2001 Canola Disease Survey in Minnesota and North Dakota
Extension Report 71, December 2001
Arthur Lamey, North Dakota State University Plant Pathologist Emeritus, Fargo, ND
Janet Knodel, NDSU North Central Research Extension Center, Minot, ND
Gregory Endres, NDSU Carrington Research Extension Center, Carrington, ND
Karen Andol, Dept. Agronomy and Plant Genetics, Univ. Minnesota, St. Paul, MN
Roger Ashley, NDSU Dickinson Research Extension Center, Dickinson, ND
Dwain Barondeau, Hettinger County Extension Agent, Mott, ND
Bill Craig, Marshall County Extension Educator, Warren, MN
Vince Crary, East Polk County Extension Educator, Fosston, MN
Zachary Fore, Univ. Minnesota Extension Service, Red Lake Falls, MN
Nathan Johnson, Kittson County Extension Educator, Hallock, MN
Scott Knoke, Benson County Extension Agent, Minnewaukan, ND
Mike Liane, Ramsey County Extension Agent, Devils Lake, ND
Terry Lykken, Towner County Extension Agent, Cando, ND
Randy Melaas, Pembina County Extension Agent, Cavalier, ND
Mark Miller, Rolette County Extension Agent, Rolla, ND
Curt Nyegaard, Roseau County Extension Educator, Roseau, MN
Howard Person, Pennington County Extension Educator, Thief River Falls, MN
Nels Peterson, Nelson County Extension Agent, Lakota, ND
Russ Severson, West Polk County Extension Educator, Crookston, MN
Introduction
Materials and Methods
Results
Discussion
Acknowledgments
Literature Cited
A field survey in Minnesota and North Dakota evaluated the incidence (percent infected stems) of aster yellows, caused by the aster yellows phytoplasma; blackleg, caused by Leptosphaeria maculans (Phoma lingam); Sclerotinia stem rot, caused by Sclerotinia sclerotiorum; and the pod severity (percent infected pod area) of Alternaria black spot, caused by Alternaria brassicae and A. japonica. Field surveys of canola diseases were conducted previously in North Dakota in 1991 and 1993-2000 (2, 3, 4, 5, 7, 8, 9, 10) in Minnesota in 1996-2000 (5, 7, 8, 9, 10) and in South Dakota in 1996, 1997 and 2000 (5, 7, 8). In previous surveys, blackleg incidence was highest in 1991, when it was 28% in North Dakota (2, 3). The highest incidence of Sclerotinia stem rot over the years was 19% in North Dakota in 1993 (2, 3) and 19% in Minnesota in 1997 (8). Alternaria pod spot severity has been assessed only since 1996 and severity has never been high in either state. Aster yellows was common in some areas of both Minnesota and North Dakota in 1999, and fairly common in 2000, but data were not recorded in the 1999 survey.
The 2001 field survey of canola diseases was conducted in 297 fields in 30 counties of the two states. There were 252 fields in 24 counties of North Dakota and 45 fields in six counties of Minnesota that were surveyed.
Materials and Methods
The survey techniques were adapted from those used by Petrie et al. in Saskatchewan (13). The fields were inspected for disease when the crop was in the swath and the stubble was freshly cut. All fields surveyed were Argentine canola, Brassica napus. Five stems were sequentially pulled from the soil at each of eight randomly selected locations for a total of 40 stems examined in each field. The lower stems and roots of the plants were visually inspected for symptoms of blackleg and Sclerotinia stem rot. These diseases were easily identified on 6- to 18-inch stubble left in the fields after swathing.
Blackleg was identified by the black girdling lesions at or near the soil surface. Tiny black pycnidia characteristic of the virulent strain of the blackleg fungus often were associated with the lesions. In cases where the cause of the disease was in doubt, the roots of plants were split with a knife and the interior tissues inspected. If they were dark gray to black, rather than white, the root was considered to be positive for blackleg. If blackleg symptoms were only superficial with few or no pycnidia and no internal tissue discoloration, the low virulence strain of L. maculans was suspected.
Sclerotinia stem rot was identified by the bleached stems, often accompanied by a shredding of the tissues. The presence of sclerotia inside the bleached tissues was an additional confirmation of Sclerotinia.
