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Resistance to Root Rot Caused by Phytophtora Palmivora Butl. In Carica Papaya L.: Screening. Heritability. And Assessment of Groth under Nursery and Field Conditions

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Title:Resistance to Root Rot Caused by Phytophtora Palmivora Butl. In Carica Papaya L.: Screening. Heritability. And Assessment of Groth under Nursery and Field Conditions
Authors:Mosqueda-Vasquez, Raul
Date Issued:1977
Abstract:Papaya production in Hawaii is limited by the "replant problem". These studies were initiated because root rot caused by Phytophthora palmivora Butl. is an important component of this problem. Root rot resistance has been identified in the field. It is known however, that breeding for resistance by planting continuously in infested fields has some drawbacks and has not been entirely effective to date.
A reliable method of uniform inoculation is needed to accelerate breeding for resistance. Information on inheritance of resistance is lacking. Papaya plants surviving artificial inoculations are stunted in growth and it is not known if they can reach the reproductive stage and yield progenies which could be selected to accumulate resistance.
A group of papaya lines was chosen which had shown resistance and susceptibility to root rot in previous studies.
An existing method of inoculation that makes use of small chambers where young cotyledonary stage seedlings are suspended in a zoospore suspension of the pathogen was tested in a series of experiments. The method was discarded because results failed to correlate well with field results.
A new method of screening for resistance was therefore developed using the P170 isolate of P. palmivora as inoculum. This method consisted of growing papaya seedlings in pots with vermiculite in a greenhouse for one month. A zoospore suspension of the desired concentration was poured into the pots. Mortality counts and disease rating of the seedlings were taken one month after inoculation. Survivors were then transplanted into the field.
By this inoculation procedure it was seen that: (1) developmental resistance in papaya begins to appear after the second week after germination; (2) in one week old seedlings of 'Higgins', a concentration as low as 200 sporangia/plant produced 50% mortality; (3) developmental resistance increased as seedlings became older and apparently was totally developed by the time seedlings were 2 months old; (4) comparisons among papaya lines inoculated with increasing concentrations of inoculum demonstrated that resistance exists; (5) papaya lines one month old, inoculated with a uniform concentration of inoculum, could be separated in three groups: resistant: Line 8, 'Waimanalo'-23, 'Waimanalo'-24 and Line 40; moderately resistant: 4 5-T22 and 'Kapoho'; and susceptible: 'Higgins'. The correlation between greenhouse and field mortality was 0.9355, which was highly significant and supports the reliability of the method.
A 5 x 5 half diallel crossing system was used to estimate heritability. All progenies were inoculated in the greenhouse by the described procedure. Although the two methods of statistical analyses used gave different values, both showed agreement on the presence of a highly significant additive genetic variation for root rot resistance in the population studied.
All inoculated papaya lines were severely defoliated following inoculation in the nursery, but significant differences between lines were observed in plant height and stem diameter. The growth of Line 8 was not affected by the pathogen.
When transplanted into an infested field, resistant lines 'Waimanalo'-23 and 'Waimanalo'-24 recovered in growth 3 months and 4 months respectively after transplanting. No yield data were obtained but it was observed that surviving seedlings were able to produce mature fruit from which seed could be obtained to grow progenies for further selection.
A breeding program to introduce resistance into the current papaya cultivar used in Hawaii is proposed. This would begin with backcrossing to incorporate resistance, followed by phenotypic recurrent selection to accumulate resistance while keeping variability. Elite resistant plants could then be selected out of variable progenies. These would then be inbred to produce homozygous lines for use as varieties.
Appears in Collections: Ph.D. - Horticulture

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