3.3 Results
3.3.2 Stability of genotypes for Alternaria blight severity, total storage root fresh mass and harvest
3.3.2.1 Stability for Alternaria blight reaction
The highest average yield gains of 61.2, 58.8 and 55.3% in response to fungicide treatment were recorded by MBR 536, Namusoga, and Magabali, respectively (Table 3.4). The lowest yield gains of 15.2, 20.4 and 22.6% were recorded by NKA318L, Araka Red and BND145L, respectively. With respect to Alternaria blight severity, treatment with fungicide resulted in variable reductions in severity among genotypes across seasons and sites (Table 3.3).
NASPOT 1 recorded the lowest percentage reduction in disease severity of 40.8% between the Alternaria inoculated and fungicide treated plants. Kigaire recorded the highest percentage disease reduction of 63.6%.
Correlations between Alternaria blight severity and TRY were calculated using the AUDPC values and yield of the Alternaria inoculated plants. Both the AUDPC values and the TRY were meaned over the genotypes and seasons. There were negative but non-significant correlations between Alternaria blight severity and mean TRY across the genotypes during seasons 2010B and 2011A at Namulonge and 2011A and 2011B at Kachwekano (Table 3.5).
3.3.2 Stability of genotypes for Alternaria blight severity, total storage root fresh mass
Table 3.5 Correlation between Alternaria blight severity scores (expressed as area under the disease progress curve values) of inoculated plants and yield meaned over genotypes and seasons
NAM1 -
NAM2 0.595 -
NAMY1 -0.206 0.003
NAMY2 -0.017 -0.054
KACY2 -0.029 0.047 -0.091 -0.092
KACY3 0.019 -0.090 -0.178 -0.128
NAM1 NAM2 KAC2 KAC3
NAM1 = area under disease progress curve (AUDPC) at Namulonge 2010B; NAM2 = AUDPC at Namulonge 2011A;
NAMY1 = Yield at Namulonge 2010B; NAMY2 = Yield at Namulonge 2011A; KAC2 = AUDPC at Kachwekano 2011A;
KAC3 = AUDPC at Kachwekano 2011B; KACY2 = Yield at Kachwekano 2011A; KACY3 = Yield at Kachwekano 2011B
The rank order of the performance of the genotypes changed across the six environments (Appendix 3.4). However, some genotypes were consistently ranked as resistant and others were consistently ranked as susceptible. A genotype with the highest AUDPC mean AMMI estimate was considered to be the most susceptible and was ranked last (30th) while the genotype with the lowest AUDPC was the most resistant and was ranked first. NASPOT 1 was the most susceptible genotype in four of the six environments and ranked second most susceptible in the other two environments. New Kawogo and MBR 536 were the most susceptible genotypes at Namulonge 2010B and Namulonge 2011A, respectively. NASPOT 7 was the second most susceptible genotype in four of the environments. Shock was the most resistant genotype in four of the environments and NASPOT 3 the most resistant in the other two environments. Kigaire exhibited consistency in resistance to the disease and was second most resistant in two environments and third most resistant in three of the environments.
In the AMMI biplot (Figure 3.2), susceptible genotypes were scattered in quadrants I and II while resistant genotypes were scattered in quadrants III and IV. Genotypes close to the horizontal line have low interaction with the environments and are therefore stable whereas the further away genotypes are from the horizontal line the more unstable they are. The most stable genotypes for Alternaria blight with above average mean AUDPC values and susceptibility were NASPOT 1, Sowola 6, NASPOT 4 and NASPOT 10 O. The most stable genotypes with below average mean values and thus resistant were Magabali, BND145L, NASPOT 8, Namusoga, Tanzania and NKA259L. Genotypes MBR 536, NASPOT 2, NKA318L, Malagalya and NASPOT 7 were the furthest away from the horizontal line and therefore the least stable for Alternaria
blight severity. BND145L and NASPOT 10 O were in opposite quadrants to each other thus their contributions to the interaction SS were in opposing directions.
Genotypes Bwanjule, NASPOT 11, NASPOT 3 were specifically adapted to environment Namulonge 2011B. Dimbuka, Araka Red, NASPOT 7 were relatively stable and adapted to environment Namulonge 2011B. NKA318L, NASPOT 2 and MBR 536 were relatively unstable with specific adaptation to Kachwekano 2010B and Kachwekano 2011B, respectively. New Kawogo was relatively unstable with above average AUDPC value with low interaction with Kachwekano 2010B and Kachwekano 2011A. None of the environments was stable for Alternaria blight; however, Namulonge 2011A, Namulonge 2011B, Kachwekano 2010B, Kachwekano 2011B were relatively more stable than Namulonge 2010B and Kachwekano 2011B.