At each of eight random sampling locations, the swath was inspected for symptoms of aster yellows. An attempt was made to randomly sample five plants at each stop, but this was not feasible, so counts of aster yellows might better be considered estimates than accurate counts. Aster yellows symptoms used to identify the disease included bladder-like upper pods, a witches' broom or proliferation of tissues on the upper portions of the plant, and a purpling of the upper tissues. Since purpling can also be a characteristic of certain hybrids or of sulfur deficiency, this characteristic was used in conjunction with other symptoms and in comparison with other plants in the swath.
Ten pods were inspected for Alternaria black spot at each of four random locations per field for a total of 40 pods. Pod severity (percent of pod area affected) was assessed using a visual scale of 1%, 5%, 10% and 20% devised by Conn et al. (1). Pod severity and incidence in northwestern North Dakota was estimated on the basis of which figure best fit the ten pods being examined, rather than by inspection of individual pods.
Incidences of aster yellows, blackleg and Sclerotinia stem rot were calculated as (number infected/40) X 100. The average severity of black spot in each field was calculated by adding the number of pods per category (1%, 5%, 10%, 20%) times the category (1, 5, 10, 20) and dividing by 40 (the total number of pods examined). A sample calculation follows for a field with 15 pods at 1%, 2 at 5% and 1 at 10%: [(15 X 1%) + (2 X 5%) + (1 X 10%)]/40 = 0.875%.
Counties to be surveyed were determined based on Farm Service Agency records of acreage in 1999 and 2000, with the objective of conducting the survey in the counties with the greatest acreage (Table 1). Acreages from both 1999 and 2000 were used since wet planting conditions reduced acreages of certain counties in some years. An attempt was made to visit at least 10 fields in any county surveyed so there would be an adequate sample size to compare results among counties. Exceptions were made in several counties due to limited canola acreage in 2001. More than 10 fields were surveyed in some counties with large acreages; the objective was to survey one field for each 5,000 acres in the county. This resulted in 30 fields surveyed in Cavalier County, 20 in Bottineau County, 17 in Ward County, 16 in McLean County, 13 in Renville County and 12 in Towner County.
Results
The 2001 survey included all of the Minnesota and North Dakota counties surveyed in 2000 and some additional counties in North Dakota, but did not include the South Dakota counties surveyed in 2000 (Table 1). Counties not previously surveyed included Eddy, McKenzie, Nelson and Pembina counties in North Dakota.
Table 1. Fields surveyed in 2001. ------------------------------------------------ North Dakota No. of Minnesota No. of County Fields County Fields ------------------------------------------------ Benson 9 Kittson 7 Bottineau 20 Marshall 11 Burke 5 Pennington 8 Cavalier 30 Polk 4 Eddy 2 Red Lake 10 Divide 5 Roseau 5 Foster 7 Minn. total 45 Hettinger 10 McHenry 10 McKenzie 2 McLean 16 Mountrail 10 Nelson 10 Pembina 4 Pierce 10 Ramsey 10 Renville 13 Rolette 18 Sheridan 10 Stutsman 10 Towner 12 Ward 17 Wells 10 Williams 2 N.D. total 252 ---------------------------------------------------
Aster yellows incidence was 2.2% in Minnesota and 1.8% in North Dakota, approximately
half the incidence in 2000 for both states (Table 2). The highest incidences were 5% in
Roseau county, Minnesota; 4.7% in Benson County, North Dakota; 4.1% in Pennington County,
Minnesota; and 3.2% in Rolette County, North Dakota. These figures were approximately half
those of the top counties in 2000. No aster yellows was recorded in 11 counties in North
Dakota with the majority of those counties located in the western part of the state.
