Mechanism of inheritance of resistance to pythium root rot disease and traits for tolerance to low soil fertility in common bean
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Date
2015
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Sokoine University of Agriculture
Abstract
The bean root rot disease mainly Pythium spp. is a major problem affecting bean
production especially in the major bean producing areas of the Great lakes region
including southwestern Uganda, Rwanda, western Kenya, northern and the southern
highlands of Tanzania. The problem of Pythium bean root rot appears to be made
worse by declining soil fertility, resulting from intensive land cultivation by the
small holder farmers. Probably for a more sustainable farming system, breeding of
common bean genotypes tolerant to both the bean root rot disease and low soil
fertility problem would be a welcome improvement. This requires information on
the suitable resistant or tolerant germplasm and their nature of inheritance of the
resistance genes, which is currently not understood. This research was therefore
undertaken in order to: (a) investigate the inheritance of resistance to bean root rot
disease caused by Pythium spp. in two common bean genotypes RWR 1946 and
RWR 2075, and (b) identify the allelic relationship of the resistance genes in these
genotypes and RWR 719 (a previously characterized Pythium root rot resistance
source). In addition, these studies also aimed at: (c) identifying genotypes tolerant to
both Pythium root rot disease and low soil fertility (low P and Al toxicity), and (d)
determining early generation inheritance of selected low phosphorus tolerance-
related traits in common bean genotypes RWR 1946 and RWR 2075. Inheritance of
resistance to Pythium root rot investigated in the F 1, F 2 and backcross populations
revealed a single dominant gene that could fully express in several backgrounds and
was present at the same locus in the genotypes RWR 1946, RWR 2075 and RWR
719. On the other hand, phenotypic evaluation of the selected known low soil
fertility tolerant or susceptible genotypes to identify new sources of Pythium root rotiii
resistance revealed that the BILFA nursery is a potential source of Pythium root rot
resistance. Assessment of the leaf area, shoot and root dry weights, total root length,
lateral and basal roots production, shoot P concentration and P uptake under varying
phosphorus availability was performed on 13 common bean genotypes. Results
confirmed that genotypes RWR 1946 and RWR 2075 were tolerant to low soil
phosphorus availability and responsive to added phosphorus. Unfortunately, when
the same genotypes were evaluated under high aluminum saturation of up to 55.2%,
they were sensitive to aluminium toxicity. Parental genotypes RWR 1946, RWR
2075, K 132 and their F 1 s crosses were evaluated under low and high phosphorus
availability to determine early generation inheritance of low phosphorus tolerance
related traits. Results revealed that increased lateral and basal root production, total
root length and higher shoot growth as traits for low phosphorus tolerance were
heritable and were to a great extent likely be due to additive genes. The findings of
this study are important because genotypes with tolerance to both Pythium root rot
disease and low soil phosphorus constraints have been verified. Tolerance or
resistance to such two important stresses makes them very good breeding materials
since the problem of declining soil fertility is on the increase in the Great lakes
region and consequently with likely outbreaks of the bean root rot disease.
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Keywords
bean production, pythium root, bean root, bean disease, bean breeding