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.
Printed in Brazil Germplasm of Cucurbitaceae in Brazil
The cucurbit species represent an important agribusiness in Brazil. Main species are Cucurbita moschata, C. maxima, Cucumis melo, C. sativus, Citrullus lanatus, and Sechium edule which were introduced by African slaves and Indians. More recently, European, American and Japanese immigrants introduced some improved genotypes which were spread across the country. There are some public and private breeding programs whose genetic variability is limited. In traditional agriculture however, there is a great genetic variability which gave base to the establishment of four germplasm banks that comprise more than 4700 accessions, involving over fifteen species of which only 25% have been evaluated. Several useful genes were identified. It is necessary to further the evaluation of the rescued genetic variability and to assess the need for new collections in order to have cucurbit germplasm for long-term conservation and for breeding programs in Brazil.
Systemic Infections for the Assay of Plant Viruses
The first infectivity assays of plant viruses relied on the percentage of inoculated plants becoming systemically infected (22, 31). The use of syste­ mic infections for assays became unfashionable, however, after the discovery of local lesions and the demonstration that their numbers depended on virus concentration (23) . Continued emphasis on local lesions over the years led to the widely held belief that systemic infections could not be used for a re­ liable and accurate assay of plant viruses. There was never any good statisti­ cal evidence for this viewpoint. I n fact, no thorough investigation has been published of the accuracy of "systemic assays" of plant viruses, as I shall term those assays based on the development of systemic symptoms in inoculated plants. McKinney (31 ) successfully used systemic assays in an early investiga­ tion of the properties of tobacco mosaic virus. He inoculated pla ' nts by pin pricks through a wad of cotton soaked in inoculum and wedged in a leaf axil. This was apparently an efficient inoculation procedure because it always gave 100% infection with an undiluted extract from infected to­ bacco, and usually gave 100% with a 1: 1000 dilution. McKinney made no attempt to analyze his results statistically to obtain an estimate of the virus concentration and of the error, but he did use dilution curves and recognized the principle of obtaining relative virus concentrations from dilutions giving identical results. Recalculation of his results shows that the percentage of infection decreased with dilution, as expected for a Poisson distribution. McKinney also stated the following seven necessary conditions for an in­ fectivity assay, conditions which still hold for both systemic and local lesion assays. 1. The virus must lend itself to study in expressed fluids without losing its potency in a short time or upon dilution. 2. The plant must be easily grown throughout the year. 3. The plant must be uniformly susceptible and the symptoms definite. 4. The plants for a single experiment must be