Dr Christopher Teh, soil biologist and senior lecturer at Universiti Putra Malaysia answers gardening questions in Star2’s monthly gardening column, Ready, Set, Grow!, which is brought to you by social enterprise Eats, Shoots & Roots. Go to questions.eatsshootsandroots.com to send in your queries.

Can you please tell me how to get rid of those tiny white pests that dwell on the underside of leaves? The whole leaf gets coated with them and then it withers and drops soon after. – Lee Kok Tham

These white pests you are seeing are most likely aphids. They can be controlled by water-spraying them off your plants with a spray bottle or spraying them with water mixed with neem oil. You have to do this repeatedly until the aphid population is gone. You can also plant garlic or onion around your plants, as these two crops tend to control aphids.

I have several chilli plants in pots and my first harvest was very good. However, with the second generation plants, the fruits are quite nice but do not ripen well. Just before ripening to a nice red colour they all start to turn slightly brownish with a fungus-like growth before rotting. Is it due to the soil or what is the problem? Is there any remedy? – TGC

It is likely you may have over-watered your plants. Overwatering can cause fruit pods to fall, flowers to abort/drop, or no flowers. It can also cause fungal diseases. I suggest you discard all the soil from the pot and start again, this time watering less frequently than you did before.

Depending on where you place the pot, you will have to adjust the watering level. Generally, you should water once every two to three days. But you should only water when the soil is slightly damp, not when the soil is very wet. Consequently, the soil you use is also a factor that affects your watering frequency. Your soil should be freely draining. If it is not, you can add perlite or sand into your potted soil (mix them with the soil first).

If the leaves wilt, you are not watering enough. But do not rely on just touching the soil surface to see how much water there is in the pot. Very often, the soil surface may look very dry, but stick a finger into the soil, and you will feel that the soil underneath is still moist.

I would like to know why my lime tree isn’t fruiting despite putting manure and trimming it? – Vasanthi

I suggest you do not add manure directly into the soil or on your plant, but compost the manure first. Besides killing any dangerous pathogens in the manure, composting makes the nutrients more readily available to your plant than applying it directly, without composting.

Lime trees require high N (nitrogen) and P (phosphorus), so I suggest applying fertilisers rich in those elements.

Lime trees also require little watering – once a week, rather than every day. Check the moisture of your soil to work out a good watering frequency. The soil should be only slightly damp for watering. If the leaves start to fall, it means you need to water more frequently.

How do I get rid of nematodes in my garden? My plants grow well initially but their leaves turn yellow and growth is stunted eventually. When I pull out the dying plant, I see rounded lumps on the roots. – Yun Chow

One of the obvious signs of nematode infection is, indeed, knotted roots or distorted tubers.

To get rid of this infection, you need to till (turn over) your garden often to expose the nematodes. After tilling, cover the area with black plastic to heat up the soil underneath. Do this a couple of times until you see the nematode attacks stop.

If you are planting short term crops like vegetables, it is recommended that after every harvest, you remove the whole plant, including the roots.

Crop rotation (where you plant different crops throughout the year) is especially effective in controlling nematodes. Do not always plant the same crop.

Beans, tomatoes, and potatoes are generally resistant to nematodes.