The Ultimate Guide to Plant Diseases and How To Treat Them

So, you’ve patiently taken care of your beloved plants, watered them well in the summer and carefully protected them during the winter frost. You’ve made sure that they’re appropriately fertilised, pruned, and receive sufficient shade and sun… only to one day discover that they have succumbed to a disease.

As disheartening as this may be, you’ve come to the right place for guidance on the different types of common diseases that can attack your plant and the best ways to treat them.

We’ll also look at the diseases that affect outdoor plants compared to indoor ones, as well as the types of plant diseases that are commonly found here in Australia. So let’s dive right in!

Non-infectious Plant Diseases

There are two overarching categories of plant diseases: infectious and non-infectious. Environmental factors are the main cause of non-infectious plant diseases. Common sources of disease include damage from insects, birds, and snails, as well as poor soil conditions, nutrient deficiency, overwatering or under-watering, sun scorch, and injury caused by frost or salt.

Symptoms of non-infectious plant diseases are similar to those of infectious diseases and often include plant weakening and wilting, yellowing of leaves, stunted growth, and root rot. A quick and easy way to determine whether a disease is infectious or not is to observe whether the same symptoms are occurring on different species of plants in the same location. If so, it’s most likely a non-infectious disease, as infectious diseases tend to affect only one species or members of a species of plants.

Non-infectious diseases, on the other hand, cannot be transmitted from plant to plant, and plants often bounce back to good health once the harmful environmental condition has been removed or improved.

Infectious Plant Diseases

The symptoms of infectious plant diseases are more complex than those of non-infectious ones. They appear in stages, typically worsening over time if not treated, and they only affect certain species of plants, as opposed to non-infectious diseases, which appear on many different plant species at the same time.
Plant infections are often spread by wind, water, insects, soil, and tools, and these infections can hang around for months, even years. There are three main causes of infectious plant diseases: fungi, bacteria, and viruses.

Fungal Plant Diseases

Fungi are thought to be responsible for 75% to 85% of diseases that occur in plants. The reason why fungal diseases are so pervasive is because of their long-living, tenacious, and recurrent nature. Some fungi can even survive indefinitely in soil and withstand extreme temperatures, starvation, and toxic chemicals.

Below are the common types of fungal diseases that can infect your plants:

Powdery Mildew

Powdery Mildew plant example
Image By Jeff Kubina - Own work, Public Domain

It’s easy to identify powdery mildew from the white/greyish spots or patches that it produces on the surfaces of plants. Occasionally, this fungus also appears as a thick white cobweb on leaves and stems, and at times also on flowers and fruits. Powdery mildew affects thousands of different outdoor and indoor plants, including roses, parlour palms, monsteras, and pileas.

Powdery mildew thrives in shady, humid areas with poor air circulation and temperatures of around 16°C to 27°C. This means outbreaks occur mostly during summer. It produces new spores about every 3 to 14 days that are transported by wind hundreds of kilometres away and, quite unusual for fungi, don’t need moisture to grow and infect plants. In fact, prolonged exposure to water can actually destroy powdery mildew spores.

If left untreated, powdery mildew can cause stunted new growth of plant stems and flowers, as well as infected leaves drying up and falling prematurely. It can also cause fewer blooms in ornamentals and a decrease in fruit production. Powdery mildew rarely causes plant death, however; rather, it disfigures plants and can be quite a nuisance because it can survive from season to season.

How To Treat Powdery Mildew
You can use mild, natural products to treat powdery mildew. It very rarely requires chemicals or fungicides.
  • Water the leaves:spraying the affected areas with plain water once a week can eradicate the spores because they can’t tolerate a wet environment. However, do not keep the leaves wet for too long as this may cause other diseases to develop.
  • Remove infected leaves: to curb the spread of the fungus, prune off all leaves that have signs of powdery mildew on them and collect any fallen leaves because they could be further sources of infection.
  • Use baking soda and non-detergent soap mixture: although it’s more effective when used as a preventative measure, this mixture also helps to control powdery mildew once it has already developed. Mix 1 tablespoon of baking soda with 1/2 a teaspoon of liquid, non-detergent soap, and 1 gallon of water. Spray an even coat of the mixture once a week to every surface of the plant including the stem and the undersides of the leaves.
  • Spray with milk: believe it or not, full cream milk is actually effective against powdery mildew. A mixture of 1 part milk to 10 parts water sprayed once every 7 to 14 days can significantly help to prevent infection before it appears. Alternating between a baking soda spray and a milk spray helps to prevent the fungus from becoming resistant to the treatments.
  • Prune and space plants: increasing air circulation by thinning foliage and spacing your plants helps to treat powdery mildew.
  • Disinfect garden tools: make sure to sterilise your pruning tools and any other gear that comes into contact with infected plants to avoid spreading the infection.

