Comparison of disease severity and incidence at different growth stages of naturally infected canola plants under field conditions by pycnidiospores of Phoma lingam as a main source of inoculum

Figures & data

Fig. 1. Mean disease severity (a) and disease incidence (b) for the plants infected at 1, 0 (cotyledon), 1, 3 (3-leaf) and 1, 6 (6-leaf) stages of cultivar ‘Westar’. The experiment was carried out in a 50 × 60 m plot from June 1^st to July 7^th in 2004 for five weeks and from May 27^th to August 10^th in 2005 for 10 weeks at the Carman Research Station, Carman, Manitoba. Ten plants at 1, 0; 1, 3; and 1, 6 stages were each placed in a phoma stem canker-infested canola field for one week. After one week of field exposure, the plants were returned to greenhouse until the pod-filling stage. Stem disease severity was evaluated using a 0-5 scale where 0 = uninfected, 1 = lesion area less than 25% of the cross-section area of the crown, 2 = 25 to 50%, 3 = >50% girdled, stem firm, 4 => 50% girdled, stem weak, and 5 = plant dead (West et al., 2002). The square root transformation was applied to mean disease severity.

Table 1.  Mean fruiting bodies* density on stubble pieces, mean weakly concentration of ascospores^†, phoma stem canker severity and incidence^‡ of canola plants infected at three different growth stages in each week at the Carman Research Centre in Manitoba for 2004 and 2005

				Spore sampler
Year	Week	Pycnidia	Pseudothecia	Burkard	Rotorod	DSC	DSL	DSR	DIC	DIL	DIR
2004	1	4.0	0	0	0	4.2	2.9	1.8	100	100	80
2004	2	4.1	0.002	0	0	2.8	1.5	1.3	77	50	50
2004	3	3.8	0.003	0	0	4.7	2.9	1.9	100	100	90
2004	4	4.1	0	0	0	1.2	0.7	0.4	50	66	22
2004	5	3.8	0	0	0	2.4	1.8	1.8	80	90	70
2005	1	4.6	0	0	0	2.4	2.2	1.6	100	100	100
2005	2	3.7	0.02	0.028	0	3.6	2.6	3.4	100	100	100
2005	3	3.7	0	0	0	3.0	2.7	2.6	100	88	100
2005	4	3.1	0	0	0	1.3	0.7	0.4	70	33	44
2005	5	3.6	0	0.008	0	4.0	3.2	3.4	100	100	88
2005	6	3.7	0	0	0	4.1	2.7	2.1	100	90	66
2005	7	4.1	0.004	0.008	0	2.3	1.1	0.9	100	75	77
2005	8	3.7	0	0	0	3.6	2.4	2.6	100	100	100
2005	9	4.1	0	0	0	1.5	1.8	0.9	60	55	40
2005	10	4.1	0	0	0	2.0	2.4	0.7	60	88	60
Notes: * The density of fruiting bodies considered as a total number of pycnidia or pseudothecia per cm^2 of stubble pieces. It was calculated by dividing total number of fruiting bodies counted on 10 cm of each stubble piece on surface area of the stubble piece. The surface area of each piece was calculated as, in which L is the length of 10 cm of each piece and C is circumference of each piece which was measured using flexible seamstress tape.
^† The mean weakly concentration of air-borne ascospores (ascospores/m^3/week) was the average concentration of ascospores calculated by two types of sporetraps per week. Spore concentration for rotorod spore sampler was calculated by the rod rotations per minute (2400), the rotation constant (0.097), the sampling period in minutes per day (120 min). This concentration for seven-day Burkard spore trap was calculated as the number of spores per section, divided by the throughput at the orifice of the Burkard trap, which was 0.6 m^3/h.
^‡ Mean disease severity (0-5 scale) in the plants at 1, 0 stage (DSC), at 1, 3 stage (DSL), and at 1, 6 stage (DSR); disease incidence (percentage) in plants at 1, 0 stage (DIC), at 1, 3 stage (DIL) and at 1, 6 stage (DIR).

Table 2.  Analysis of variance for the effects of treatment, year and their interaction on the mean disease severity and disease incidence of blackleg on the canola plants infected at three different growth stages of cultivar ‘Westar’ in 2004 and 2005*

	Mean Disease Severity^†			Disease Incidence (%)^‡
Source of variation	df	MS	F Value	df	MS	F Value
Treatment	2	8.13	26.11***	2	0.21	8.71***
Year	1	1.76	0.34	1	0.21	1.38
Year × Treatment	2	0.46	1.49	2	0.05	0.40
Week (Year)	13	5.20	16.69***	13	0.29	11.90**
Week × Treatment (Year)	26	0.31	0.79	...	.....	......
Error	390	0.39	......	26	0.02	......
Notes: * Value followed by ** and *** are significantly different at 0.001 and 0.0001, respectively.
^† Stem disease severity was evaluated at the pod-filling stage of canola. Stem disease severity was evaluated using a 0-5 scale where 0 = uninfected, 1 = lesion area less than 25% of the cross-section area of the crown, 2 = 25 to 50%, 3 = >50% girdled, stem firm, 4 => 50% girdled, stem weak, and 5 = plant dead (West et al., 2002). The square root transformation was applied to mean disease severity.
^‡ Disease incidence was evaluated by counting all plants with stem lesion or canker dividing by total plant count per treatment and multiplying by 100. Arc sin-square root transformed data were used for analysis.

Table 3.  Mean comparison of pycnidia and pseudothecia of Leptosphaeria maculans on blackleg infected canola stubble pieces in each week at the Carman Research Centre in Manitoba for pooled data*

Fruiting bodies	2004	2005
Pseudothecia	8.1×10^−7 ± 0.0001 a	2.44×10^−5 ± 0.0005a
Pycnidia	3.98 ± 0.0001 b	3.86 ± 0.0005b
*Back-transformed value of mean density of fruiting bodies. Mean density of fruiting bodies of L. maculans identified on canola infested debris at Carman Research Station, Manitoba. Thirty-five blackleg-infected stubble pieces were selected randomly from the field each week during the experiment in 2004 and 2005 to examine for the presence of pycnidia or pseudothecia. The surface area of each piece was calculated as, in which L is the length of each piece (10 cm of each piece was selected for counting the fruiting bodies) and C is circumference of each piece which was measured using flexible seamstress tape. The square root transformation was applied to mean density of fruiting bodies.

West , J.S. , Fitt , B.D.L. , Leech , P.K. , Biddulph , J.E. , Huang , Y.-J. and Balesdent , M. H. 2002 . Effects of timing of Leptosphaeria maculans ascospore release and fungicide regime on phoma leaf spot and phoma stem canker development on winter oilseed rape (Brassica napus) in southern England . Plant Pathol. , 51 : 454 – 463 .

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