Abstract
Resistance to witches' broom disease (WB) in cocoa, caused by Moniliophthora perniciosa, was investigated in 56 cross families. Twenty-four micro-grafted clones (16 parental and 8 control or reference clones) were also inoculated, including 16 parental clones representing various genetic groups. The clones and seedlings were tested for resistance to the disease using an optimised agar-droplet inoculation method in screen house experiments with plants of one-year-old, and by inoculating one bud per plant. For clones, three replicates of three plants each were used and for seedlings two replicates of 25 seedlings). Resistance components recorded on an individual plant basis were infection frequency (IF = % of swellings or brooms), broom frequency (BF, %), incubation period (IP, days to express first symptoms) and symptom severity (expressed as broombase diameter or BBD in mm). As expected from earlier work with the same type of agar-droplet inoculation method, average IF and BF were high. This allowed for better distinction among other post-penetration resistance parameters. Seedling BF was approximately 100%, whereas this parameter varied widely for clones (22 to 100%). Significant differences were observed among the clones and seedling progenies both for IP and BBD. For the clones, BBD was negatively correlated with IP (r = - 0.68) and positively correlated with IF (r = 0.62). IF and IP were not significantly correlated (r = - 0.22). For 14 clones tested in the screen houses, it was possible to compare the resistance components with field results (% vegetative infection). BBD was correlated with field levels of resistance (r = 0.71), whereas the correlation between IF and field resistance was lower (r = 0.42), though still significant. IP was not correlated with field results (r = - 0.16). These results clearly show that BBD is an important trait to be used in selection for WB resistance. There was a strong linear relationship between mid parental values and progeny means for BBD (r2 = 0.85) and for IP (r2 = 0.78), indicating strong heritability and additive effects. The strong additive effects were also confirmed for the combined NCII design analyses, whereas the effect of dominance was not significant. The variation for IP and BBD within cross progenies was continuous and normally distributed, including for the progenies of SCA 6, suggesting that resistance in vegetative tissue to WB is polygenic in nature. The presence of transgressive segregation within many seedling families augurs well for the possibility to accumulate genes for resistance by individual plant selection within crosses among resistant parents, (50% of the parental clones had been chosen because of their known levels of resistance to WB). It is recognised that variations in host, pathogen and environmental factors all may variously affect the repeatability of resistance testing. Precautions to be taken when using the agar-drop method in both seedlings and in clones are outlined. The implications of the findings on breeding of cocoa aiming at combining good yield with high levels of resistance to witches' broom are discussed.
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Claims 19
Claim · model-written summary
Clones and seedling progenies differed significantly in incubation period and broombase diameter.
- Taxon
- cocoa
Theobroma cacaoe0 - Material
- clones and seedling progeniese1
- Outcome
- IP and BBDe1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Resistance to witches' broom disease (WB) in cocoa”
e1“Significant differences were observed among the clones and seedling progenies both for IP and BBD”
Claim · model-written summary
Average infection frequency and broom frequency were high in the agar-droplet test.
- Taxon
- cocoaTheobroma cacaoe0
- Outcome
- average IF and BFe1
- Qualifier
- were highe1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Resistance to witches' broom disease (WB) in cocoa”
e1“average IF and BF were high”
Claim · model-written summary
Seedling broom frequency was approximately 100%.
- Taxon
- cocoaTheobroma cacaoe0
- Outcome
- Seedling BFe1
- Value
- approximately 100%e1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Resistance to witches' broom disease (WB) in cocoa”
e1“Seedling BF was approximately 100%”
Claim · model-written summary
Broom frequency varied widely among clones, from 22 to 100%.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Resistance to witches' broom disease (WB) in cocoa”
e1“Seedling BF was approximately 100%, whereas this parameter varied widely for clones (22 to 100%)”
Claim · model-written summary
Broombase diameter was negatively correlated with incubation period among clones (r = -0.68).
- Taxon
- cocoaTheobroma cacaoe0
- Material
- clonese1
- Outcome
- BBDe1
- Direction
- negatively correlatede1
- Value
- r = - 0.68e1
- Qualifier
- with IPe1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Resistance to witches' broom disease (WB) in cocoa”
e1“For the clones, BBD was negatively correlated with IP (r = - 0.68) and positively correlated with IF (r = 0.62)”
Claim · model-written summary
Broombase diameter was positively correlated with infection frequency among clones (r = 0.62).
- Taxon
- cocoaTheobroma cacaoe0
- Material
- clonese1
- Outcome
- BBDe1
- Direction
- positively correlatede1
- Value
- r = 0.62e1
- Qualifier
- with IFe1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Resistance to witches' broom disease (WB) in cocoa”
e1“For the clones, BBD was negatively correlated with IP (r = - 0.68) and positively correlated with IF (r = 0.62)”
Claim · model-written summary
Infection frequency and incubation period were not significantly correlated (r = -0.22).
- Taxon
- cocoaTheobroma cacaoe0
- Material
- clonese1
- Outcome
- IF and IPe2
- Direction
- not significantly correlatede2
- Value
- r = - 0.22e2
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Resistance to witches' broom disease (WB) in cocoa”
e1“For the clones, BBD was negatively correlated with IP (r = - 0.68) and positively correlated with IF (r = 0.62)”
e2“IF and IP were not significantly correlated (r = - 0.22)”
Claim · model-written summary
Broombase diameter was correlated with field levels of resistance (r = 0.71) for 14 clones in screen houses.
