadian Plant Disease Survey 70: 19-22.

2. Gugel R.K. and Petrie G.A. 1992. History, occurrence, impact and control of blackleg of rapeseed. Canadian Journal of Plant Pathology 14: 36-45.

3. Kaminski D.A., Morrall R.A.A. and Duczek L.J. 1996. Survey of canola diseases in Saskatchewan, 1995. Canadian Plant Disease Survey 76: 99-102.

4. Morrall R.A.A. and Dueck J. 1983. Sclerotinia stem rot of spring rapeseed in western Canada. Proceedings of 6th International Rapeseed Conference, Paris, France, 17-19 May. 957-962.

5. Platford R.G. 1996. Distribution, prevalence and incidence of canola diseases in Manitoba in 1995.

6. Canadian Plant Disease Survey 76: 103-105.