Inheritance of resistance to the papaya ringspot virus-watermelon strain from two different accessions of winter squash Cucurbita maxima Duch.
Inheritance of resistance to the papaya ringspot virus-watermelon strain (= PRSV-W, syn. watermelon mosaic virus-1 = WMV-1) was studied for two Cucurbita maxima accessions – ABL-010 and Redlands Trailblazer – considered to be resistant to the potyvirus. ABL-010 (ABL) derives its resistance from Brazilian accessions of C. maxima. Redlands Trailblazer (RT) is an Australian cultivar of C. maxima, the resistance of which traces back to C. ecuadorensis. In both cases, virus could be recovered from the inoculated (symptomless) plants via back inoculation onto susceptible C. pepo cultivar Caserta, indicating a tolerance type of resistance. Both ABL and RT were crossed to each other and to the standard PRSV-W-susceptible cultivar Buttercup (B). For each one of the crosses (ABL ×> B), (ABL × RT) and (RT × B), parental lines, F1, F2 and backcrosses to both the parents (BC1P1 and BC1P2) were tested for PRSV-W resistance after mechanical inoculation. Seedlings were inoculated at the first true leaf stage with a known PRSV-W isolate in 0.02 M phosphate buffer (pH 7.0) plus sodium sulphite 0.01%. Plants were scored for virus symptoms 28 days after inoculation; symptom severity being rated with a scale from 0 (= no symptoms) to 5 (= plants with severe mosaic and leaf distortions). Broad and narrow-sense heritabilities of PRSV-W resistance were 0.44/0.36, 0.59/0.59 and 0.58/0.58 for crosses (ABL × B), (ABL × RT) and (RT × B), respectively. Resistance imparted by ABL in cross with susceptible B could be explained by the action of three genes with partial dominance. Resistance imparted by RT in cross with B was found to be under control of at least two genes with additive effects. Susceptible plants were found in segregating populations of the cross ABL × RT (transgressive segregation), indicating that at least one of the loci involved in the control of resistance in ABL was not allelic to their counterpart loci in RT. The resistance imparted by RT appears to be of higher level than that imparted by ABL.
Inheritance of resistance to papaya ringspot virus type W in hybrids between Cucurbita ecuadorensis and C. maxima
Papaya ringspot virus type W (PRV-W) causes foliar mosaic, deformed fruit and reduced yield in crops of Cucurbita maxima cv. Queensland Blue in Queensland. Cucurbita ecuadorensis is highly resistant to PRV-W. Using crosses between these 2 cucurbits we studied inheritance of resistance to PRV-W in mechanically inoculated parental, F1, F2, backcross and inbred backcross populations under glasshouse and/or field conditions. Disease severity was assessed on each plant as the proportion of the young leaf area which was chlorotic. Inheritance of resistance, following inoculation of cotyledons with either of 2 isolates that differed in the severity of symptoms produced, was most consistently described as polygenic. Analysis of means of generations indicated additive gene effects were predominant. However simpler inheritance could not be entirely discarded as a 13:3 digenic model based on epistasis and modifiers described some distributions. No strong associations existed between high resistance to PRV-W and undesirable characteristics so that transfer of high levels of resistance from C. ecuadorensis to commercially acceptable lines of C. maxima should be possible.
Plant Virology: The Principles
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