Cotyledon and true-leaf pathogenicity tests
The disease index of 44 radish accessions screened at cotyledons and true-leaves for DM resistance separated the accessions into four phenotypic categories (Table 4). At cotyledon stage 37 accessions (84%) were resistant, 5 accessions (11%) partially resistant, and 2 accessions (5%) highly susceptible. In resistant accessions 43 to 100% of the plants were in classes 1–2, 0 to 52% in classes 3–4, and 0 to 21% in classes 5–6. Partially resistant accessions presented between 27 and 42% of plants in classes 1–2, 36 to 73% in classes 3–4, and 0 to 36% in classes 5–6. The two accessions Rd197 and Rd208 classified as highly susceptible did not register any plants in classes 1–3, 6 and 8% in classes 4, and 92 and 94% in classes 5–6 respectively.
Table 4
Number of plants per interaction-phenotype classes (IP classes), total number of plants evaluated and disease index (DI) values of forty-four radish accessions tested for downy mildew resistance at cotyledon and true-leaf stages with H. brassicae isolate R10.
Code | Cotyledon | Total | DI | | Leaves | Total | DI | |
| IP classes | | | | IP classes | | | |
| 1 | 2 | 3 | 4 | 5 | 6 | | | | 1 | 2 | 3 | 4 | 5 | 6 | | | |
Rd001 | 14 | 3 | 2 | 1 | 0 | 0 | 20 | 1.5 | R | 13 | 5 | 0 | 0 | 0 | 0 | 18 | 1.3 | R |
Rd002 | 18 | 1 | 1 | 3 | 0 | 0 | 23 | 1.5 | R | 15 | 5 | 0 | 0 | 0 | 0 | 20 | 1.3 | R |
Rd003 | 15 | 1 | 4 | 3 | 0 | 1 | 24 | 2.0 | R | 16 | 4 | 0 | 0 | 0 | 0 | 20 | 1.2 | R |
Rd004 | 14 | 1 | 3 | 1 | 3 | 2 | 24 | 2.3 | R | 17 | 1 | 0 | 0 | 0 | 0 | 18 | 1.1 | R |
Rd005 | 16 | 2 | 6 | 0 | 0 | 0 | 24 | 1.6 | R | 4 | 14 | 2 | 0 | 0 | 0 | 20 | 1.9 | R |
Rd011 | 10 | 5 | 6 | 1 | 0 | 0 | 22 | 1.9 | R | 5 | 8 | 1 | 3 | 2 | 1 | 20 | 2.6 | PR |
Rd012 | 10 | 0 | 6 | 6 | 1 | 1 | 24 | 2.6 | PR | 2 | 6 | 4 | 5 | 2 | 1 | 20 | 3.1 | PR |
Rd013 | 14 | 1 | 6 | 1 | 0 | 0 | 22 | 1.7 | R | 2 | 13 | 5 | 0 | 0 | 0 | 20 | 2.2 | R |
Rd014 | 13 | 2 | 4 | 4 | 0 | 1 | 24 | 2.1 | R | 0 | 10 | 4 | 2 | 3 | 1 | 20 | 3.1 | PR |
Rd015 | 13 | 0 | 7 | 3 | 0 | 0 | 23 | 2.0 | R | 4 | 15 | 1 | 0 | 0 | 0 | 20 | 1.9 | R |
Rd017 | 12 | 2 | 6 | 4 | 0 | 0 | 24 | 2.1 | R | 14 | 6 | 0 | 0 | 0 | 0 | 20 | 1.3 | R |
Rd020 | 10 | 0 | 9 | 3 | 0 | 1 | 23 | 2.4 | R | 4 | 14 | 0 | 0 | 0 | 0 | 18 | 1.8 | R |
Rd108 | 11 | 0 | 6 | 6 | 0 | 1 | 24 | 2.5 | R | 10 | 0 | 0 | 3 | 1 | 5 | 19 | 3.0 | PR |
Rd111 | 11 | 1 | 2 | 7 | 1 | 0 | 22 | 2.4 | R | 13 | 6 | 1 | 0 | 0 | 0 | 20 | 1.4 | R |
Rd175 | 17 | 0 | 3 | 3 | 0 | 0 | 23 | 1.7 | R | 8 | 11 | 1 | 0 | 0 | 0 | 20 | 1.7 | R |
Rd176 | 12 | 3 | 7 | 1 | 0 | 0 | 23 | 1.9 | R | 10 | 8 | 0 | 0 | 0 | 2 | 20 | 1.9 | R |
Rd177 | 15 | 1 | 7 | 1 | 0 | 0 | 24 | 1.8 | R | 12 | 5 | 1 | 0 | 2 | 0 | 20 | 1.8 | R |
Rd180 | 12 | 0 | 6 | 6 | 0 | 0 | 24 | 2.3 | R | 16 | 3 | 0 | 0 | 1 | 0 | 20 | 1.4 | R |
