Ask the Plant Doctor! How much to water, composting and common fungal diseases

Spray insecticidal soap to get rid of powdery mildew on plants. Photos: 123rf.com

Do you have a question about plants or how to maintain your garden? Send your gardening questions to the Plant Doctor!

Email your questions to [e-mail] with "Plant Doctor" in the subject field. Questions may be edited for brevity and clarity.

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Dear Plant Doctor, I noticed that some of my plants are covered in a white cottony substance, and then the plants start to rot. I notice that my Japanese rose is especially vulnerable, but my ferns are unaffected. What should I do? – Mel Goh

Several reasons can lead to the appearance of white dust or talcum-like powder on your plants. However, your observation that your rose plant is especially vulnerable suggests that powdery mildew could be the problem. This common fungal disease affects many plants, with roses being particularly susceptible.

Powdery mildew thrives in conditions of low light and poor airflow, as well as humid air with dry soil. This can occur when plants are too close together or when plant-watering mainly wets the foliage but not the soil.

To control this disease, increase sunlight exposure and improve air circulation by spacing plants further apart or trimming excess foliage to reduce crowding. Water directly onto the soil, not splashing the leaves. Remove and dispose of any infected plant parts, including those that have fallen to the ground.

Avoid shaking infected plants to reduce spore spread, and sanitise all cutting tools used on infected plants.

Insecticidal soap, available online, can help control this disease. Follow the instructions on the bottle. Addressing the infection early will yield the best results.

Other probable causes include pests, such as mealybugs, woolly aphids and scale insects. Insecticidal soap is also effective in controlling them.

The bokashi method of composting works well for those living in small spaces.

For those staying in a small place or an apartment, what is the best and most simple way of making compost? – Karen Teh

In limited spaces, Bokashi composting is a convenient alternative to composting. While composting relies on oxygen-dependent decomposition, Bokashi uses fermentation in a low-oxygen environment.

Bokashi requires at least 30% moisture content in the organic materials, whereas composting is much more sensitive to moisture levels. Additionally, Bokashi needs smaller amounts of materials, often kitchen scraps, compared to composting, which requires not only specific ratios of certain materials but a larger pile of materials (ideally, over 30kg).

Bokashi fermentation is faster than composting, typically completed within six weeks. However, it necessitates the addition of microbes, such as Effective Microorganisms (EM), and an airtight container.

The fermentation process will produce a strong smell that some may perceive as

sweet and sour and others as rancid.

The quality of the end product of Bokashi depends on the materials used, and the high table salt content from kitchen scraps could harm your garden over time.

Various Bokashi methods can be found online, and Bokashi kits are even available for purchase, usually including a 20-litre airtight bucket, molasses and EM.

The leaves on my plant are turning yellow. When I Googled it, overwatering and underwatering are cited as the main reasons. How do I know whether I am overwatering or underwatering my plants? – Ida Karim

Do a simple, rough test: To assess your watering, use a hand trowel to dig out about 10cm to 20cm of soil. Hold the soil in your hands, without squeezing, and observe its moisture. If water drips through your fingers, it is overwatered. If you squeeze the soil and water only drips out at that point, it indicates adequate watering.

But if no water drips out even after squeezing, this indicates underwatering. Analogously, adequately moistened soil feels like a damp sponge. Water should only come out when we squeeze the sponge. Plants that are underwatered will tend to wilt, but overwatered plants may also wilt if the soil is waterlogged for long periods, causing root rot.

But it is very important to realise that yellowing leaves could also be caused by nutrient deficiency (likely in nitrogen or sulfur) or insufficient sunlight.

Dr Christopher Teh heads the Dept of Land Management, Faculty of Agriculture, Universiti Putra Malaysia.