Table 3.6 AMMI analysis for Alternaria blight severity, total storage root fresh mass and harvest index for 30 sweetpotato genotypes evaluated in six environments
AUDPC TRY HI
Source of variation df SS MS %Total SS
%GEI
SS SS MS %Total
SS
%GEI
SS SS MS %Total
SS
%GEI SS
Total 1079 994490 866 69244 64.2 39.03 0.0362
Treatments 179 433694 2428*** 43.6 37235 208.0*** 53.8 17.62 0.0984*** 45.1
Genotypes (G) 29 187073 6451*** 18.8 6434 221.9*** 9.3 4.65 0.1604*** 11.9
Environments (E) 5 80204 16041*** 8.1 15195 3039.1*** 21.9 6.65 1.3303*** 17.0
Interaction (G x E) 145 167417 1512*** 16.8 15605 107.6*** 22.5 6.31 0.0435*** 16.2
IPCA1 33 79163 2399*** 47.3 7874 238.6*** 50.5 3.26 0.0989*** 51.7
IPCA2 31 50614 1633*** 30.2 3522 113.6*** 22.6 1.42 0.0458*** 22.5
Residuals 81 37639 463 22.5 4209 52.0 27.0 1.63 0.0201 25.8
Error 887 481654 543 26834 30.2 17.74 0.0201
*** = significant at P<0.0001; AUDPC = area under disease progress curve for Alternaria blight severity; TRY = total storage root fresh mass (t ha-1); HI = harvest index; df = degrees of freedom; SS = sum of squares; MS = mean square; %Total SS = percentage of total sum of squares; %GEI SS = percentage of genotype x environment interaction sum of squares; IPCA = interaction principal component analysis
Table 3.7 AMMI analysis for percentage dry mass composition, weevil damage and sweetpotato virus disease severity for 30 genotypes evaluated in six environments
Source of variation DM% Weevil damage SPVD
Source df SS MS %Total
SS
%GEI
SS SS MS %Total
SS
%GEI
SS SS MS %Total
SS
%GEI SS
Total 1079 153562 142.3 1654.8 1.53 2884.3 2.67
Treatments 179 29014 162.1 18.9 1238.8 6.92*** 74.9 2087.0 11.66*** 72.4
Genotypes (G) 29 6644 229.1* 4.3 76.7 2.64*** 4.6 242.3 8.36*** 8.4
Environments (E) 5 1001 200.1 0.7 977.7 195.54*** 59.1 1455.6 485.19*** 50.5
Interaction (G x E) 145 21369 147.4 13.9 184.4 1.27*** 11.1 389.1 4.47*** 13.5
IPCA1 33 15212 461.0*** 71.2 83.8 2.54*** 45.4 249.4 7.56*** 64.0
IPCA2 31 4244 136.9 19.9 57.4 1.85*** 31.1 101.5 3.27*** 26..0
Residuals 81 1913 23.6 9.0 43.2 0.53 23.4 38.1 1.66
Error 887 118702 136.8 389.4 0.48 623.2 1.08
*** = significant at P<0.0001; * = significant at P<0.05; DM% = percentage dry mass composition; SPVD = sweetpotato virus disease; df = degrees of freedom; SS = sum of squares; MS = mean square; %Total SS = percentage of total sum of squares; %GEI SS = percentage of genotype x environment interaction sum of squares; IPCA = interaction principal component analysis
Figure 3.2 Biplot of mean area under disease progress curve (AUDPC) for Alternaria blight severity and the first interaction principal component axis (IPCA1) scores for 30 sweetpotato genotypes evaluated in six environments
Key
Genotypes
TANZA = Tanzania; NAMU = Namusoga; SILKL = Silk Luwero; SEMA = Semanda; NP2 = NASPOT 2; SOW6 = Sowola 6; NK = New Kawogo; NP1 = NASPOT1; NP4 = NASPOT4; NP10 = NASPOT 10 O; MAGA = Magabali; NP8 = NASPOT 8; KIG = Kigaire; BWANJU = Bwanjule; NP11 = NASPOT 11; NP3 = NASPOT3; MALAGA = Malagalya; AR = Araka Red; NP7 = NASPOT7; DIM = Dimbuka
o
Environments
NAM1 = Namulonge 2010B; NAM2 = Namulonge 2011A; NAM3 = Namulonge 2011B; KAZ1 = Kachwekano 2010B;
KAZ2 = Kachwekano 2011A; and KAZ3 = Kachwekano 2011B
The biplot provides a useful diagrammatic overview of the interaction patterns of the genotypes and environments and their relative stability levels. However, for ranking purposes the AMMI model does not provide an integrated measure of stability based on scores for the first two important IPCAs. To rank the genotypes more holistically in terms of stability and performance the ASV and GSI for each genotype were calculated.
