PAPAYA - Tropical Fruits - page 309

petiole hybrids have been produced but the yield of the commercial cultivars has
not been achieved.
A common papaya disease is the root, stem and fruit rot caused by
Phytophthora palmivora Butl., which is especially severe during wet seasons.
Early studies in Hawaii involved repeated selection for seedling mortality and
vigour, and replanting each advance generation in the same fi eld resulted in a
few highly tolerant lines (Mosqueda-Vazquez et al. , 1981). Cultivars showing
high tolerance to the Phytophthora root rot have also exhibited tolerance
to stem canker and fruit rot caused by the same organism. Other fruit
diseases that appear to be important enough to warrant breeding ef orts are
anthracnose and chocolate spot, both caused by strains of C. gloeosporioides ,
and stem-end rot caused by Phoma caricae-papayae .
The aphid-transmitted papaya ringspot virus (PRV) (papaya mosaic
virus or distortion ringspot virus) of dif erent strains often limits commercial
production in most papaya-growing areas. Tolerant lines have been created,
and a dioecious cultivar named 'Carifl ora' with strong tolerance to PRV
has been released (Conover et al. , 1986). Using this material, several highly
tolerant hermaphroditic 'Solo' selections have been made in Hawaii. More
rapid tolerance is achieved using molecular biology and the transformation of
papaya with the virus coat protein that confers resistance (Fitch et al. , 1992).
Two resistant 'Solo' lines, one with yellow fl esh ('Rainbow') and the other with
red fl esh ('SunUp'), have been released in Hawaii.
The hermaphroditic papaya is naturally self-pollinating, and continuous
inbreeding has not shown inbreeding depression. Also, F 1 hybrids between
'Solo' lines have not shown hybrid vigour, probably due to a close genetic
relationship, with many genes in common, or to the fact that the vigour is
only expressed under poor growing conditions. This narrowness in germplasm
has been confi rmed for ten Hawaii cultivars and three non-Hawaii cultivars
by DNA analysis at 80% similarity. Heterosis has been observed in F 1 s of
crosses between 'Solo' and widely dif erent papaya accessions and between
interspecifi c crosses involving V. caulifl ora × V. monoica and V. goudotiana × V.
monoica (Mekako and Nakasone, 1975; Manshardt and Wenslaf , 1989a,b).
Cultivars
Wide variability is shown by the papayas grown in various countries; with
few exceptions, most cannot be classifi ed as horticultural 'cultivars' (Table
11.4). Plantings are usually heterogeneous and seeds are obtained from open-
pollinated fruit. A number of horticultural cultivars have been reported to
produce relatively uniform progenies (Table 11.5). Stabilizing characteristics
in dioecious cultivars is more dii cult than in hermaphroditic ones, as the
genotype of the male with respect to the fruit characteristics is unknown. In
hermaphroditic cultivars, proper selection and self-pollination can stabilize