Rd181 | 11 | 1 | 4 | 7 | 0 | 1 | 24 | 2.5 | R | 10 | 7 | 2 | 0 | 0 | 0 | 19 | 1.6 | R |
Rd182 | 15 | 0 | 7 | 0 | 0 | 0 | 22 | 1.6 | R | 19 | 0 | 1 | 0 | 0 | 0 | 20 | 1.1 | R |
Rd183 | 15 | 2 | 5 | 2 | 0 | 0 | 24 | 1.8 | R | 8 | 7 | 0 | 0 | 1 | 4 | 20 | 2.6 | PR |
Rd184 | 16 | 0 | 4 | 1 | 0 | 1 | 22 | 1.7 | R | 17 | 2 | 1 | 0 | 0 | 0 | 20 | 1.2 | R |
Rd186 | 13 | 3 | 4 | 4 | 0 | 0 | 24 | 2.0 | R | 6 | 14 | 0 | 0 | 0 | 0 | 20 | 1.7 | R |
Rd187 | 15 | 2 | 2 | 2 | 0 | 1 | 22 | 1.8 | R | 20 | 0 | 0 | 0 | 0 | 0 | 20 | 1.0 | R |
Rd188 | 13 | 6 | 2 | 1 | 0 | 0 | 22 | 1.6 | R | 10 | 9 | 1 | 0 | 0 | 0 | 20 | 1.6 | R |
Rd189 | 16 | 3 | 4 | 1 | 0 | 0 | 24 | 1.6 | R | 2 | 15 | 1 | 2 | 0 | 0 | 20 | 2.2 | R |
Rd190 | 14 | 3 | 2 | 5 | 0 | 0 | 24 | 1.9 | R | 4 | 14 | 1 | 1 | 0 | 0 | 20 | 2.0 | R |
Rd191 | 11 | 6 | 0 | 3 | 0 | 0 | 20 | 1.8 | R | 5 | 14 | 1 | 0 | 0 | 0 | 20 | 1.8 | R |
Rd192 | 13 | 2 | 5 | 3 | 1 | 0 | 24 | 2.0 | R | 2 | 18 | 0 | 0 | 0 | 0 | 20 | 1.9 | R |
Rd193 | 18 | 2 | 1 | 0 | 0 | 0 | 21 | 1.2 | R | 2 | 18 | 0 | 0 | 0 | 0 | 20 | 1.9 | R |
Rd194 | 10 | 6 | 4 | 3 | 0 | 0 | 23 | 2.0 | R | 4 | 14 | 2 | 0 | 0 | 0 | 20 | 1.9 | R |
Rd195 | 13 | 5 | 0 | 5 | 1 | 0 | 24 | 2.0 | R | 8 | 10 | 2 | 0 | 0 | 0 | 20 | 1.7 | R |
Rd196 | 18 | 4 | 0 | 0 | 0 | 0 | 22 | 1.2 | R | 19 | 0 | 0 | 0 | 0 | 0 | 19 | 1.0 | R |
Rd197 | 0 | 0 | 0 | 2 | 3 | 19 | 24 | 5.7 | HS | 0 | 0 | 0 | 0 | 0 | 10 | 10 | 6.0 | HS |
Rd198 | 13 | 3 | 3 | 3 | 1 | 0 | 23 | 2.0 | R | 17 | 3 | 0 | 0 | 0 | 0 | 20 | 1.2 | R |
Rd201 | 4 | 0 | 5 | 5 | 1 | 0 | 15 | 2.9 | PR | 1 | 3 | 7 | 4 | 3 | 2 | 20 | 3.6 | PR |
Rd202 | 24 | 0 | 0 | 0 | 0 | 0 | 24 | 1.0 | R | 11 | 8 | 1 | 0 | 0 | 0 | 20 | 1.5 | R |
Rd203 | 22 | 0 | 2 | 0 | 0 | 0 | 24 | 1.2 | R | 0 | 18 | 2 | 0 | 0 | 0 | 20 | 2.1 | R |
Rd204 | 14 | 1 | 5 | 3 | 0 | 0 | 23 | 1.9 | R | 8 | 9 | 2 | 0 | 0 | 0 | 19 | 1.7 | R |
Rd205 | 4 | 0 | 3 | 8 | 0 | 0 | 15 | 3.0 | PR | 6 | 3 | 3 | 4 | 1 | 1 | 18 | 2.7 | PR |
Rd206 | 9 | 0 | 2 | 4 | 0 | 0 | 15 | 2.1 | R | 5 | 6 | 6 | 2 | 1 | 0 | 20 | 2.4 | R |
Rd207 | 5 | 0 | 2 | 8 | 0 | 0 | 15 | 2.9 | PR | 1 | 2 | 5 | 9 | 3 | 0 | 20 | 3.6 | PR |
Rd208 | 0 | 0 | 0 | 1 | 4 | 11 | 16 | 5.6 | HS | 0 | 0 | 1 | 3 | 0 | 16 | 20 | 5.6 | HS |
ER | 0 | 3 | 1 | 3 | 2 | 2 | 11 | 3.9 | PR | 2 | 15 | 3 | 0 | 0 | 0 | 20 | 2.1 | R |
Total | | | | | | | 964 | | | | | | | | | 858 | | |
At cotyledon stage a total of 24 observations were made (8 observations x 3 replicates) per accession. At true-leaf stage a total of 20 observations were made (10 observations x 2 replicates) per accession. |
The accessions were separated into four phenotypic categories at cotyledon and true-leaf stages: R = Resistant (DI ≤ 2.5), PR = Partially Resistant (2.5 < DI ≤ 4.0), S = Susceptible (4.0 < DI ≤ 5.0), and HS = Highly Susceptible (5.0 < DI ≤ 6.0). |