The ASV ranked NASPOT 1, Namusoga and NASPOT 8 with values of 0.63, 0.75 and 0.81 as the most stable and MBR 536, NASPOT 11 and Malagalya with values of 11.09, 5.29 and 5.14 as the least stable for Alternaria blight (Table 3.8). The GSI ranked Tanzania and Namusoga as the best genotypes combining stability and resistance to Alternaria blight.
Table 3.8 Mean stability rankings of 30 sweetpotato genotypes for Alternaria blight severity (expressed as AUDPC values) for ASV and GSI indices across six environments meaned for spray treatments
Genotype Mean AUDPC Rank ASV Rank GSI Rank
Araka Red 98.9 24 1.91 9 35 19
BND145L 87.2 11 2.10 10 24 8
Bwanjule 72.3 4 2.86 16 20 6
Dimbuka 106.7 28 4.07 22 49 28
Ejumula 87.0 10 3.01 18 33 17
Kigaire 71.7 3 3.18 19 21 7
Magabali 89.0 16 1.25 7 25 10
Malagalya 86.3 9 5.14 28 47 27
Mbl 170 88.1 13 2.39 12 19 4
MBR 536 92.8 19 11.09 30 36 21
Namusoga 82.0 8 0.75 2 11 2
New Kawogo 114.3 29 3.56 21 50 29
NKA103M 87.6 12 2.91 17 30 14
NKA259L 89.5 17 2.46 13 29 12
NKA318 94.5 20 4.63 25 42 25
NASPOT 1 129.2 30 0.63 1 31 16
NASPOT 10 O 95.1 21 0.93 4 27 11
NASPOT 11 75.8 5 5.29 29 37 23
NASPOT 2 97.7 22 4.34 23 43 26
NASPOT 3 80.8 7 4.37 24 36 22
NASPOT 4 105.3 27 1.15 6 33 18
NASPOT 7 105.2 26 5.11 27 52 30
NASPOT 8 90.7 18 0.81 3 24 9
OTADA 88.3 14 1.76 8 18 3
Semanda 89.0 15 3.54 20 30 15
Shock 69.1 1 4.68 26 29 13
Sowola 6 101.3 25 2.16 11 35 20
SPK004 98.9 23 2.79 15 37 24
Tanzania 71.2 2 1.14 5 6 1
Silk Luwero 79.3 6 2.76 14 19 5
Mean 90.8
ASV = AMMI stability value, GSI = Genotype selection index
The environments were also ranked by the ASV and the GSI. The ASV ranked Namulonge 2011B as the most stable environment for Alternaria blight and Namulonge 2010B as the least stable. The GSI ranked Namulonge 2011A and Kachwekano 2010B as the most stable with low disease pressure and Kachwekano 2011A and Kachwekano 2011B as the least stable with high disease pressure (Table 3.9).
Table 3.9 Mean stability ranking of the six test environments for Alternaria blight severity
Environment Mean AUDPC Rank ASV Rank GSI Rank
Kachwekano 1 91.04 3 6.0647 2 4 1
Kachwekano 2 95.61 4 10.1770 5 9 5
Kachwekano 3 99.95 6 10.5150 4 10 6
Namulonge 1 83.88 2 11.9200 6 8 4
Namulonge 2 75.72 1 6.3888 3 4 1
Namulonge 3 98.76 5 4.1676 1 6 3
ASV = AMMI stability value; smallest ASV is the most stable and given rank 1; largest ASV is the most unstable and given rank 6; Kachwekano1 = 2010B; Kachwekano 2 = 2011A; Kachwekano 3 = 2011B; Namulonge 1 = 2010B;
Namulonge 2 = 2011A; Namulonge 3 = 2011B