Expert Tips for Healthy Houseplants: Preventing Diseases and Pests

Expert Tips for Healthy Houseplants: Preventing Diseases and Pests

Plants, like us and our pets, are living things and like us, react to good care. Maintaining good, healthy indoor plants should be a pleasurable task rather than too much of a chore.

Most have some basic needs and by knowing what they are and by recognising any problems early on, your plants will develop in a strong and healthy way.

Get to know your plants: Be observant

Get to know your plants, so that when problems arise, examine it/them carefully and ask yourself – are the environmental conditions satisfactory? Many problems are not caused by a single factor, but rather the result of several factors coming together to cause an unhealthy or unattractive looking plant.

Diseases are often secondary symptoms caused by poor environmental conditions and/or care. Diseases are typically caused by fungi, bacteria or viruses and while there are options to control them (eg fungicides), it is often more effective to tackle the primary issue caused by poor environment or care.

These primary issues involve one or a combination of unsatisfactory light, soil, temperature, water and fertilizer.

How to identify plant diseases

The warning signs of such Secondary Problems are always visible!

  • Growth slows, is stunted or becomes spindly
  • Leaves may change colour, especially yellow or brown
  • Leaves may become brittle or dead and fall off
  • Leaves may show spots, usually dark red, brown or black
  • Leaves may change shape or appear eaten, especially around the edges
  • Leaves may show white powdery blotches or a fuzzy grey mould
  • Stems may become soft or mushy with black matter visible at the base or on the soil
  • Rootrot can occur through bacterial infection

 disease leaf

What are these diseases and how to deal with them.

It is important to note that if you feel your plants are suffering from a disease, bacteria or a virus, you should isolate them immediately. Wash your hands after touching any affected plant and sterilize your garden tools with one part bleach to 9 parts water after dealing with each infected plant.

Powdery Mildew

This is a fungal disease, identified by a white powder on the leaves. It thrives in shady, warm, dry conditions. Spray with a fungicide such as neem oil, a lime-sulphur mix or as an inexpensive but effective option, baking soda. Remove any dead debris from the soil, ensure adequate air circulation and reasonable light. To help prevent fungal diseases, water the soil only, being careful not to splash the leaves.

Grey mould (Botrytis)

Another fungal disease, often threatening Begonias and Cyclamens or plants that have been recently pruned or re-potted. It resembles a fuzzy grey mould. Remove any dead leaves, provide good air circulation. Again, baking soda (Sodium Bicarbonate) is a good fix as it is an organic and eco-friendly remedy. A copper fungicide spray is a suitable alternative for any fungal issue.

Sooty Mould

Looks like soot or ash and this fine black powder can interfere with the plant’s ability to photosynthesise. This is often caused by pests such as thrip. Remove the damaged leaves and mix a tablespoon of washing up liquid to 2 litres of water to eliminate the thrip by wiping down the plant with this mix. Wash this off after 15 minutes.

White Mould

This tends to grow in fuzzy spots that spread, whereas Powdery Mildew powders the surface like flour. It is due to damp, warm, poorly lit conditions or from weeds. Again, to cure, use any of a good fungicide, washing-up soap, Neem oil, or baking soda. To reduce the threat of white mould, ensure your plant is in good draining soil, that the soil only is watered and that overcrowding does not take place.

Crown and Stem Rot

A fungal disease which takes over quickly and affects the whole plant above the soil and often accompanied by rootrot below. The signs are that the plant discolours quickly (red, brown to black) and the leaves will wilt and wither. This is a humidity-loving fungus that lives in poorly draining soil and once it has a hold, it is difficult to fix.

Remove the afflicted parts of the plant, apply a good fungicide and re-pot in new clean soil in a new container. To prevent this in the future, ensure the plant’s soil drains well, that you water the soil directly (not the stems or leaves), sterilize your tools and keep your plants strong with nutrient-rich soil and a good fertiliser.

Plant Rust

Reveals itself with dusty orange patches; this is usually spread by water over the leaves. A copper spray or sulphur powder will sort this out.


A soil-borne fungal disease, caused by using/re-using infected soil, letting dead debris remain on the soil’s surface, or improper conditions such as soggy soil or over-crowding. It shows itself by the leaf tips turning yellow, then brown around the edges and then dying.

Any of the previously recommended treatments for fungal diseases will be appropriate here, but of benefit regardless, is to not over-water or allow dead debris to remain in contact with the plant.

Bacterial infections/Viruses

These can reveal themselves in the form of leaf spots, blights, cankers and so on. Similar looking to fungal diseases, they can be due as much to poor environmental conditions than anything else.

Usually over-watering, temperature and humidity at too high a level, poor drainage affecting the roots and not enough air circulation are the causes. This goes back to your management of the environment.

The solutions involve adjusting the plant’s position, as well as the temperature and humidity in the room and your watering routine.


Common pests and how to deal with them.

As you will know from your outdoor garden, especially if you have a vegetable garden, there are a range of insects from the obvious Cabbage white butterfly larvae, snails and slugs to tiny bugs that enjoy a good feed at our expense. There are pests that will threaten the wellbeing of your houseplants too, including fungus gnats, spider mites, ants, mealy bugs, thrip, aphids, and an odd looking armoured insect called scale.

By regularly observing your plants – and don’t forget the underside of the leaves or at the base of stems – you will be able to ‘nip these in the bud.’ Try using organic or natural remedies as much as possible.

