Research on Papaya Decline*


 By Eric Dillingham
 Research Assistant
 C.V.I. Agricultural Experiment Station


 The papaya was first described by the Spanish chronicler
 Oviedo in 1526 on the Caribbean coast of Panama and Columbia.
 It was soon found growing throughout the tropics and became
 adapted to many tropical regions, particularly in areas with fertile
 soils and abundant rainfall.

 Papayas are herbaceous dicotyledonous plants which may
 produce fruits for more than 20 years. When cultivated, plants
 usually have a single trunk, but several bunches may develop as
 the plants become older. Trees grown under optimum conditions
 of fertility and rainfall may reach heights of 30 feet or more.
 The trunk has soft, pulpy wood and the leaves are deeply lobed
 with hollow soft petioles. The melon-like fruit varies considerably
 in size, shape, flavor and color and hangs close to the trunk. The
 papaya is very nutritious since the fruit contains high amounts of
 vitamin A, fair quantities of vitamin C, some riboflavin and niacin
 and is a good source of calcium, phosphorous and iron. Papain
 which is collected from the green fruit, is used in the manufacture
 of meat tenderizers.

 The papaya plant can exhibit three sex types characterized
 by their flowers which are either male, female or hermaphrodite.
 Though it is possible for a papaya plant to have more than one
 type of flower, usually they express only a single sex. There are
 tendencies for papaya plants to sometime change sex type due
 to unfavorable climate conditions such as drought or high tem-
 peratures. The papaya thrives best under warm conditions with
 abundant rainfall. It prefers well-drained fertile soils and cannot
 tolerate strong winds or flooding, since these conditions associated
 with the Virgin Islands, can reduce optimum growth and fruit
 yields.

 Since 1955 a disease known as 'papaya decline' has limited
 commercial production of certain desirable varieties such as the
 Solo. Originally from Barbados, Solo owes its consistency in
 character (sweet, uniform size and dark pink in color) to a high
 degree of natural self-pollination of its bisexual flowers. This
 continuous selection of pear-shaped fruits produced by bisexual
 plants, has maintained Solo relatively unchanged. Improved selec-
 tions, such as Sunrise Solo, have resulted from vigorous breeding
 work. Unfortunately, the Solo is not resistant to the St. Croix
 decline and production of this variety has been limited in the
 Virgin Islands for this reason.


 Since 1966, the St. Croix papaya decline has been under
constant study by the U.S. Department of Agriculture and the
College of the Virgin Islands Agricultural Experiment Station.
It was first necessary to isolate the actual cause of the decline and
determine if it is fungus, bacterium or virus. Many scientists have
described the symptom of the decline disease (the appearance of
water soaked lesions on the trunk called 'greasy spot') and the
detrimental effect it has on the papaya plant. They have isolated


*Papaya research is partially funded under Section 406, Tropical
and Subtropical Agriculture, US. Department of Agriculture.


fungi such as Phytophthora parasitica, Corynespora crassicola,
Fisarium sp. and Pythium sp. and hypothesized that one of these
fungi or a combination of them is causing the papaya plants to
decline.

 Currently papaya experiments at the experiment station are
seeking to find a feasible way to control the decline, with three
experiments in progress working toward possible control measures.
Since the decline seems to attack plants that are not healthy, one
experiment is to provide the best possible growing conditions.
The soil has been fumigated to destroy any existing soil diseases
and sulfur has been applied to help the natural soil nutrients
become more available to the plants. This experiment is currently
in progress.

 A second experiment consisting of 24 papaya varieties from
different parts of the world is underway at the experiment station.
These plants show promising results since some of them exhibit
natural resistance to the decline. After obtaining a resistant variety
we hope to cross-breed it with a more desirable variety and obtain
papaya that grows under the unique conditions of the Virgin
Islands and produces a desirable fresh fruit.

 The third experiment consists of Sunrise Solo papayas grown
with different chemical fungicides, insecticides and minor ele-
ments. It is thought that these chemicals which are used on other
crops might help to control the fungi and allow the Sunrise Solo
to be cultivated in the Virgin Islands.


Symptoms of papaya decline disease are lesions on the trunk
called 'greasy spot.'