Table 2. Aster yellows incidence in 2001. ------------------------------------------------------- District Incidence (%) County Incidence (%) ------------------------------------------------------- Minnesota (NW) 2.2 Kittson 0.4 Marshall 0.2 Pennington 4.1 Polk 5 Red Lake 1.5 Roseau 5 ------------------------------------------------------- North Dakota (NE) 4.4 Cavalier 1.4 Nelson 1.8 Pembina 0 Ramsey 11 Towner 10 ------------------------------------------------------- North Dakota (C) 0.5 Eddy/Foster 1.3 Sheridan 0 Stutsman 0 Wells 0.8 ------------------------------------------------------- North Dakota (NC) 1.9 Benson 4.7 Bottineau 0.8 McHenry 0 Pierce 1 Rolette 3.2 ------------------------------------------------------- North Dakota (NW) 0.5 Burke 0 Divide 0 Mountrail 0 Renville 1.0 Ward 0.9 Williams 0 ------------------------------------------------------- North Dakota (WC) 0 McKenzie 0 McLean 0 ------------------------------------------------------- North Dakota (SW) 0 Hettinger 0 ------------------------------------------------------- North Dakota (State) 1.8 -------------------------------------------------------
Blackleg incidence was 1.6% in Minnesota and 1.8% in North Dakota (Table 3). This
represents a lower incidence in North Dakota and a higher incidence in Minnesota than in
2000. Incidence in North Dakota varied by county, with the highest averages 6.5% in Towner
County, 4.8% in Cavalier County, 3.5% in Pierce County and 3.3% in Benson County. The
highest incidence in Minnesota was 3.8% in Pennington County. Blackleg incidence varied by
crop reporting district, with the highest incidence in northeast North Dakota with an
incidence of 3.7%, followed by central North Dakota with 1.7% and Minnesota with 1.6%
(Figure 1). Samples taken in the 2000 survey and tested at Melfort, SK, Canada, were
primarily the virulent strain of the blackleg fungus (M. Keri, personal communication).
Table 3. Blackleg incidence in 2001 ------------------------------------------------------- District Incidence (%) County Incidence (%) ------------------------------------------------------- Minnesota (NW) 1.6 Kittson 0.4 Marshall 2 Pennington 3.8 Polk 1.3 Red Lake 1.3 Roseau 0 ------------------------------------------------------- North Dakota (NE) 3.7 Cavalier 4.8 Nelson 1.8 Pembina 0 Ramsey 0 Towner 6.5 ------------------------------------------------------- North Dakota (C) 1.7 Eddy/Foster 0.8 Sheridan 2 Stutsman 1 Wells 3 ------------------------------------------------------- North Dakota (NC) 1.5 Benson 3.3 Bottineau 0.6 McHenry 0.5 Pierce 3.5 Rolette 0.8 ------------------------------------------------------- North Dakota (NW) 0.7 Burke 0 Divide 0 Mountrail 0.5 Renville 1 Ward 1.2 Williams 0 ------------------------------------------------------- North Dakota (WC) 0.4 McKenzie 0 McLean 0.5 ------------------------------------------------------- North Dakota (SW) 0 Hettinger 0 ------------------------------------------------------- North Dakota (State) 1.8 -------------------------------------------------------
Figure 1. Blackleg by District in 2001.
In 2001, the average Sclerotinia stem rot incidence was 14.1% for both Minnesota and North
Dakota (Table 4). This is lower than the 17.0% for North Dakota and 17.8% for Minnesota in
2000. The incidence in North Dakota varied by county and by crop reporting district, with
the highest incidences 40% in Wells County, 38% in Sheridan County, and 28% in Stutsman
County, all in the central crop reporting district, and 28.8% in Ramsey County, in the
northeast crop reporting district (Figure 2). The highest incidence in Minnesota was 25%
in Pennington County. The highest incidence for a crop reporting district was 32% for the
central district of North Dakota, followed by 17% for the northeast district of North
Dakota (Figure 3).
Table 4. Sclerotinia Stem Rot Incidence in 2001. ------------------------------------------------------- District Incidence (%) County Incidence (%) ------------------------------------------------------- Minnesota (NW) 14.1 Kittson 2.5 Marshall 18.6 Pennington 25 Polk 16.3 Red Lake 6.3 Roseau 16.5 ------------------------------------------------------- North Dakota (NE) 17 Cavalier 15 Nelson 13.8 Pembina 0 Ramsey 28.8 Towner 19.5 ------------------------------------------------------- North Dakota (C) 32 Eddy/Foster 21.1 Sheridan 38 Stutsman 28 Wells 40 ------------------------------------------------------- North Dakota (NC) 11.8 Benson 12.8 Bottineau 9.4 McHenry 9.5 Pierce 14 Rolette 14 ------------------------------------------------------- North Dakota (NW) 5.7 Burke 6 Divide 8 Mountrail 1 Renville 10.2 Ward 5 Williams 0 ------------------------------------------------------- North Dakota (WC) 1 McKenzie 0 McLean 1.1 ------------------------------------------------------- North Dakota (SW) 7.8 Hettinger 7.8 ------------------------------------------------------- North Dakota (State) 14.1 -------------------------------------------------------
Figure 2. Sclerotinia Stem Rot in 2001.