A similar response was observed on the plants inoculated on the 1st and 2nd leaves, once 34 accessions (77%) were classified as resistant, 8 accessions (18%) as partially resistant, and 2 accessions (5%) as highly susceptible. In resistant accessions 55 to 100% of the plants were in classes 1–2, 0 to 40% in classes 3–4, and between 0 and 10% in classes 5–6. Partially resistant accessions showed 15 to 75% of the plants in classes 1–2, 0 to 70% in classes 3–4, and between 11 and 32% in classes 5–6. The two highly susceptible accessions Rd197 and Rd208 did not register plants in classes 1–2, 0 and 20 % in classes 3–4, and 100 and 80% in class 6 respectively (Table 4).
Root evaluation with two H. brassicae isolates
Unlike the cotyledons and true-leaves that were tested with isolate R10 only, roots were tested with isolates R10 and R6, in two independent experiments. There was a highly significant correlation (r = 0.81, P < 0.000) between DI induced by the two isolates (Fig. 2) and most of the accessions showed the same general pattern of resistance when inoculated with each isolate (Table 5).
Table 5
Number of plants per interaction-phenotype classes (IP classes), total number of plants evaluated and disease index (DI) value of radish accessions tested for downy mildew resistance on the roots with H. brassicae isolates R10 and R6.
Code | Isolate R10 | Total | DI | | Isolate R6 | Total | DI | |
| IP classes | | | | IP classes | | | |
| 1 | 2 | 3 | 4 | | | | 1 | 2 | 3 | 4 | | | |
Rd001 | 22 | 0 | 0 | 0 | 22 | 1.0 | R | 19 | 0 | 0 | 0 | 19 | 1.0 | R |
Rd002 | 23 | 0 | 0 | 0 | 23 | 1.0 | R | 23 | 1 | 0 | 0 | 24 | 1.0 | R |
Rd003 | 21 | 1 | 0 | 0 | 22 | 1.0 | R | 23 | 0 | 0 | 0 | 23 | 1.0 | R |
Rd004 | 21 | 0 | 0 | 0 | 21 | 1.0 | R | 19 | 0 | 0 | 0 | 19 | 1.0 | R |
Rd005 | 10 | 4 | 6 | 1 | 21 | 1.9 | PR | 4 | 5 | 11 | 3 | 23 | 2.6 | S |
Rd011 | 17 | 2 | 2 | 2 | 23 | 1.5 | PR | 10 | 2 | 7 | 2 | 21 | 2.0 | PR |
Rd012 | 4 | 2 | 2 | 1 | 9 | 2.0 | PR | - | - | - | - | 0 | nt | |
Rd013 | 13 | 3 | 4 | 2 | 22 | 1.8 | PR | 16 | 4 | 2 | 1 | 23 | 1.5 | PR |
Rd014 | 15 | 4 | 1 | 0 | 20 | 1.3 | PR | 11 | 1 | 7 | 4 | 23 | 2.2 | S |
Rd015 | 17 | 2 | 1 | 1 | 21 | 1.3 | PR | 9 | 6 | 1 | 1 | 17 | 1.6 | PR |
Rd017 | 18 | 3 | 2 | 1 | 24 | 1.4 | PR | 9 | 4 | 5 | 4 | 22 | 2.2 | S |
Rd020 | 12 | 10 | 0 | 1 | 23 | 1.6 | PR | 6 | 3 | 5 | 1 | 15 | 2.1 | S |
Rd108 | 13 | 5 | 0 | 1 | 19 | 1.4 | PR | 7 | 5 | 5 | 2 | 19 | 2.1 | S |
Rd111 | 19 | 1 | 0 | 0 | 20 | 1.1 | PR | 20 | 2 | 0 | 0 | 22 | 1.1 | PR |
Rd130 | 22 | 0 | 0 | 0 | 22 | 1.0 | R | 11 | 0 | 0 | 0 | 11 | 1.0 | R |
Rd175 | 7 | 0 | 2 | 11 | 20 | 2.9 | S | 12 | 0 | 1 | 9 | 22 | 2.3 | S |
Rd176 | 9 | 1 | 3 | 9 | 22 | 2.5 | S | 9 | 1 | 3 | 8 | 21 | 2.5 | S |
Rd177 | 16 | 2 | 2 | 1 | 21 | 1.4 | PR | 12 | 2 | 2 | 3 | 19 | 1.8 | PR |
