Aug 112018

A common error gardeners make is to assume that appearance of coloured areas on leaves is caused by natural variegation, fungal infection or nutrient deficiency.  In many cases, the variegated colour of leaves of some shrubs, indoor plants, lettuces, herbs is a natural mutation.  Happily, this is not a destructive mutation, but one that results in less chlorophyll in some parts of the leaves, causing a paler and mottled effect.  Sometimes, keen gardeners have selected these plants and bred from them. These variegated plants can be lower in vigour and shorter-lived than their ‘normal’ relatives but this hasn’t affected their popularity as garden plants.

Variegation Due to Plant Viruses

There is, however, variegation caused by some sort of plant problem. Although some garden pests, such as aphids can cause leaf yellowing, this colour change is not the only sign of an infestation.  A more problematic cause of colour change is infection by disease-causing viruses or virus-like viroids and phytoplasmas, of which there are hundreds.  Shown at the left is the leaf symptoms of cucumber mosaic virus (image courtesy of

Fortunately, trees and Australian native plants have few virus disease problems and, interestingly, viruses in wild-growing plants don’t seem to develop disease, although they have not been studied as well as cultivated plants.  It is in cultivated plants, mainly food crops, that virus infection is of most concern. Because viruses and virioids are very simple “organisms” with DNA or RNA coated by a protein layer. They cannot survive for long or multiply outside of a host so they are ‘obligate parasites’ – they can only increase in number inside a living organism.  Once inside the plant, the plant is harnessed into producing more virus.  The plant cannot grow properly because its energy and nutrients are diverted away from normal activities like photosynthesis.

Shown here are the leaf and fruit symptoms of tomato spotted wilt virus (images courtesy of North Carolina Co-operative Extension).

Virus infection may result in a variety of symptoms – leaf colour variegation (which may be mottling, stripes or sport) is just one.  Viruses may infect all parts of the plant but symptoms are generally most obvious on young foliage.  Some result in ringspots, such as ringspot virus on Cymbidium orchids; bronzing, such as that caused by tomato spotted wilt virus; malformations, as caused by potato leafroll virus; and wilting, also caused by tomato spotted wilt virus.


It is common practice to use the common name of a virus, rather than its scientific name (unlike the use of botanic names of plants).  Viruses are named based on the first plant on which it is studied, followed by the most obvious symptom of the disease on that plant, followed by the virus group to which it belongs.  For example, apple mosaic virus is the common name of the apple mosaic ilarvirus, and tulip breaking virus is the common name of the tulip breaking potyvirus.  Many viruses have a wide host range, not just the plant it is first studied on, and the symptoms can vary.



Plant viruses, like those infecting humans and other animals are spread by some sort of contact. More than 20% of virus diseases are spread via seed where the virus is mostly in the cells required for growth, but some are in the seed coat.


Some are spread by vectors such as aphids and thrips.  Shown here is rose mosaic virus.  This is likely to be spread by aphids (image courtesy of The Utah Department of Agriculture and Food, Division of Plant Industry).  Some are carried by single celled organisms such as Polymyxa betae which transmits Beet necrotic yellow vein virus by infecting plant roots.


If plant leaves become damaged by cultivation or animals feeding, sap containing virus may be transmitted another plant.  This has been observed with cucumber mosaic virus.


Although rare, nematodes and fungi are also implicated.  Parasitic plants can spread viruses. Tobacco mosaic virus can even survive in dead infected tobacco leaves in cigarettes.

All virus diseases will be transferred from infected parent plants through new bulbs, corms, tubers, stolons, and cuttings.


As we know from human virus epidemics, animal viruses are almost impossible to control except by avoiding exposure, mounting our own immune system to fight off the infection or by vaccination. Plant don’t have the same options – once a plant is infected it remains infected.  Some viruses do not infect the growing tip of a plant (the meristem) and this is often used to advantage by growers. The unaffected tissue from the meristem of a plant can be tissue-cultured and can result in virus-free plants.

There are no chemical or biological controls.

So prevention is the only effective control. Methods include:

  • Removing infected dying plants and destroying them so that vectors such as aphids do not spread the virus to healthy plants.
  • Keeping weeds away – they can host viruses, so keep the surrounds weed-free.
  • Paying attention to your own hygiene because some viruses can be spread by handling, so hand-washing between plants can be a necessity.
  • Sterilising pruning and cutting implements after every plant or plant group – dip or wipe tools with methylated spirits
  • Growing disease-resistant varieties – many resistant varieties of crops of economic value have been developed.
  • Insect -proofing greenhouses. Professional growers often rely on insect- proof greenhouses to keep out possible virus-carrying vectors.
  • Quarantining plants for suspect locations.

There are many virus diseases not yet in Australia, so quarantine is an important method of preventative control.  Please adhere to quarantine legislation – you never know what a plant or seed might be carrying.


Jun 262018

On average, every Australian spends around 90% of their time indoors, be it at work, at home, at the shops or the local pub – a figure that is quite astonishing.          But, if you think that’s frightening – a number of studies have found that the air indoors (where we spend most of our time) is generally more polluted than the air outside (where we spend about 10% of our time – or even less in some months)!  Having plants growing indoors is an easy and attractive way of cleaning up this pollution.

A class of chemicals called Volatile Organic Compounds (VOCs) are the main indoor air pollutants. VOCs are present in many common office and indoor items including carpets, paints, wall panels, wood products (especially particle board) and furnishings, and can be pretty bad news. Including such baddies as formaldehyde, benzene, xylene, and toluene, VOCs decrease the indoor air quality and are a significant contributor to “sick building syndrome”, a term used to describe a range of symptoms thought to be caused by the office or home environment.  Carbon dioxide and carbon monoxide also pollute indoor air.