Figure 3. Sclerotinia Stem Rot by District in 2001.
The survey in Cavalier County was completed at two different times: 20 fields were
surveyed on August 15-16 and 10 fields were surveyed on August 28. The fields surveyed on
August 28 were late planted fields that had not been swathed on August 15-16. The 20 early
fields averaged 20% Sclerotinia incidence; the 10 late fields averaged 6%.
Alternaria black spot severity was 0.37% in Minnesota and 0.66% in North Dakota; these figures are twice those in 2000 for North Dakota and about the same for Minnesota. Severity by county varied from 0.1% in Hettinger County in the southwest to as high as 1.2% in Mountrail and Ward Counties and 1.7% in Renville County in northwest North Dakota. In Minnesota severity ranged from as low as 0.06% in Marshall County to as high as 1.75% in Polk County (Table 5).
Table 5. Alternaria black spot severity on the pods in 2001 ----------------------------------------------------------- District Incidence (%) County Incidence (%) ----------------------------------------------------------- Minnesota (NW) 0.37 Kittson 0.19 Marshall 0.06 Pennington 0.72 Polk 1.75 Red Lake 0.20 Roseau 0.23 ----------------------------------------------------------- North Dakota (NE) 0.43 Cavalier 0.36 Nelson 0.18 Pembina 0.43 Ramsey 0.31 Towner 0.89 ----------------------------------------------------------- North Dakota (C) 0.35 Eddy/Foster 0.34 Sheridan 0.21 Stutsman 0.38 Wells 0.41 ----------------------------------------------------------- North Dakota (NC) 0.57 Benson 0.04 Bottineau 0.9 McHenry 0.8 Pierce 0.4 Rolette 0.44 ----------------------------------------------------------- North Dakota (NW) 1.2 Burke 0.8 Divide 0.4 Mountrail 1.2 Renville 1.7 Ward 1.2 Williams 1 ----------------------------------------------------------- North Dakota (WC) 1.17 McKenzie 1 McLean 1.19 ----------------------------------------------------------- North Dakota (SW) 0.10 Hettinger 0.10 ----------------------------------------------------------- North Dakota (State) 0.66 -----------------------------------------------------------
Discussion
Blackleg, caused by L. maculans, was first found in North Dakota in 1991. Some isolates of the blackleg pathogen were identified as the highly virulent pathogenicity group PG-2, which is common in Canada (2, 6). Average incidence in North Dakota was 27.7% in 1991, which was the last year that the highly susceptible cultivar `Westar' was grown (Figure 4). By 1993 `Westar' had been replaced by moderately susceptible cultivars, and blackleg incidence was 3.4% in 1993, 6.2% in 1994, 6.6% in 1995 and lower thereafter. One reason for the slightly higher incidences in 1994 and 1995 is that a few fields of Polish canola, Brassica rapa, were surveyed in those years; all cultivars of B. rapa are susceptible. Although there was change in varieties grown, with some moderately resistant and resistant varieties being grown in recent years, differences in environmental conditions and the use of fungicide seed treatment may also have influenced blackleg incidence.
Figure 4. Blackleg Incidence, 1991-2001.
Major economic incidences of blackleg were found in two North Dakota fields, one in Towner
County with 48% blackleg and one in Cavalier County, with 38%. This is similar to 1999 and
2000. Many lesions were girdling the stems. Petrie (12) determined that there is a 0.7%
yield loss for each 1% of stems with girdling lesions, so the losses in these fields were
estimated at 34% and 27%, respectively. Four other fields in North Dakota had smaller
losses; there were two in Cavalier County with 28% and 20%, one in Sheridan County with
20% and one in Pierce County, with 20% incidence. Losses for 28% and 20% incidence are
estimated at 20% and 14%, respectively.
Sclerotinia was the most prevalent and serious disease problem in 2001, as it was in all previous years, except 1991. Average Sclerotinia incidence was 14.1% in both Minnesota and North Dakota, compared to 17.8% in Minnesota and 17.0% in North Dakota in 2000. Compared to other years, 2001 was an average year, with a Sclerotinia incidence that was at about the mid-point between the lowest and the highest infection levels in other years (Figure 5).
