My periwinkle plants used to grow and bloom well, but their stems have recently started rotting in the middle. This happened to three different plants in separate pots. Why?

Lee Chin Chin

Your plants are exhibiting symptoms of fungal disease, which periwinkles are prone to.

First, ensure the plants are grown under sufficient sunlight. Perwinkles prefer direct sunlight. Make sure they are not overcrowded - good air circulation and ample sunlight penetration to most parts of the plant can help to reduce incidence of disease.

Applying a systemic fungicide such as benomyl may help.

Proper sanitation is also necessary. Remove and dispose of all diseased plants. Avoid re-using infected soil. If you must, sterilise the soil beforehand. Clean flower pots before re-using to grow the same plant again.

I have a 2m-tall soursop plant. It flowers, but the blooms drop after a few days and bear no fruit. What should I do to make it fruit?

John Tan

A likely reason behind the lack of fruit production could be due to the lack of natural pollinators in an urban environment to help pollinate the plant's flowers.

Hand pollination is often employed to encourage fruit to grow.

Soursop flowers have both male and female reproductive organs, but are protandrous - meaning the male parts mature faster and shed pollen before the female parts become receptive to pollen germinating.

You would need to collect the pollen from the flower when it first opens in the morning. Store it overnight in a refrigerator.

The next day, after the stigma - a bulb that sits on a long stalk in the centre of the flower - becomes receptive, transfer the pollen: Dab some on a fine artist brush and brush it against the stigma.

If your plant is too exposed or needs more humidity, fruit production issues might arise. Grow more plants around the soursop plant but take care not to cast shade on it.

Alternatively, install a misting system. Adjust the duration and frequency of misting to avoid making it too damp for your plant.

Answers by Dr Wilson Wong, a certified practising horticulturist and founder of Green Culture Singapore (www.greenculturesg.com), a website for plant lovers. He is also an NParks-certified park manager.

Tip

Leave enough space between your plants. Good air circulation and ample sunlight exposure are necessary to ensure healthy growth and can help reduce disease.

Growing plants too close together may lead to lack of sunlight. Leaves may die, leading to bald patches. Moisture will also not evaporate, contributing further to the spread of diseases.