Table 4.

Disease severity index (DSI_b) and disease incidence (DI_b, percentage of diseased plants) of clubroot in bioassays performed in a growth chamber with soil from the field trials 2018/2019, linked to the number of gene copies (logarithm at calculation) of Plasmodiophora brassicae for susceptible Cultivar mix and clubroot-resistant cultivars cv. Mentor, cv. Alister, and cv. Archimedes. Values are mean for all samples from each plot (n = 4). Sites were selected based on field abundance (gene copies g⁻¹ soil) of P. brassicae DNA (Simrishamn 600,000; Tomelilla 370,000; Kumla 50,000).

Treatment	DSIb (0–100)		DIb (%)		Gene Copies (No. g soil−1)		DSIb × Gene Copies	DIb × Gene Copies
Gene copies P. brassicae, average
Simrishamn					442,300	a *
Tomelilla					38,000	b
Kumla					3700	b
P-value					<0.001
Coeff. of var. †					113
Field wise
Simrishamn
Cultivar mix	15.8	a	19.4	a	633,900	a	ns	ns
cv. Mentor	4.6	a	5.6	b	471,500	ab	ns	ns
cv. Alister	16.2	a	19.2	a	299,900	b	ns	ns
cv. Archimedes	4.0	a	5.8	b	364,300	ab	ns	ns
p-value	0.020		0.020		0.015
Coeff. of var.	141		134		4.0
Tomelilla
Cultivar mix	54.6	a	61.8		48,400	a	ns	ns
cv. Mentor	22.9	b	25.7		42,400	ab	ns	ns
cv. Alister	29.8	b	33.7		27,300	c	ns	ns
cv. Archimedes	14.6	b	16.9		35,200	bc	ns	ns
p-value	<0.001		ns		<0.001
Coeff. of var.	71.2		66.2		2.9
Kumla
Cultivar mix	2.5		2.5	a	3400		ns	ns
cv. Mentor	0.7		0.7	ab	3500		ns	ns
cv. Alister	0.0		0.0	b	4000		ns	ns
cv. Archimedes	1.7		1.7	b	3600		ns	ns
p-value	ns		ns		ns
Coeff. of var.	323		323		75

^† Coefficient of variation. * Different letters indicate significant differences according to Tukey’s HSD-test (p < 0.05).