Figure 5. Sclerotinia Incidence, 1991-2001.
The highest state-wide incidence of Sclerotinia stem rot in North Dakota was in 1993, with
an incidence of 18.7%. July rainfall that year was very high. The lowest incidence of
Sclerotinia stem rot in North Dakota was in 1991, with an incidence of 7.1%, a year with
low July rainfall. The highest state-wide incidence of Sclerotinia stem rot in Minnesota
was in 1997 with an incidence of 18.8%, and the lowest was in 1998, with an incidence of
10.6%.
The Sclerotinia data from Cavalier County indicate that infection levels were higher for earlier planted canola (20%) in that county than that for late planted canola (6%). There was a dry period in early to mid July that might have reduced the Sclerotinia incidence for canola flowering at that time.
Data on Canadian yield trials with Sclerotinia indicate a 0.5 to 0.7% yield loss for each 1% of infected plants (11, 15). Since only plant stubble was inspected and infections farther up the plant could not be assessed, the higher figure of 0.7% was used to estimate yield losses. Thus, incidences of 14.1% in Minnesota and North Dakota represent estimated yield losses of 9.9%. Assuming an average yield potential of 13 cwt/A in North Dakota and an average price of $9.47/cwt (loan rate used), losses were approximately 1.29 cwt/A or $12.22/A; losses on 1,293,000 North Dakota acres (Farm Service Agency data) are estimated at $15,800,460, which is $2,330,940 less than the estimated losses in 2000. Assuming a yield potential of 14.5 cwt/A in Minnesota and an average price of $9.12/A (loan rate), losses were approximately 1.44 cwt/A or $13.13/A; losses on 73,762 Minnesota acres (Minnesota Agricultural Statistics Service data) are estimated at $968,495, much less than that estimated in 2000. Total losses for the two states are estimated to be $16,768,955, or about 80% of the estimated losses in 2000.
Fields with over 30% Sclerotinia were considered to be fields with an economic loss, since this incidence represents an estimated 21% loss. There were 39 of 289 surveyed fields in North Dakota, or 14%, with an economic loss (Table 6). This is about two thirds the percent of fields with an economic loss in 2000. There were seven of 45 surveyed fields in Minnesota, or 16%, with an economic loss. This is about two thirds of the percent of fields with an economic loss in Minnesota in 2000. Average incidence in the fields with over 30% Sclerotinia was 47.9% for the 39 surveyed fields in North Dakota and 43.8% for the seven surveyed fields in Minnesota. The percent infection for the fields with an economic loss in 2001 was slightly greater than the percentages for Minnesota and North Dakota in 2000.
Table 6. Percent of fields with an economic loss
(>30% incidence) from Sclerotinia stem rot in 2001.
---------------------------------------------------------------
Fields with Fields with
Economic Loss Economic Loss
--------------- ---------------
No. of % of No. of % of
District Fields Fields County Fields Fields
---------------------------------------------------------------
Minn. (NW) 7/45 15.6 Kittson 0/7 0
Marshall 3/11 27.3
Pennington 3/8 37.5
Polk 1/4 25.0
Red Lake 0/10 0
Roseau 0/5 0
---------------------------------------------------------------
N.D. (NE) 14/66 21.2 Cavalier 5/30 16.7
Nelson 0/10 0
Pembina 0/4 0
Ramsey 5/10 50.0
Towner 4/12 33.3
---------------------------------------------------------------
N.D. (C) 19/39 48.7 Eddy/Foster 2/9 22.2
Sheridan 6/10 60.0
Stutsman 5/10 50.0
Wells 6/10 60.0
---------------------------------------------------------------
N.D. (NC) 6/67 9.0 Benson 0/9 0
Bottineau 2/20 10.0
McHenry 1/10 10.0
Pierce 2/10 20.0
Rolette 1/18 5.6
---------------------------------------------------------------
N.D. (NW) 0/52 0 Burke 0/5 0
Divide 0/5 0
Mountrail 0/10 0
Renville 0/13 0
Ward 0/17 0
Williams 0/2 0
---------------------------------------------------------------
N.D. (WC) 0/18 0 McKenzie 0/2 0
McLean 0/16 0
---------------------------------------------------------------
N.D. (SW) 0/10 0 Hettinger 0/10 0
---------------------------------------------------------------
N.D. (State) 39/289 13.5
---------------------------------------------------------------
In Minnesota, Alternaria black spot pod severity in 2001 was similar to 2000, but in North
Dakota it was about twice as great in 2001 as in 2000 (Figure 6). Black spot severity was
low in all years, except that it was moderately low in North Dakota in 1996. Alternaria
black spot may contribute to increased seed shattering and green seed. For each 1% of
black spot severity on the pods there is a 1% yield loss (14). However, incidences in
northwestern North Dakota cannot be compared with those from other areas since the method
of recording data was different (see Materials and Methods). It is possible that assessing
10 pods as a group may lead to assessments that are higher than those where 10 pods were
assessed individually.
