September 2003. Tonga asked about fruit drop in squash: was it normal to have so many fruits fail after fruit set. There are from 1-3 per plant. The plants were growing vigorously and there was no sign of sunburn and little powdery mildew, but there was some signs of fungal infections on the small fruit.
In response, Tonga was asked if checks had been made for fruit fly. In Australia, attack by the cucumber fly, Bactrocera cucumis, causes a similar problem. The simplest way to check is to take some fruit and rear out any adults. Possibly, other flies in the family Lonchaeidae and Drosophilidae could be present as well. There are no records of any species of major importance to cucurbits in Tonga, so if flies are reared out of the fruit it will be very important to have the species identified.
Another, more likely, possibility is insufficient pollinators. Honeybees are needed for pollination of cucurbits and there may not be sufficient in the area or there are competing floral sources that the bees are visiting. Bees may need to be introduced.
The simplest way to check this theory will be to take some male flowers and hand pollinate and mark a number of flowers. Mark a similar number of flowers that are not hand pollinated as a control. This needs to be done early in the morning (sunrise or just after) when the stigmas are sticky and the pollen has not been eaten or carried off by other insects. Some fruit will still probably fail to set depending on the number of fruit that each plant is carrying as there is a limit to the number of fruit that the plant will retain. However, if most fruit develop on the hand pollinated flowers and less on the unpollinbated control flowers, then it will be a good indication that bees are needed. Whether the fruit has set or not can be determined within 2 days. Improperly shaped fruit in the current crop could also be an indicator of insufficient pollinators.
Fruit go mouldy after they fail to set (with both poor pollination and fruit fly attack) and this is possibly the secondary organisms that are being seen, at least on the failed fruit.
A similar problem in New Caledonia ...........
March 2004. Two months after pollination some fruit had stop growing. In several blocks, up to 60 percent of fruit are like this. At maturity the fruit are: a) small; b) without seeds; and c) sometimes half a fruit has no seeds and other part has full seeds. It was thought the problem was associated with bees, but some crops on one side of a river are healthy, while those on the other side have 70% of fruit with poor growth. Both places received the same fertilisers and pesticides, applied the same day.
Almost certainly a pollination problem and not a disease. It was suspected that either the beehive was on the other side of the river, or that the grower used an insecticide at the wrong time. Some pyrethroids can have a repellant effect as well as insecticidal.