PRSV and ZYMV are members of the genus Potyvirus and family Potyviridae. Most potyviruses have limited geographical distributions, but PRSV and ZYMV infect cucurbits all around the world (Lecoq et al., 1998). Both viruses are easily transmitted in a nonpersistent manner via aphid feeding. Hosts for PRSV include commercial crops of Caricaceae and Cucurbitaceae (Tripathi et al., 2008), whereas ZYMV is generally limited to the latter. Olarte-Castillo et al. (2011) judged PRSV to be the most important virus disease on cucurbits in the tropics and subtropics. ZYMV was described first in Italy by Lisa et al. (1981); since then, it has been reported in all cucurbit growing areas. Desbiez and Lecoq (1997) list more than 50 locations in Europe, Africa, Asia, Oceana, and North America were ZYMV has been reported.
Viral infections of cucurbit crops were identified in Puerto Rico as early as the 1930s by Cook (1936) and later by Adsuar and Cruz Miret (1950). Surveys in 1981 to 1992 indicated a high incidence of viral diseases around commercial cucurbit farms in Puerto Rico (Escudero, 1992). ZYMV was confirmed in Puerto Rico in 1996 (Lecoq et al., 1998). In 2001 and 2002, a survey of cucurbit crops in Puerto Rico with virus symptoms showed 69% of all samples infected by ZYMV and 59% of samples infected with PRSV (Paz-Carrasco and Wessel-Beaver, 2002). Infection with PRSV and ZYMV appeared to lower yields, especially in summer squash (Cucurbita pepo L.) and tropical pumpkin, although no data are given to document this observation.
Tropical pumpkin (C. moschata) is grown worldwide in the lowland (below 3000 m) humid tropics. In Puerto Rico, where tropical pumpkin is consumed daily by many people, the crop has consistently been the second most important vegetable crop on the island in economic value (Departamento de Agricultura de Puerto Rico, 2015).
In response to the challenges of susceptibility of C. moschata to potyviruses, plant breeders have attempted to identify sources of resistance. Provvidenti et al. (1983), in reference to ZYMV resistance in Cucurbita, mentions that “a few sources of resistance have been found.” Munger and Provvidenti (1987) state that the 1983 publication referred to ‘Nigerian Local’. According to written correspondence between Provvidenti and L. Wessel-Beaver dated 24 Apr. 2000, seed of the original accession of ‘Nigerian Local’ was obtained from Dr. Igwegbe, University of Nigeria, Nsukka, Anabra State in 1983 (L. Wessel-Beaver, personal communication). Brown et al. (2003) concluded that resistance in ‘Nigerian Local’ to both PRSV and ZYMV was controlled by a single gene, but later studies found PRSV resistance (McPhail-Medina et al., 2012) and ZYMV resistance (Pachner et al., 2011) in ‘Nigerian Local’ to be multigenic. Costa (1974) reported the Brazilian genotype ‘Menina’ to be resistant to PRSV. Paris et al. (1988) also found ‘Menina’ to be resistant to ZYMV, although it is not clear if this is the same genotype used by Costa (1974) since Paris et al. (1988) reported the genotype to be from Portugal. Pachner et al. (2011) demonstrated that resistance to ZYMV in ‘Menina’ is conferred by one gene. This same study concluded that the Puerto Rican cultivar ‘Soler’ carries a recessive gene conferring moderate resistance to ZYMV.
Most studies of PRSV and ZYMV mention that these viruses have a negative impact on yield and other traits in cucurbit crops. However, a review of the literature yields few studies that document these impacts, especially in Cucurbita. Demski and Chalkley (1972) reported up to 43% yield loss in summer squash (C. pepo) inoculated with “watermelon mosaic virus” but it is unclear if this refers to what is now known as PRSV (=WMV-1) or WMV (a separate potyvirus). In New Zealand, Fletcher et al. (2000) inoculated C. maxima ‘Delica’ with ZYMV at an early stage of growth in the field and observed 48% yield loss along with a 62% reduction in number of fruits. Losses were less when inoculations occurred later in the season. Pacheco et al. (2003) observed that biomass was reduced up to 74% when measured 40 d after cotyledons of squash (C. pepo) were inoculated with a severe strain of PRSV compared with being inoculated with a mild strain of PRSV. ‘Menina’ (likely the same C. moschata cultivar used in our study) did not show any differences when inoculated with severe vs. mild strains of PRSV. They did not continue the study on field-grown plants. Kumar et al. (2008), observed up to a 97% reduction in yield and 75% reduction in fruit diameter when working in India with C. moschata inoculated with PRSV.
There are no reports in the literature where the three sources of resistance to PRSV and ZYMV, ‘Nigerian Local’, ‘Menina’, and ‘Soler’ (ZYMV resistance only), have been evaluated together in the field. Nor are there reports on how well greenhouse evaluations for potyvirus resistance are predictive of response in the field. Therefore, our objectives of this research were 1) to compare the development of virus symptoms due to PRSV and ZYMV in susceptible and resistant genotypes of tropical pumpkin from inoculation in the greenhouse to mature plants in the field, 2) to determine whether greenhouse evaluations of PRSV and ZYMV are predictive of a genotype’s expression of virus infection in the field, and 3) to document the effect of PRSV and ZYMV on flowering, yield, and fruit quality of tropical pumpkin.
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