Figure 6. Alternaria Black Spot Severity, 1996-2001.
Relatively low levels of aster yellows occurred in 2001, compared to higher levels in 2000
and in 1999. Apparently there was a smaller influx of the aster leafhopper, vector of the
aster yellows phytoplasma, in 2001 as compared to 2000.
Acknowledgments
This survey was supported by a USDA grant to the University of Minnesota, on behalf of the Minnesota Canola Council. The field assistance of IPM scouts Nathan Carlson, Kelly Novak, and Holly Semler is greatly appreciated. The assistance of Agnes Vernon for publication layout is gratefully acknowledged.
Literature Cited
1. Conn, K.L., J.P Tewari and R.P. Awasthi. 1990. A Disease Assessment Key for Alternaria Black Spot in Rapeseed and Mustard. Can. Plant Dis. Sur. 70(1):19-22.
2. Lamey, H.A. 1995. Blackleg and Sclerotinia Disease of Canola in North Dakota in 1991 and 1993. Plant Disease 79:322-324.
3. Lamey, A. 1995. Blackleg and Sclerotinia Disease Surveys of Canola in North Dakota, 1991-1994. NDSU Extension Rept. 23. 4 p.
4. Lamey, A. 1996. North Dakota Disease Survey, 1995. NDSU Extension Rept. 32, 4 p.
5. Lamey, A. 1997. 1996 Canola Disease Survey in Minnesota, North Dakota and South Dakota. NDSU Extension Rept. 34. 4 p.
6. Lamey, H.A., and D.E. Hershman, 1993. Canola (Brassica napus L.) blackleg caused by Leptosphaeria maculans in North Dakota. Plant Disease 77:1263.
7. Lamey, A., J. Knodel, K. McKay, G. Endres, Z. Fore, K. Andol and M. Draper. 2001. 2000 Canola Disease Survey in Minnesota, North Dakota and South Dakota. NDSU Ext. Rept. 63: p. 1-7.
8. Lamey, A. and K. McKay. 1998. 1997 Canola Disease Survey in Minnesota, North Dakota and South Dakota. NDSU Extension Rept. 39. 4 p.
9. Lamey, A., K. McKay and J. Knodel. 1998. 1998 Canola Disease Survey in Minnesota and North Dakota. NDSU Extension Rept. 49. 4 p.
10. Lamey, A., K. McKay, J. Knodel, G. Endres, K. Andol, and Z. Fore. 2000. 1999 Minnesota and North Dakota Canola Disease Survey. NDSU Extension Rept. 60. 5 p.
11. Morrall, R.A. A., J. Dueck and P.R. Verma. 1984. Yield Losses Due to Sclerotinia Stem Rot in Western Canadian Rapeseed. Can. J. Plant Pathol. 6:265.
12. Petrie, G.A. 1986. Blackleg and Other Diseases of Canola in Saskatchewan in 1984 and 1985. Can. Plant Dis. Surv. 66(2):51-53.
13. Petrie, G.A., K. Mortensen and J. Dueck. 1985. Blackleg and Other Diseases of Rapeseed in Saskatchewan, 1978 to 1981. Can. Plant Dis. Surv. 65(2):35-41.
14. Stonehouse, D. 2000. Alternaria Black Spot Is Still Out There. Canola Guide May 2000:17.
15. Thompson, J.R., P.M. Thomas and I.R. Evans. 1984. Efficacy of Aerial Application of Benomyl and Iprodione for the Control of Sclerotinia Stem Rot of Canola (Rapeseed) in Central Alberta. Can. J. Plant Pathol. 6:75-77.
Extension Report 71, December 2001