Rust Disease

The name of this disease comes from the colour of the spores that appear on plants once they’ve been infected. Usually causing orange patches or raised spots on leaves, rust disease can also appear in yellow, dark brown, purple, and white forms.

Plants susceptible to this fungus include roses, carnations, chrysanthemums, sunflowers, geraniums, fuchsia, myrtle, and berry bushes. White rust, also known as white blister, is more common in cruciferous vegetables such as cabbage, cauliflower, broccoli, kale, and bok choy. Some of the effects of rust on plants include leaves turning brown prematurely and dying, stunted plant growth, and distorted stems and leaves.

Rust disease grows in cool temperatures; therefore, infections often arise during the autumn and spring months. The exception is white rust, which prefers humid conditions and is more common in coastal areas. But what all rust fungi have in common is that they thrive in wet environments. This means that the longer a plant remains wet after being watered, the more likely it will develop rust.

How to Treat Rust Disease
Although rarely fatal to plants, it can be difficult to treat rust disease once it has infected a plant. Therefore, management of the disease and prevention of infection are the best ways to deal with this fungus.
  • Remove infected leaves: prune off diseased leaves and collect fallen foliage immediately, then either put them in a bag for disposal or burn them to destroy the spores. Do not compost them.
  • Buy disease-free plants: before bringing home any seedlings or plants, first check them for signs of rust disease. Even then, experts recommend quarantining new plants for at least 3 weeks in a separate area of your home to make sure they’re not infected.
  • Avoid overhead watering: water your plants from the base instead of overhead to reduce the leaves’ exposure to water because rust needs water to germinate.
  • Provide sufficient air circulation: air circulation is important in preventing all fungal diseases; therefore, regularly prune leaves, space your plants, and remove weeds that may cause overcrowding.
  • Sterilise garden tools: make sure to disinfect your tools each time you use them to prevent spreading the infection.
  • Fungicides: if you opt for the fungicide route, make sure you apply it before any infections appear. This is because they’re ineffective against this fungus once it has developed. You can use either copper or sulphur powder. Mix the powder with water according to the package instructions and spray all of the plant’s surfaces.
  • Plant resistant cultivars: purchasing rust-resistant varieties of various plants is another option to avoid the disease.

Damping-Off Disease

Have you ever planted seeds and wondered why they don’t grow? Or noticed that your seedlings suddenly wilt and die? The culprit could be a fungus that causes damping-off disease. Damping-off disease is an infection that attacks the leaves, stems, and roots of plants at very early stages of growth before they become strong enough to fight it off.

Commonly occurring in seedlings produced in greenhouses, damping-off disease lives in soil and arises when the seedbed is overcrowded or the soil over-watered. Temperatures below 20°C before plant germination and placing seeds too deep in the soil can also cause it as this delays germination.

Apart from the decay of seeds and seedlings, other symptoms of damping-off disease include the growth of a white fungus on the soil, mould on stems and leaves, and, on some occasions, leaf spotting.

How to Treat Damping-off Disease
Unfortunately, once seeds or seedlings become infected with the fungus that causes damping-off disease, it’s not possible to treat or save them. To prevent and manage damping-off disease, here are a few steps you can take:
  • Sterilise seeding kit: the fungus that causes damping-off disease lives in soil; therefore, old seed trays, pots, benches, and tools may contain traces of contaminated soil. Make sure to disinfect them, or better yet buy new equipment, if the old ones have been in contact with the disease.
  • Use clean potting soil: if you’ve experienced problems with damping-off disease in your garden, do not use the same soil when planting new seeds. Instead, purchase well-draining, pasteurised potting mixture. If planting in a garden, use raised beds.
  • Avoid overwatering: fungi that cause damping-of disease thrive in wet soil
  • Don’t overcrowd seedlings: air-circulation is important in preventing damping-off disease.
  • Shallow planting: planting seeds too deeply will delay germination and increase the chances of damping-off disease developing.
  • Use fungicides: spraying the soil and seedlings with organic fungicides such as copper oxychloride can be effective in controlling and preventing the disease.