Rd180 | 14 | 5 | 2 | 2 | 23 | 1.7 | PR | 10 | 5 | 5 | 2 | 22 | 2.0 | PR |
Rd181 | 13 | 3 | 1 | 2 | 19 | 1.6 | PR | 9 | 6 | 3 | 3 | 21 | 2.0 | PR |
Rd182 | 17 | 2 | 1 | 1 | 21 | 1.3 | PR | 16 | 2 | 2 | 1 | 21 | 1.4 | PR |
Rd183 | 13 | 2 | 3 | 5 | 23 | 2.0 | PR | 7 | 2 | 7 | 3 | 19 | 2.3 | S |
Rd184 | 17 | 2 | 0 | 0 | 19 | 1.1 | PR | 17 | 2 | 1 | 1 | 21 | 1.3 | PR |
Rd186 | 18 | 2 | 2 | 1 | 23 | 1.4 | PR | 15 | 5 | 3 | 0 | 23 | 1.5 | PR |
Rd187 | 17 | 1 | 3 | 1 | 22 | 1.5 | PR | 9 | 0 | 5 | 4 | 18 | 2.2 | S |
Rd188 | 20 | 0 | 1 | 0 | 21 | 1.1 | PR | 16 | 4 | 0 | 0 | 20 | 1.2 | PR |
Rd189 | 9 | 6 | 3 | 4 | 22 | 2.1 | S | 12 | 1 | 3 | 0 | 16 | 1.4 | PR |
Rd190 | 16 | 4 | 1 | 2 | 23 | 1.5 | PR | 13 | 5 | 1 | 1 | 20 | 1.5 | PR |
Rd191 | 14 | 1 | 1 | 3 | 19 | 1.6 | PR | 10 | 4 | 6 | 1 | 21 | 1.9 | PR |
Rd192 | 13 | 2 | 5 | 1 | 21 | 1.7 | PR | 10 | 4 | 4 | 1 | 19 | 1.8 | PR |
Rd193 | 12 | 4 | 3 | 2 | 21 | 1.8 | PR | 11 | 3 | 3 | 2 | 19 | 1.8 | PR |
Rd194 | 6 | 6 | 5 | 4 | 21 | 2.3 | S | 2 | 1 | 3 | 15 | 21 | 3.5 | HS |
Rd195 | 7 | 3 | 4 | 3 | 17 | 2.2 | S | 9 | 4 | 5 | 4 | 22 | 2.2 | S |
Rd196 | 12 | 2 | 1 | 3 | 18 | 1.7 | PR | 3 | 5 | 8 | 3 | 19 | 2.6 | S |
Rd197 | 3 | 0 | 1 | 2 | 6 | 2.3 | S | 1 | 1 | 1 | 3 | 6 | 3.0 | S |
Rd198 | 3 | 1 | 4 | 10 | 18 | 3.2 | HS | 1 | 1 | 3 | 14 | 19 | 3.6 | HS |
Rd201 | 8 | 3 | 4 | 6 | 21 | 2.4 | S | - | - | - | - | 0 | nt | |
Rd202 | 24 | 0 | 0 | 0 | 24 | 1.0 | R | - | - | - | - | 0 | nt | |
Rd203 | 23 | 0 | 0 | 0 | 23 | 1.0 | R | - | - | - | - | 0 | nt | |
Rd204 | 19 | 0 | 0 | 0 | 19 | 1.0 | R | - | - | - | - | 0 | nt | |
Rd205 | 17 | 1 | 0 | 0 | 18 | 1.1 | PR | - | - | - | - | 0 | nt | |
Rd206 | 18 | 1 | 1 | 2 | 22 | 1.4 | PR | - | - | - | - | 0 | nt | |
Rd207 | 13 | 3 | 2 | 5 | 23 | 2.0 | PR | - | - | - | - | 0 | nt | |
Rd208 | 16 | 4 | 3 | 0 | 23 | 1.4 | PR | - | - | - | - | 0 | nt | |
ER | 9 | 2 | 1 | 3 | 15 | 1.9 | PR | 4 | 3 | 4 | 1 | 12 | 2.2 | S |
Total | | | | | 920 | | | | | | | 702 | | |
nt – not tested. At root stage a total of 24 observations were made (8 observations x 3 replicates) per accession and isolate. F accession = 13.78 (p = 0.000), F isolate = 25.39 (p = 0.000), F accession x isolate = 1.73 (p = 0.005). Accessions were separated into four phenotypic categories based on DI values: R = Resistant (DI ≤ 1.0), PR = Partially Resistant (1.0 < DI ≤ 2.0), S = Susceptible (2.0 < DI ≤ 3.0), and HS = Highly Susceptible (3.0 < DI ≤ 4.0). |
Isolate R6 was more aggressive than R10 inducing DI values equal or higher in all accessions, with the exception of the accessions Rd013, Rd175, and Rd189. However, no significant differences were observed in the virulence between isolates in the same accession. Five accessions (Rd001, Rd002, Rd003, Rd004 and Rd130) were resistant at the roots to both isolates and accession Rd198 was highly susceptible to both (Fig. 2).
