One of the major aims of the Barbados Society of Technologists in Agriculture (BSTA) is discussion of matters concerning agriculture, disseminating information to its members and offering advice to the Government and other interested agencies. In carrying out its mission, the BSTA often organises â€œThink Tanksâ€ which bring together individuals who are considered knowledgeable on a specific topic to discuss the topic, find solutions to problems and give recommendations.
One such Think Tank was held on Viruses in November 2007 in response to the growing problem of disease in papayas. Acreage of the crop had diminished substantially in recent years and the supply of the local fruit was so low that imports had to be made from Trinidad. The Think Tank group represented the areas of entomology, pathology, agronomy, breeding and management.
Researchers in the 1980s had identified Bunchy Top as the major disease of papaya in Barbados. The first symptom is a faint mottling of the upper leaves which progressively become more chlorotic, especially in interveinal areas. Leaves and petioles show reduced growth, and become rigid. Oily spots are often present in the upper parts of stems and on petioles. Apical growth ultimately ceases, which, with the shortening of internodes, imparts a “bunchy top” appearance to affected plants. The oldest leaves progressively abscise, leaving fewer, more stunted leaves at the top. Eventually, plants may be killed. If fruits are set on infected plants, they may be tasteless and unmarketable.
Early research indicated that the problem was caused by a virus, but later work indicated that the causal agent was a mycoplasma-like organism (MLO) transmitted by the papaya leafhopper (Empoasca papayae) and possibly also E. dilitaria .In Barbados, farmers were advised to adopt an avoidance strategy . The crop was grown, sprayed with insecticides in an effort to control the insect vectors and harvested for a few months before being destroyed and replaced by a follower crop. This worked well for a number of years, but around 2004, a large papaya producer noted an upsurge in â€œBunchy Topâ€ like symptoms in spite of the fact that the plants appeared free of insects. The result was a 90% loss in plants and a non-economic yield This was the worst outbreak of the disease observed at the plantation for over 7 years. The disease subsequently occurred in a second field planted slightly later, in spite of weekly spraying with various alternated combinations of insecticides and fungicides.
The Ministry of Agriculture Entomologist was consulted and agreed that although the disease was prevalent, the plants were free of insects. Another scientist suggested that the insects may have been transmitting the disease, then moving on to nearby weeds. The recommendation was treatment with Bioneem which would repel the insects before they transmitted the disease.
There was a dramatic positive response to the treatment. The spread of the disease in the fruiting field was halted and the overall losses in this field were in the vicinity of 10%. However, the response was not sustained in later planted fields, and severe losses continued to be encountered. It appeared therfpre that the problem was caused by a combination of factors.
No positive identification of the disease organism was made and no research had apparently been carried out locally on this problem for many years.
A literature search was done to collect relevant information on the topic and contact was made by BSTA with a researcher in Florida who indicated that the causal agent of the problem is a Rickettsia type bacterium. Researchers at UWI noted that roots of many papaya plants with Bunchy Top had nematode galls and also that plants with a low colonisation by beneficial soil fungi showed a greater degree of disease symptoms.
Those present at the Think Tank studied this information and recommended that Symbex and compost be added to soils to enhance the beneficial soil organisms, that soil be treated with Neemex which is reported to control nematodes without harming beneficial soil organisms, and that more careful attention be paid to other agronomic practices including boron, zinc and calcium application. In addition to application of the insect repellent Bioneem, the application of Biocide, a bactericide, was also introduced. These recommendations have been followed by at least two large farmers and we at BSTA are pleased to note that supply of papaya fruit is once more meeting demand, and importation is no longer necessary.
However, Phytophthora, a disease which flourishes in hot, wet weather and causes root rot and die back of crops, is affecting some papaya fields, mostly in the red soils of the island. The application of Vigor Cal Phos has been recommended to control this problem. Farmers must also apply chemicals like Bravo to control Anthracnose which is also prevalent in the wet season . Harvesting fruit at the â€œturningâ€ stage combined with dipping in a Benlate solution after harvest will also help in the supply of good quality fruit to the market.
The BSTA will continue to host Think Tanks to discuss issues and attempt to assist with solving problems which have an impact on local agriculture.