4.3 Results
4.3.2 Comparison of means among generations
Analysis of variance of ALS severity scores from the six generations showed highly significant differences among the generations for all the crosses (Table 4.3)
Table 4.3: ANOVA for angular leaf spot severity scores among generations in the crosses, Wairimu x Mexico 54, Super-rosecoco x Mexico 54 and Wairimu x G10909
Treatment df SS Mean square Fpr
Wairimu x Mexico 54 5 2724.670 544.934 <0.001 Super-rosecoco x Mexico 54 5 3709.319 741.864 <0.001 Wairimu x G10909 5 2955.735 591.147 <0.001 df = degrees of freedom, SS = sum of squares, Fpr = F probability
Results of the mean separation using the Tukey’s studentized range test are presented in Table 4.4. For all the three crosses, the disease severity scores of P2, BC1P2 and F1 were not significantly different from each other, while the severity scores of P1, BC1P1 and F2, were significantly different from each other and from the severity scores of P2, BC1P2 and F1
generations.
For the cross Wairimu (S) x Mexico 54 (R) the susceptible parent, Wairimu (P1) had a disease severity score of 8.1. The resistant parent, Mexico 54 (P2), had a disease severity score of 2.0.
The F1 disease severity score was 2.2 and it was not significantly different from BC1P2 and P2. The F2 disease severity score was 4.0 and it was significantly different from P1, P2, F1, BC1P1 andBC1P2. The backcross to the susceptible parent, BC1P1, had a disease severity score of 6.8 and was significantly different and lower than that of its recurrent parent P1. The backcross to the resistant parent BC1P2 had a disease severity score of 2.2 and it was higher but not significantly different from the resistant parent P2.
For the cross Wairimu (S) x G10909 (R) the susceptible parent, Wairimu (P1), had a disease severity score of 7.9. The resistant parent, G10909 (P2), had a disease severity score of 1.9.
The F1 disease severity score was 2.1 and it was not significantly different from the BC1P2 and P2. The F2 disease severity score was 5.1 and it was significantly different from P1, P2, F1, BC1P1 andBC1P2. The backcross to the susceptible parent, BC1P1, had disease severity score of 6.9 and was significantly different and less than that of its recurrent parent P1. The backcross to the resistant parent BC1P2 had a disease severity score of 2.1 which was not significantly different from the resistant parent P2.
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For the cross Super-rosecoco (S) x Mexico 54 (R), the susceptible parent, Super-rosecoco (P1), had a disease severity score of 8.0. The resistant parent, Mexico 54 (P2), had a disease severity score of 1.7. The F1 disease severity score was 2.0 and it was not significantly different from the BC1P2 and P2. The F2 disease severity score was 5.6 and it was significantly different from P1, P2, F1, BC1P1 and BC1P2. The backcross to the susceptible parent, BC1P1, had a disease severity score of 7.1 and was significantly different and less than that of its recurrent parent P1. The backcross to the resistant parent BC1P2 had a disease severity score of 1.8 which was not significantly different from the resistant parent P2.
Table 4.4: Tukey’s studentized range test for comparison of angular leaf spot disease severity score means ± standard errors in three S x R crosses
Generation Wairimu(S) x Mexico 54 (R)
Wairimu (S) x G10909 (R)
Super-rosecoco (S) x Mexico 54 (R)
P1 8.1 ±0.11 A 7.9 ±0.12 A 8.1 ±0.12 A
BC1P1 6.8 ±0.17 B 6.9 ±0.15 B 7.1 ±0.17 B
F2 4.0 ±0.11 C 5.1 ±0.09 C 5.6 ±0.11 C
F1 2.2 ±0.13 D 2.1 ±0.12 D 2.0 ±0.13 D
BC1P2 2.2 ±0.07 D 2.7 ±0.07 D 1.8 ±0.08 D
P2 2.0 ±0.10 D 1.9 ±0.10 D 1.7 ±0.09 D
Means followed by the same letter for each cross are not significantly different at P<0.05. R and S = Resistant and susceptible, respectively.
The frequency distributions for the three crosses are presented in Figures 4.4, 4.5 and 4.6. For the cross Wairimu (S) x Mexico 54 (R), the frequency distribution for P1 (Wairimu) was skewed to the right (higher disease severity score), while that of P2 (Mexico 54) was skewed to the left (lower disease severity score). The distribution of the BC1P1 was skewed to the right, while that of BC1P2 was skewed to the left, similar to their recurrent parents. The F1 plants had disease severity scores that were similar to the resistant parent, hence skewed towards the P2. The F2 generation showed a more continuous distribution.
For the cross Wairimu (S) x G10909 (R) the frequency distribution for P1 (Wairimu) was skewed to the right (higher disease severity score), while that of P2 (G10909) was skewed to the left (lower disease severity score). The distribution of the BC1P1 was skewed to the right, while that of BC1P2 was skewed to the left, similar to their recurrent parents. The F1 plants had disease
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severity scores that were similar to the resistant parent hence skewed towards the P2. The F2 generation showed a more continuous distribution.
The cross Super-rosecoco (S) x Mexico 54 (R) had a frequency distribution for P1 (Super- rosecoco) that was skewed to the right (higher disease severity score), while that of P2 (Mexico 54) was skewed to the left (lower disease severity score). The distribution of the BC1P1 was skewed to the right, while that of BC1P2 was skewed to the left, similar to their recurrent parents.
The F1 plants had disease severity scores that were similar to the resistant parent hence skewed towards the P2. The F2 generation showed a more continuous distribution.
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ALS disease severity score (1-9 rating scale)
Figure 4.4: Frequency distribution of angular leaf spot disease severity scores for six generations of the cross Wairimu (S) x Mexico 54 (R)
Wairimu (P1) x Mexico 54 (P2)
F2
P2 BC1P2
P1 BC1P1
F1
0 60
40
0 20
4 8
0 40
2 4 6 8
20
2 60
40
60
6 20
No. of Plants
110
ALS disease severity score (1-9 rating scale)
Figure 4.5: Frequency distribution of angular leaf spot disease severity scores for six generations for the cross Wairimu (S) x G10909 (R)
Wairimu (P1) x G10909 (P2)
F2
P2
BC1P1 BC1P2
F1
P1
60
20
4 8
0 40 80
2 4 6 8
40
2 60
80
60
40
20
0 0
80
6 20
No. of PlantsNo. of Plants
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ALS disease severity score (1-9 rating scale)
Figure 4.6: Frequency distribution of angular leaf spot disease severity scores for six generations of the cross Super-rosecoco (S) x Mexico 54 (R)
Super-rosecoco (P1) x Mexico 54 (P2)
F2
P2
BC1P1 BC1P2
F1
P1
60
0
4 8
0 60 90
60
2 4 6 8
30
2 90
30
0
6 30
90
No. of Plants
112