The CSIRO estimates that the health cost of poor indoor air quality could be as high as $12 billion annually1.  Indoor plants reduce the losses occurring through increased absenteeism and reduced productivity in the workplace.

If that happens in workplaces, just think of what happens at home!

How Plants Protect People

Plants indoors can significantly reduce both VOCs and carbon dioxide from the air, meaning cleaner, greener, happier and more productive homes and offices2.  But how do they do it?   Removal of VOCs from the air indoors is actually done by the plant as well as the millions of micro-organisms in the soil. It has been shown that potting mix alone was able to remove some level of VOC’s from the air, although the amount removed significantly increased with a plant in the pot (due to the increased level of soil microbial activity that comes with having a real plant in the pot)2.  Having just three indoor plants in 10m² of office space can go a long way indeed to improving indoor air quality.

When 28 different plants were compared the most efficient at removing VOCs were Hemigraphis alternate (red flame ivy), Hedera helix (common ivy) Hoya carnosa (porcelain flower or wax plant) and Asparagus densiflorus (foxtail fern) had the highest removal efficiencies for all pollutants; Tradescantia pallida (“wandering Jew”) was able to remove all but one of the VOCs tested 3.  Don’t think, though, that these are the only effective plants (and don’t plant them outdoors as they become weedy).  Those findings show that there are differences in plant effectiveness and suggest that it would be wise to have a mix of different species.

And in modern buildings with windows that are sealed to minimize unwanted temperature changes (“tight buildings”) levels of carbon dioxide are also higher than outdoors4, coming from air we breathe out, and can cause restlessness and sleepiness.  Carbon monoxide can also be a little higher, generated by heating, smoking, incense and candles4.  Since plants are responsible for removing these gases from the air, having them indoors not only helps save the planet, but helps make us less lethargic and generally feeling better!

Some research suggests that indoor plants, either because they bring people in contact with nature or because they improve indoor air quality can also increase creative task performance and lift mood.

So, let’s have a look at some readily available indoor plants, their light requirements, and their ideal positioning in the home or office.

Indoor Plant Care

Just like their friends outside, indoor plants need proper care because they can suffer from a variety of issues that can affect their health, well-being and appearance.

Too much “love”

This is the most common affliction of indoor plants. Yes, that’s right; many of us kill our indoor plants with love, either by overwatering or overfeeding them.


Indoor plants are best fed once per month in the growing season or if flowering and then almost not at all during autumn and winter. However, this depends on what you feed them.


Watering needs to be monitored at all times. One of the best ways to do this is by putting your indoor plants into a self-watering pot, one that regulates the uptake of water into the soil. This way, over and under watering are no longer issues, and indoor plant ownership becomes a far less stressful endeavour.


It is also recommended that the leaves of indoor plants are periodically wiped with a damp cloth, on both the upper and lower surfaces. This helps remove dust that can build up and clog plant pores, removes any insects or eggs that may be present, and keeps the plant foliage looking shiny and healthy. It might be a bit tedious, but is worth it in the long run.

Light exposure

Rotate your plants every now and then (yes, turn them around on the spot) to keep growth nice and even by giving them even access to the source of light. Keep an eye on your indoor plants, especially over winter, as potential pest problems can present themselves in the cooler months.

Are there any downsides?

Unfortunately, indoor plants can be extremely harmful (often fatal) if ingested by pets, so it is advisable to keep indoor plants out of reach of puppies and kittens especially, but also adult dogs and cats.


1. Wood RA, Orwell RL, Tarran J, Torpy F, Burchett M. 2008. Indoor plants: improving the indoor environment for health, well-being and productivity. ISHS Acta Horticulturae 790: VIII International People-Plant Symposium on Exploring Therapeutic Powers of Flowers, Greenery and Nature.
2. Wolverton BC, Wolverton JD. 1993. Plants and soil microorganisms: Removal of formaldehye, zylene and ammonia from the indoor environment. Journal Mississippi Acacemy of Sciences 38 (2): 11 – 15.
3. Dong Sik Yang, DS, Pennisi SV, Son K-C, Kays SJ. 2009. Screening Indoor Plants for Volatile Organic Pollutant Removal Efficiency. HortScience 44 (4) 1377 – 1381.
4. The Centre for Australian Weather and Climate Research. 2010. Indoor Air Project Part 1: MAIN REPORT. Indoor Air in Typical Australian Dwellings.

May 262018

There are many prompts that lead people into wanting to get into gardening and grow their own food.  One of SGA’s enthusiastic volunteers tells us of her pathway into sustainable gardening.

“I lived in a high-rise apartment in Hong Kong for 26 years until I came to Australia.  Apple trees were in the fairy tale books and vegetables were in the supermarket aisles. Continue reading »

May 262018

If it is important to you to reduce your exposure to artificial chemicals in food, it can be quite expensive to buy organic fruit and veggies and it may not be possible grow all of them yourself.  Production that is Certified Organic, means that as well as not being exposed to residues of chemical pesticides and herbicides, you are doing something for the environment because farmers and gardeners are not permitted to label food “Organic” if they use artificial fertilisers.   But if you are on budget, like most of us, how do you decide which organic foods to prioritise when you go shopping or which to grow yourself? Continue reading »

Apr 272018

While the southern parts of Australia have donned winter pyjamas and flannelette sheets, the northern states are still revelling in warm, and mostly sunny autumn days. Regardless of the conditions in your little patch of paradise, there is still loads to do this month. Get set for those produce plants that need the cooler weather to grow. Continue reading »

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