THE 1996‑98 WESTERN CANADA DISEASE SURVEY; DO AGRONOMIC PRACTICES AFFECT DISEASES OR VICE‑VERSA?

R.A.A. Morrall¹, D.A. Kaminski² and L‑A. Kaminski³

¹Dept. Biology, Univ. Saskatchewan, Saskatoon  S7N 5E2,CDN. (morrall@sask.usask.ca)

²Westco Fertilizers, 4015 Thatcher Ave., Saskatoon  S7R 1A3, CDN. (da.kaminski@westcoag.com)

³Agriculture and Agri‑Food Canada, Saskatoon  S7N 0X2, CDN. (kaminskil@em.agr.ca)

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

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.

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.

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.

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.

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

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