Papaya Ringspot Virus-watermelon Strain Research Articles

Incidence of pests and arthropod natural enemies in zucchini grown with living mulches

The use of living mulches to reduce the density of silverleaf whitefly, Bemisia argentifolii Bellows and Perring, and melon aphid, Aphis gossypii Glover, and the conveyances of aphid-transmitted virus, papaya ringspot virus-watermelon strain (PRSV-W), and squash silverleaf disorder on zucchini, Cucurbita pepo L. was evaluated during 1995 and 1996 in Oahu, Hawaii. Living mulches used were buckwheat ( Fagopyrum esculentum Moench) and yellow mustard ( Sinapis alba L.). Mean densities of aphids were significantly lower in zucchini grown with living mulch both years of the study. In contrast, whiteflies were lower only during the second year. The severity of squash silverleaf disorder was significantly higher in bareground zucchini compared with living mulch-diversified zucchini during both experiments. The percentage of zucchini plants displaying PRSV-W during the early season was significantly lower on zucchini grown in living mulches during both years of the study. The delay in PRSV-W in living mulch resulted in a higher percentage of fruits by weight displaying viral symptoms in bareground zucchini. Reductions in pest activities in living mulch-diversified zucchini resulted in higher marketable yields only during one year of the study.

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.

Melon PI 414723 Is Resistant to Papaya Ringspot Virus Watermelon Strain

PI 414723 has received much attention from melon (Cucumis melo L.) breeders, pathologists, and entomologists for resistances to zucchini yellow mosaic and watermelon mosaic viruses, including resistances to virus multiplication and subsequent transmission by the melon aphid, powdery mildew resistance, and melon aphid (Aphis gossypii Glover). PI 414723 was derived from PI 371795, which was a contaminant in cucumber (Cucumis sativus L.) PI 175111 collected in 1948 by Walter N. Koelz in Mussoorie, Uttar Pradesh, India (altitude 1829 m). Its fruit, which have soft flesh and rind that split at maturity, are used in soups and stews, and the seeds are roasted and eaten. PI 414723, PI 371795, and the related Ames 20219 and progeny 92528a were resistant to California and Florida isolates of papaya ringspot virus watermelon strain (PRSV-W). Plants were either symptomless, or they exhibited local lesions, systemic necrosis, or systemic spots. Resistance to PRSV-W is conditioned by a single dominant gene. Allelism with Prv1 (PI 180280, Rajkot, Gujarat, India), Prv2 (PI 180283, Bhavnagar, Gujarat, India), Nm (`Vedrantais, Fance), and a recently described gene for PRSV-W resistance in PI 124112 (Calcutta, India) is yet to be determined.

Occurrence of Mosaic Viruses in Melons in the Central Valley of California

Three melon-growing regions in the Central Valley of California were surveyed for the incidence and severity of watermelon mosaic virus 2 (WMV2), zucchini yellow mosaic virus (ZYMV), cucumber mosaic virus (CMV), and papaya ringspot virus-watermelon strain (PRSV-W), during 1988 and 1989. WMV2 was the most prevalent virus in all three regions (Yolo/Sutter/Yuba counties, Stanislaus County, and Merced/Fresno counties), both in the number of sites with infected plants and in the proportion of symptomatic plants within each site. CMV and ZYMV were detected in fewer sites and generally infected 20% or fewer symptomatic plants within a site. In Stanislaus County, ZYMV, which had not previously been reported in the Central Valley, was detected in a higher number of sites and a higher proportion of plants per site in 1989 than in 1988. The increased incidence of ZYMV is of concern because this virus is severely pathogenic. PRSV-W was detected in low levels in the growing areas of Stanislaus and Merced/Fresno counties. This study suggests that management strategies for dealing with virus diseases in the agriculturally diverse Central Valley will be specific to each particular region.

149 DIFFERENTIAL REACTIONS OF MELONS TO ISOLATES OF PAPAYA RINGSPOT VIRUS WATERMELON STRAIN

In 1963, melon (Cucumis melo L.) plant introductions (PI) 124112 and PI 180280 were reported variable in response to inoculation with the T-1 or Freitag's isolates of watermelon mosaic virus. Most plants were symptomless, but some had small pinpoint necrotic lesions on cotyledons and leaves. The T-1 and Freitag's isolates of watermelon mosaic were later designated watermelon mosaic virus 1, and more recently renamed papaya ringspot virus watermelon strain (PRSV-W). When inoculated with California or Florida isolates of PRSV-W in 1993, WMR 29 a western U.S. shipping type melon derived from PI 180280 was symptomless (incompatible reaction) and SDS-immunodiffusion assays were negative. In contrast when inoculated with the same PRSV-W isolates, PI 124112 had incompatible reactions characterized by wilting, local lesions, systemic necrotic spots and necrosis and negative SDS-immunodiffusion assays.

Nature of resistance in Cucumis melo cvs to papaya ringspot virus type W

Comparative studies were made of the reactions of a 'resistant' (Cinco), a susceptible (Planters Jumbo) and a Cinco x Planters Jumbo (Cinbo) cultivar of muskmelon to papaya ringspot virus-watermelon strain (PRSV-W). Neither Cinbo nor Cinco supported levels of virus that were detectable by biotin-ELISA, but PRSV-W was detected in individual cells of the three cultivars. In inoculated leaves of Planters Jumbo most cells were infected, but in both Cinbo and Cinco only 3-7% of cells were infected. When cotyledons of Cinbo were inoculated, mild systemic symptoms were observed, but plants remained symptomless when true leaves were inoculated. Virus was recovered from both cotyledon - inoculated Cinbo grown at 25 or 40�C and true leaf-inoculated Cinbo, but only when the latter were grown at 25�C. There was no difference in the percentage of infected cells in these leaves despite the differences in symptom expression. Cinco was highly resistant and only occasionally showed mild systemic symptoms on cotyledon-inoculated plants grown at 25�C. Unlike those of Cinbo, leaves with symptoms were not infective, but there was no difference in the percentage of infected cells in these leaves compared to those in Cinbo. For both Cinbo and Cinco, several lines of evidence are presented that suggest PRSV-W is restricted to a few cells throughout the plant.