- Taxon
- cocoaTheobroma cacaoe0
- Material
- 14 clonese1
- Outcome
- BBDe2
- Value
- r = 0.71e2
- Qualifier
- with field levels of resistancee2
- Setting
- screen housese1
Not reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Resistance to witches' broom disease (WB) in cocoa”
e1“For 14 clones tested in the screen houses”
e2“BBD was correlated with field levels of resistance (r = 0.71)”
Claim · model-written summary
The correlation between infection frequency and field resistance was lower (r = 0.42) but still significant.
- Taxon
- cocoaTheobroma cacaoe0
- Material
- 14 clonese1
- Outcome
- correlation between IF and field resistancee2
- Direction
- lowere2
- Value
- r = 0.42e2
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Resistance to witches' broom disease (WB) in cocoa”
e1“For 14 clones tested in the screen houses”
e2“the correlation between IF and field resistance was lower (r = 0.42), though still significant”
Claim · model-written summary
Incubation period was not correlated with field results (r = -0.16) for 14 clones.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Resistance to witches' broom disease (WB) in cocoa”
e1“For 14 clones tested in the screen houses”
e2“IP was not correlated with field results (r = - 0.16)”
Claim · model-written summary
Mid-parental values showed a strong linear relationship with progeny means for broombase diameter (r2 = 0.85).
- Taxon
- cocoaTheobroma cacaoe0
- Outcome
- progeny means for BBDe1
- Value
- r2 = 0.85e1
- Qualifier
- indicating strong heritability and additive effectse1
- Comparator
- mid parental valuese1
Not reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Resistance to witches' broom disease (WB) in cocoa”
e1“There was a strong linear relationship between mid parental values and progeny means for BBD (r2 = 0.85) and for IP (r2 = 0.78), indicating strong heritability and additive effects”
Claim · model-written summary
Mid-parental values showed a strong linear relationship with progeny means for incubation period (r2 = 0.78).
- Taxon
- cocoaTheobroma cacaoe0
- Outcome
- IPe1
- Value
- r2 = 0.78e1
- Qualifier
- indicating strong heritability and additive effectse1
- Comparator
- mid parental valuese1
Not reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Resistance to witches' broom disease (WB) in cocoa”
e1“There was a strong linear relationship between mid parental values and progeny means for BBD (r2 = 0.85) and for IP (r2 = 0.78), indicating strong heritability and additive effects”
Claim · model-written summary
Additive effects were confirmed in the combined NCII design analyses.
- Taxon
- cocoaTheobroma cacaoe0
- Outcome
- additive effectse1
- Qualifier
- combined NCII design analysese1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Resistance to witches' broom disease (WB) in cocoa”
e1“The strong additive effects were also confirmed for the combined NCII design analyses, whereas the effect of dominance was not significant”
Claim · model-written summary
The effect of dominance was not significant in the combined NCII design analyses.
- Taxon
- cocoaTheobroma cacaoe0
- Outcome
- effect of dominancee1
- Qualifier
- combined NCII design analysese1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Resistance to witches' broom disease (WB) in cocoa”
e1“The strong additive effects were also confirmed for the combined NCII design analyses, whereas the effect of dominance was not significant”
Claim · model-written summary
Variation for IP and BBD within cross progenies was continuous and normally distributed.
- Taxon
- cocoaTheobroma cacaoe0
- Material
- cross progeniese1
- Outcome
- variation for IP and BBDe1
- Qualifier
- continuous and normally distributede1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Resistance to witches' broom disease (WB) in cocoa”
e1“The variation for IP and BBD within cross progenies was continuous and normally distributed, including for the progenies of SCA 6”
Claim · model-written summary
Resistance in vegetative tissue to witches' broom is polygenic in nature.
- Taxon
- cocoaTheobroma cacaoe0
- Outcome
- resistance in vegetative tissue to WBe1
- Qualifier
- polygenic in naturee1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Resistance to witches' broom disease (WB) in cocoa”
e1“suggesting that resistance in vegetative tissue to WB is polygenic in nature”
Claim · model-written summary
Transgressive segregation is present within many seedling families.
- Taxon
- cocoaTheobroma cacaoe0
- Material
- seedling familiese1
- Outcome
- transgressive segregatione1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Resistance to witches' broom disease (WB) in cocoa”
e1“The presence of transgressive segregation within many seedling families augurs well for the possibility to accumulate genes for resistance by individual plant selection within crosses among resistant parents”
Claim · model-written summary
Individual plant selection within crosses among resistant parents is suggested to accumulate resistance genes.
- Taxon
- cocoaTheobroma cacaoe0
- Outcome
- individual plant selection within crosses among resistant parentse1
- Qualifier
- to accumulate genes for resistancee1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Resistance to witches' broom disease (WB) in cocoa”
e1“The presence of transgressive segregation within many seedling families augurs well for the possibility to accumulate genes for resistance by individual plant selection within crosses among resistant parents”
Claim · model-written summary
Broombase diameter is an important trait for selection for witches' broom resistance.
- Taxon
- cocoaTheobroma cacaoe0
- Outcome
- selection for WB resistancee1
- Qualifier
- BBD is an important traite1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Resistance to witches' broom disease (WB) in cocoa”
e1“These results clearly show that BBD is an important trait to be used in selection for WB resistance”