Botrytis Blight (Grey Mould)

Botrytis Blight (Grey Mould) Example
​Image by Michelle Grabowski, University of Minnesota Extension - Horticulture,

Interestingly enough for a fungus, Botrytis blight can’t directly infect healthy leaves and stems. The only way it can infect plants is by first attacking their aging flowers and then using that opportunity to spread to healthy parts of the plant through the flower’s fallen petals.

Also known as grey mould, this fungus prefers flowers with large, thick petals. It causes leaves to develop brown blotches and fall off and stems to get cankers. Indoor plants, such as pileas, zebra plants, snake plants, and peace lilies are known to frequently develop infections. And when it comes to cut flowers, carnations, gerberas, hydrangeas, and orchids, among others, are particularly susceptible.

Botrytis blight thrives in wet, humid conditions and is more active in lower temperatures than higher ones but its ideal range is between 25°C and 20°C. It can continue to live on dead foliage and other plant debris and is easily dispersed by wind or splashing water.

How to Treat Botrytis Blight
It’s possible to treat and control botrytis blight without using chemicals. With proper care and management, most flowering plants will recover from the disease once dry conditions return.

  • Provide sufficient ventilation: Grey mould thrives in wet, dark environments. Therefore, regularly prune or split your plants to avoid overcrowding.
  • Keep leaves dry: Don’t water your plants in the evening, as they will take longer to dry, thus providing ideal conditions for fungal growth. Additionally, instead of overhead watering, opt for using a soaker hose to water your plants.
  • Avoid wounding your plants: weak and wounded plants are most susceptible to grey mould. Therefore, use pruning methods that will cause minimal scarring on plant stems. For instance, prune in late winter, cut just before a leaf node or, when pruning larger stems, cut as close to the main stem as possible.
  • Remove plant debris: remove fallen leaves, petals, and any wilted parts of plants immediately and burn them. Do not compost or bury them. Also, disinfect all tools that have been in contact with infected plant material and clean the soil from your boots, shoes, and gloves.
  • Purchase disease-free plants: Flowers Across Brisbane has a lovely selection of beautiful, healthy orchids that are available in either pink or white colours and you can choose between potted or boxed ones.
  • Fungicides: if you prefer to apply fungicides, the best time to use them is in early spring before the infection develops.

Verticillium Wilt

A persistent, soil-dwelling fungus that can remain in soil and dead plant material for up to 10 years, Verticillium Wilt is one of the most dreaded diseases in horticulture. It enters plants through the roots and subsequently grows in the tissue that transports water throughout the plant, leading to highly susceptible plants eventually wilting and dying.

Symptoms of the disease usually start to manifest after plants begin flowering or after they experience stress. One of the early signs of Verticillium Wilt is the yellowing and subsequent wilting of a plant’s lower leaves, typically on only one side or section of the plant. But if left untreated, the disease will continue to progress up the plant.

You can also diagnose the presence of the fungus by pruning off a stem and cutting it in half or lengthways. If you spot any dark discolouration inside in addition to the symptoms described above, you can be fairly confident that it’s Verticillium wilt. It mostly occurs in ornamental plants such as daisies, chrysanthemums, cockscombs, and geraniums.

