Plant Doctor Archive

Pumpkins, courgettes not producing

I'M growing miniature pumpkins in a reasonably sunny spot. The pumpkins start to form but soon rot. I've also had the same problem with my courgettes. Any suggestions?

IT'S possible the flowers are being infected with a fungus such as grey mould, which can grow on the old petals of a flower as they die off, especially in cool, wet conditions. It grows readily on soft plant tissue and, once established, spreads rapidly to destroy immature fruit.

You could spray with a fungicide, but you'd need to be aware of the withholding period, which is the time that should elapse between spraying and harvesting the fruit. Courgettes usually need picking several times a week, so you probably would want to avoid spraying them.

The good news is that with warmer and drier weather, the fungus is less likely to be a problem.

Another cause could be lack of pollination. Courgettes and pumpkins have separate male and female flowers and they rely on insects and bees to transfer the pollen. Cool weather can deter bees and other pollinators, but more recently varroa mite has killed off many wild bee hives and reduced the number of hives kept by amateur beekeepers, so there are fewer bees around to pollinate our garden crops.

If the female courgette or pumpkin flower doesn't get pollinated, it will eventually die off and rot, even though a small fruit may have started to develop. You can overcome this problem by hand pollinating the flower. The female flower has a small swelling immediately behind the flower, the male has none. Simply break off the male flower, remove its petals and rub the pollen-bearing part (the stamens) against the central part (stigma) of the female flower. You can pollinate several female flowers with the pollen from one male flower.

Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.