Tropical Pumpkin (Squash) Breeding
Tropical pumpkin or calabaza (Cucurbita moschata) is one of the most important vegetable crops in Puerto Rico, occupying second place in terms of the amount of revenue generated by vegetable crops. It is also consumed throughout the Caribbean and Latin America, as well in the US mainland where it is important in a growing hispanic market. Tropical pumpkin is also widely consumed in Asia, particularly in Thailand, India and the Phillipines. Currently U.S. production is limited to Puerto Rico and subtropical areas of Florida with occasional plantings in some of the northeastern states.
Despite its popularity and importance in the diet, tropical pumpkin is viewed as a minor food crop and has not received much research attention. The Agricultural Experiment of the University of Puerto Rico has maintained a small pumpkin research program for many years. The focus of the program has varied, depending on the source of funding. Much of our research has focused on screening for resistances to diseases and insects, including powdery and downy mildew, silverleaf, whitefly and melonworm. Another aspect of our research is modification of growth habit (selection for semi-bush types). The cultivar 'Soler' is a traditional vining type of tropical pumpkin. 'Taina Dorada' was selected for its semi-bush growth habit as well as its ouculinary traits (high levels of dry matter and soluable solids).
There appears to be ample market opportunities in Puerto Rico and the US mainland to expand production of tropical pumpkin. There is a need for innovative marketing of cucurbits and room for development of new cultivars of pumpkins and squash. New cultivars developed from an effective breeding program can add value for both the grower and consumer of tropical pumpkin.
Manual Pollination Technique
Hand pollinations will be most successful on plants where there are no fruit set. Remove any open-pollinated fruit before preparing a plant for pollination.
Figure 1. Pumpkins have both male (staminate) and female (pistillate) flowers. The photo below shows male flowers in various stages of maturity. Note that, unlike female flowers (Figure 3, below), the male flowers do not have an ovary (immature fruit) at the base. In the photograph below, the flowers immediately to the left of the open flower will open the next day. Notice how the flowers are swollen and have a slight yellowish color.
Figure 2. The male flower to be used the following day is covered with a waterproof bag to prevent contamination with foreign pollen. The number above the line identifies the female plant that will be pollinated with this male flower. The number below the line identifies the male parent.
Figure 3. Female flowers are easily recognized by the ovary at the base (in contrast to the male flowers in Figure 1, above). The day before the flower opens, it needs to be tied shut in order to prevent contamination by pollinizers (bees) before controlled pollination the next morning. Here a rubberband is used. Like the male flower in Figure 1, a female flower that will open the next day will be yellowish and swollen. Be careful not to place the rubber band too far down on the flower. Different colored flags are generally used to mark unopened buds, prepared pollinations, and completed pollinations.
Figure 4. Under Puerto Rico conditions, pollinating is done from sunrise until about 9:30AM. The male flower is pulled from the plant, and brought to the female flower. Both the male and female flowers are torn open.
Figure 5. Pollen from the male flower is spread over the stigma of the female flower.
Figure 6. The female flower is covered with the same bag previously used to cover the male flower. A tag describing the cross and the date is place on the stem just below the flower.
Figure 7. The bag can be removed within 1 or 2 days (the expanding fruit will tear open the bag). Set fruits double or triple in size within a few days, indicating the pollination was successful.
Mechanical Inoculation of Papaya Ringspot Virus (PRSV) and Zucchini Yellow Mosaic Virus (ZYMV)
Resistance to both of these potyviruses can be found in Cucurbita moschata 'Nigerian Local'. Resistance to PRSV is resessive while resistance to ZYMV is dominant. We are currently (2009) incorporating resistance to these two viruses into local cultivars 'Soler', 'Taína Dorada', and 'Verde Luz'.
A short video
of the mechanical inoculation technique that we use can be found on YouTube.
Pests and Diseases Affecting Pumpkin/Squash in Puerto Rico
Silvering or Silverleaf Disorder, is a physiological disorder of squash and pumpkins induced by the feeding of the silverleaf whitefly, Bemisia argentifolii Bellows and Perring. Resistance is controlled by a single recessive gene, sl (Gonzalez-Román and Wessel-Beaver, 2002). In the photograph below a silverleaf susceptible genotype is to the left, while a silverleaf resistant genotype is to the right. Note that the silverleaf resistant genotype exhibts the mottle-leaf phenotype (silver areas in the leaf vein axils) controlled by the dominant gene M.