How to Treat Verticillium Wilt
Verticillium Wilt is a tricky disease as it can continue to live in soil for years even after you uproot and remove an infected plant. Here are the best ways to control and prevent further contamination and spread of the disease:

  • Maintain good plant health: underwatering, poor soil health, and other stressors can contribute to making plants more vulnerable to the fungus. However, there are many great apps that can help you to make sure your plants receive the nutrition, watering, and shade needs they require.
  • Remove infected plants: once you confirm the presence of the disease, carefully dig up the entire plant with its root system and destroy it. Do not compost or bury it. While uprooting the plant, try to limit the movement of soil as much as possible to minimise the spread of infection.
  • Regularly check soil health: irrigation, fertilisers, fungicides, and even rain can impact soil quality, creating conditions that encourage the growth of verticillium wilt. Don’t forget to also keep an eye on soil pH as well.
  • Weed control: some weeds, such as pigweed and nightshade, are highly susceptible to verticillium wilt and may not even show any symptoms of infection. If not removed, they will continue to be recurring sources of infection.
  • Plant resistant species: many cultivars that have been bred to be tolerant or resistant to the disease. Additionally, English daisies, evening primrose, violets, sunflowers, zinnias and several other plants are naturally resistant to Verticillium wilt. You may need to plant these resistant species for as long as 15 years in areas with confirmed Verticillium wilt in order to completely eliminate the fungus. Unfortunately, there’s currently no known cure for Verticillium wilt other than time.

Black Spot

Example of Black Spot on Plant
Image by Public Domain,

So often associated with roses that it’s also known as ‘rose black spot’, black spot disease is different from other leaf spot diseases. It causes spots or blotches that have a darker colour and often with frayed edges. Due to the vast number of rose cultivars available, the disease may not present uniformly in every rose, with variances in spot size and shape, but symptoms typically remain the same.

The first sign of this fungal disease is tiny spots on the lower leaves of the plant. The spots progress throughout the plant over the course of the season and may at times develop varying levels of yellowing around them. Premature leaf fall is another early indication of the disease that may at times be the easier of the two initial symptoms to spot. If left untreated, the disease will cause the plant to develop smaller and fewer flowers and gradually weaken.

Although black spot itself doesn’t kill plants, it does cause them to become more vulnerable to other attacks and cold injury, which can lead to plant death. Black spot lives in fallen leaves, even during winter, and is most common in yellow and copper-coloured roses. The fungus is spread by wind and water splash, and it’s most active in temperatures between 20°C to 27°C.

How to Treat Black Spot

  • Remove infected leaves: immediately remove any attached or fallen leaves that have been infected by black spot. In some circumstances, you may need to dispose of the entire plant. Don’t compost them, but burn them or put them in a plastic bag for disposal.
  • Replace mulch: the fallen leaves may have infected the mulch on your plants; therefore, consider replacing it to avoid potential reinfection.
  • Plant spacing: like all fungi, black spot thrives in dark, moist, and crowded environments. So, ensure good air circulation by adequately spacing your plants and seedlings, and regularly pruning leaves.
  • Sterilise tools and pots: contaminated equipment, plant pots, and gardening gear such as shoes and gloves frequently contribute toward spreading the fungus. So, make sure to thoroughly clean and sterilise them before reusing them.
  • Plant resistant cultivars: there are now hundreds of rose varieties to select from that have been bred to be resistant to black spot disease.
  • Apply organic fungicides: Potassium bicarbonate is said to be an effective organic fungicide against black spot. Follow the instruction on the label for best results.

Fungal Leaf Spot

Indoor plants are highly susceptible to leaf spot disease. Fungi are the most common cause of the disease, with bacteria, viruses, and other pathogens responsible for some occurrences. Fungal leaf spots typically begin as small raised dark circular spots on leaves, which increase in number and size as the disease progresses.

Eventually, the plant weakens because it can no longer effectively perform photosynthesis due to its leaves being covered in spots. Serious infections can cause massive defoliation and plant dieback. Since fungal leaf spot is caused by many different kinds of fungi, though, in some instances the disease may present as brown or red spots with a dark border or black spots with a yellow border.

To determine whether the leaf spot infection is fungal, some botanists suggest sealing a few infected leaves in a damp container for 2 to 3 days, then checking whether the leaf spots have tiny black dots in them. If they do, that means the cause is fungal. You can probably rule out fungal leaf spot, though, if symptoms appear solely on leaf margins or veins.