These include Neem oil and Pyrethrin (especially for mealy bugs and spider mites) It is best to rid Scale by wiping the plant with a cotton bud/ball dipped in ‘rubbing alcohol.’ Aphids and similar can be wiped down with a soapy dishwashing detergent and water mix, but remember to rinse off afterwards.

 N.B. Not exactly pests, but young children, cats and dogs can do themselves harm if they eat any part of most houseplants!  Many houseplants contain toxins which can be dangerous for young children, cats and dogs. If there is that potential, keep your plants well above contact height and certainly discourage any wandering hands, paws or mouths!


The Environmental, or Primary Problems.

Poor environmental conditions weaken plants, making them more susceptible to disease or insect attack. So what is a poor environment for your indoor plants?


  • Purchasing a plant from an unreliable source (maybe too young or not nurtured properly)
  • Poor soil in terms of quality, nutrients and drainage
  • Dry or excess moisture conditions in the soil
  • Limited light
  • Temperature and humidity either too high or too low
  • Unsatisfactory container (too small, too large)
  • Overcrowding, leading to competition for growing space
  • Reduced oxygen availability (dusty/dirty leaves or lack of air circulation)
  • Compaction
  • Pesticide or even water toxicity

 indoor plants sunlight

How to deal with these Environmental or Primary Problems

By ensuring you provide the best possible and consistent environmental care for your plants, you will eliminate or reduce serious problems later.

 LAWNS = Light, Air, Water, Nutrients, Space .

LIGHT: All living things need some form of light, preferably natural sunlight, to survive. Plants need it for the process of photosynthesis. Houseplants prefer a diluted sunny position, so a north facing window or shelf situation is ideal. They can be given a stint on the patio, in the conservatory or re-planted outside once they have grown and ‘hardened up.’

AIR: This includes temperature and humidity. Most houseplants are fine in an indoor temperature that we as humans also like, that is a range of 12-25C. Don’t expose them to big variations as can sometimes occur in the middle of Winter or the middle of Summer or when you turn on your heating or cooling pump.

Humidity around 50% is fine for most plants and this can be increased with a small dish of water by them or by providing an occasional stint in the bathroom. Remember that air-conditioning units tend to dry out the air considerably, so a small fan may be necessary to assist air circulation.

WATER: Plants need more water in Summer than they do in Winter. Over or under-watering is one of the biggest problems, so be careful! A good guide is to push your finger 1-2 inches into the soil; if it is dry, then your plants needs watering.

Water the soil only, avoiding any splashing onto the leaves and with good quality potting mix and adequate drainage holes, your plant will enjoy its drink. Tap water in NZ usually contains fluoride or chlorine, so rain-water or bottled spring water could be tried if you feel your ordinary tap water contains too many ‘foreign’ deposits!

NUTRIENTS: Water and sunlight will keep plants alive but the nutrients or food in a fertiliser makes them thrive. A fertiliser containing organic fertiliser is best. A reliable ‘grower’ will always supply you with a plant nurtured in quality potting mix. However, plants must be fed with nutrients to maintain growth and development. The main ‘growth’ season is in Spring and early Summer, so use a good quality liquid fertiliser about every 2-3 weeks.

SPACE: Plants like and need space to grow. This is particularly important when planting outdoors as most people tend to plant their shrubs, bushes, flowers, trees and so on, too close together, so that everything as it grows may look too cluttered, but more importantly, there may be a lack of minerals and nutrients for all round growth to occur.

With your houseplants, you may want to rotate or change their position occasionally and particularly, if some appear ‘sick,’ do not allow them in close proximity to others in your collection.


Handy Tips to help your Plant collection remain healthy

  • Cleaning the leaves. Plants use sunshine to conduct photosynthesis, the metabolic process that keeps them alive. Dusty, dirty or grimy leaves limit the plant’s ability to absorb light and can provide an easy way to access disease and to hide insects. So cleaning the leaves is essential for aesthetic reasons and to limit the possibility of pests and disease. Use a disposable microfiber cloth dipped in warm water and wipe both sides of the leaves.
  • The size and type of container is important. Only purchase a plant that looks healthy and comfortable in its current container. Do not re-pot it until it has adjusted to its new surroundings in your home. Clay or terracotta pots are best as they dry out the soil faster by pulling away excess moisture from the soil. Try to find one of these with a matching ‘saucer’ underneath. If the pot becomes root-bound, re-pot it in a container an inch or two wider and deeper than its current one. Oddly enough, roots don’t like lots of space and it can increase the danger of root rot if the pot is too big.
  • If your plant has long roots or is quite mature, bottom-watering can be best. Let your plant stand in a water-filled saucer or tub for approximately 15 minutes before settling it back to its normal position.
  • This is best done in Spring, using clean, sharp secateurs. Any yellowed leaves, especially towards the base of the plant should be removed since the energy will then go into helping your plant grow, rather than in trying to keep old leaves alive.
  • The lighter the plant’s leaves, the more sun and water it will need. Due to chlorophyll – the active food-making part of the plant – dark green leaved plants are better suited to lower light situations and less water.
  • If you know what to look for, you can spot the warning signs early and act quickly and appropriately to deal with your Houseplant issues before they develop into real problem situations!


Finally, The Green Grower’sPlant Care Guide’ and its columns titled ‘New to Plants’ and ‘Understanding Houseplant Basics,’ provide very good coverage of the aspects discussed here. With that combination or by contacting The Green Grower, we are sure that you can -and will - reduce the chances of your plants becoming unhealthy for any reason.

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