In R10 inoculation, 8 accessions (18%) were resistant, 29 accessions (64%) partially resistant, 7 accessions (16%) susceptible, and only one accession (2%) highly susceptible. In resistant accessions all the plants were in class 1 (appearance of necrosis without sporulation), with the exception of Rd003 accession which presented one plant (5%) in class 2. Partially resistant accessions included between 44 and 95% of the plants in class 1, 0 to 43% in class 2, and 0 to 35% in classes 3–4. Susceptible accessions registered between 29 and 50% of plants in class 1, 0 to 29% in class 2, and 32 to 65% in classes 3–4. The only accession Rd198 classified as highly susceptible registered 17% of the plants in class 1, 5% in class 2, and 78% in classes 3–4.
In R6 inoculation, 5 accessions (14%) were resistant, 16 accessions (44%) partially resistant, 13 accessions (36%) susceptible, and two accessions (6%) highly susceptible. In resistant accessions all the plants were in class 1 (appearance of necrosis without sporulation), with the exception of Rd002 accession which presented one plant (4%) in class 2. Partially resistant accessions included between 43 and 91% of the plants in class 1, 6 to 35% in class 2, and 0 to 43% in classes 3–4. Susceptible accessions registered between 16 and 54% of plants in class 1, 0 to 26% in class 2, and 37 to 66% in classes 3–4. The accessions Rd194 and Rd198 classified as highly susceptible registered 10 and 5% of the plants in class 1, 5% in class 2, and 85 and 90% in classes 3–4 respectively.
Comparing DM resistance in different organs
A highly significant positive correlation (r = 0.81, P < 0.000) was recorded between DI values of cotyledons and true-leaves (Fig. 3a), but between cotyledons and roots (r = 0.17, P ˃ 0.05) and leaves and roots (r = 0.23, P ˃ 0.05) the correlation was not significant (Figs. 3b and 3c).
Responses were significantly different between accession and plant organ concerning resistance/susceptibility to DM. For instance, Rd208 was very susceptible in cotyledons and leaves (DI = 5.6 in both) and showed an interesting partially resistance in the roots (DI = 1.4) to isolate R10. On the contrary, Rd201 was partially resistant in cotyledons and true-leaves (DI = 2.9 and 3.6 respectively) and was susceptible in roots (DI = 2.4). Even more contrasting was the accession Rd198 (breeding line) resistance in cotyledons and leaves (DI = 2.0 and 1.2 respectively) and high susceptibility in roots to isolates R10 and R6 (DI = 3.2 and 3.6 respectively). Likewise, four accessions Rd175, Rd176, Rd194, and Rd195 were resistant in cotyledons and true-leaves (DI between 1.7 and 2.0) and susceptible in roots to both isolates (DI between 2.2 and 3.5).