How to Treat Fungal Leaf Spot
Leaf spot is caused by many different kinds of fungi. Depending on the severity and timing of the infection, treatment can vary. Below are the best ways to control and prevent further contamination and spread of the disease:

  • Remove infected and fallen leaves: the fungi that cause leaf spot can continue to live in plant material; therefore, frequently check your plants, particularly in the spring and early fall seasons, and immediately remove diseased and dead leaves.
  • Air circulation: the fungus prefers wet, crowded conditions so make sure to regularly prune your plants. If indoors, you can even use a fan to increase airflow. With outdoor plants, make sure to space your planting to allow for ventilation.
  • Keep leaves dry: avoid overhead watering and also water early in the day to minimise prolonged leaf wetness.
  • Disinfect garden tools: sterilise all tools after use and clean any equipment, clothing, and gear that comes into contact with infected plants.
  • Use clean potting soil: change the soil in the pots or in the area of your garden where fungal leaf spot has been identified. You may also want to replace any old pots in which infected plants have been grown.

Bacterial Plant Diseases

Although smaller than fungi, bacteria can’t directly infect healthy plant tissue the way fungus can. They can only enter plants through wounds or natural openings, making them more of an opportunistic pathogen.

Bacterial Leaf Spot

Similar to fungal leaf spots, bacterial leaf spot appears as dark to brownish spots with a yellow border on the lower leaves. But bacterial leaf spots also have a water-soaked appearance, usually on the underside of the leaf. In some instances, to see the water-soaked areas on the leaf, you may need to hold the leaf up to a light.
Popular houseplants such as zebra plant, Chinese evergreen, pothos, crotons, and fiddle leaf fig are all susceptible to bacterial leaf spot.

The disease is spread through infected seeds, water splash, and during plant propagation. It prefers warm and humid conditions with most of the bacteria becoming active in temperatures between 25°C and 30°C.

How to Treat Bacterial Leaf Spot

  • Remove infected leaves immediately: bacterial leaf spot infections can be localised or systemic. Quickly remove any leaf that you notice is infected; and if the infection persists, you may need to uproot and destroy the plant entirely.
  • Collect and dispose of fallen leaves: most bacteria that cause leaf spot can survive on fallen leaves, which then become sources of reinfection. Therefore, regularly check for, and remove plant debris.
  • Air circulation: prune your plants regularly to increase ventilation and space them when planting.
  • Keep leaves dry: avoid overhead watering as well as watering your plants in the evening to minimise prolonged leaf wetness.
  • Clean plant pots: to avoid reinfection, thoroughly clean and disinfect all pots that have been in contact with contaminated plants.
  • Use organic fungicides: copper fungicides are only effective in preventing the disease, not treating it. Therefore, use them early in the season before infection breaks out.

Fire Blight

Image by Marcin Łuczak - Own work, CC BY-SA 4.0,

If you grow any plants from the Rosaceae family, such as roses, apples, pears, peaches, or nectarines, be on the lookout for fire blight disease. Indigenous to North America, but now found all around the world, this bacterial disease attacks every area of a plant, causing flowers and leaves to droop, wither, and appear burnt – hence the name of the disease.

Occurring during spring and summer months, fire blight enters plants through new growth or open flowers and then spreads to the rest of the plant. Humid conditions hasten the spread of the bacteria, with temperatures between 21°C and 27°C being the most ideal for the bacteria. When the infection reaches the roots, the plant will die.

Infected plants secrete a thick bacterial liquid during wet humid weather that insects, water, and gardening tools can transfer to healthy plants. Once infected, the bacteria remain in the plant permanently until it’s uprooted and destroyed, or, if caught early, the infected new growth is pruned and burned.

How to Treat Fire Blight

  • Prune diseased wood: remove and destroy any part of the plant that exhibits fire blight, ideally in winter when the bacteria is dormant.
  • Sterilise pruning tools: the disease can be transferred from infected tools so make sure to regularly disinfect gardening tools after use.
  • Avoid excessive nitrogen fertilisation: nitrogen fertiliser encourages new growth, which increases the occurrence of infection, particularly in the summer.
  • Avoid water splash: rain and splashing water contribute to the spread of the bacteria; therefore, irrigate your plants carefully with a soaker hose.

Aster Yellows

Aster Yellows example
Image from Missouri Botanical Garden

Sounding more like the name of a lovely flower than a disease, aster yellows is actually quite a disruptive and chronic plant infection. Its name comes from the fact that it commonly affects members of the Asteraceae family, which is the largest of all flowering plant families. Sunflowers, daisies, chrysanthemums, and marigolds all belong to this diverse plant group.

Aster yellows disease causes a range of significant damage to plants, including preventing blooms from developing colour, producing sterile flowers, and turning leaves yellow or white. It also causes stunted or deformed plant growth. Its only redeeming feature is that it rarely kills plants.

This bacterial disease lives in plant sap and is largely spread by aster leafhoppers, a sap-sucking migratory insect that is most active during the summer months. The aster leafhoppers feed on the sap by inserting their mouth probes into the tissue and transfer the infection when they move on to feed on the next plant.

How to Treat Aster Yellows

  • Uproot and destroy diseased plants: there’s no treatment for aster yellows, so once plants are infected the best move is to remove and destroy the entire plant.
  • Control aster leafhoppers: the best ways to manage aster leafhoppers in your garden are by installing mesh fabric, hanging yellow sticky traps because they’re attracted to the colour, and removing garden debris as the insects live in them during the winter.
  • Remove weeds: perennial weeds are susceptible to infection and contribute to spreading the disease, so make a habit of checking for, and removing, weeds in your garden.
  • Inspect purchased plants: carefully examine plants before you buy them to make sure that they are disease-free.
  • Plant resistant cultivars: consider planting species that are resistant to the disease, such as woody ornamentals, geraniums, cockscombs, and impatiens.

Bacterial Wilt

Similar to leaf spot diseases, plant wilt is a very common symptom of both fungal and bacterial diseases. Even hot weather and underwatering can cause plants to wilt. In addition to leaves wilting, followed by them turning yellow, both types of infections also cause stunted plant growth and eventually plant death in severe cases.

Luckily, however, there’s a very quick test for determining whether the symptoms detected in your plants are bacterial wilt or not. “Bacterial streaming” is a very quick and simple method that involves snipping off a stem from the plant and dipping it in water. If you notice a cloudy secretion oozing from the cut end of the stem that means that it is infected with bacterial wilt.

Geraniums, pothos, potatoes, tomatoes, and eggplant are most at risk of bacterial wilt. Due to the disease living in soil, activities that put contaminated soil or infected plants in direct contact with healthy plants cause it to spread. Therefore, asexual plant propagation involving an infected plant, such as using cuttings, layering, root division, grafting, or budding will cause the infection to spread.

How to Treat Bacterial Wilt

  • Uproot infected plants immediately: remove and destroy the infected plant material. And because there’s a high chance that the disease is also in the soil, it’s better to re-plant resistant cultivars.
  • Buy disease-free seeds: make sure the seeds you purchase are certified as disease-free to avoid introducing the disease into your garden.
  • Quarantine newly purchased susceptible plants: keep all new plants that are vulnerable to bacterial wilt in a separate area for at least 3 weeks after purchase to monitor their development and whether they exhibit any disease.
  • Remove susceptible weeds: weeds such as nightshade and stinging nettle are hosts of bacterial wilt; therefore, remove and discard them to prevent the spread and recurrence of infection.
  • Restrict water splash: the bacteria can be spread by water that moves soil from infected plants to healthy ones.
  • Keep gardening tools clean: regularly wash your gloves and disinfect tools that come in contact with the disease.

Crown Gall

Ranging in size from 0.3 cm to 30 cm in diameter, crown galls are rough, abnormal swellings that grow at the section where the plant stem meets the roots. They are more than unsightly masses, however, and can lead to plant death, particularly of younger ones. Galls are initially cream-coloured and darken over time as they feed on the nutrients intended for the plant, causing the plant to weaken and eventually die.

The bacteria that cause crown gall can survive in soil and inside the galls themselves for many years. Infections can only enter plants through wounds, such as those made by pruning and insects or animals, and are spread through irrigation, garden tools, and water splash. The bacteria can also spread when older galls on herbaceous plants decay and fall off, causing the bacteria to be released into the soil.

Plants susceptible to crown gall include ornamentals such as roses, geraniums, fuchsia, peonies, and chrysanthemums; fruits like peaches, apples, grapevines, and blackberries; and a variety of vegetables including beetroots and courgettes.

How to Treat Crown Gall

  • Uproot infected plants: plants do not recover from crown gall; therefore, the best approach after infection is removing and disposing of diseased plants.
  • Examine seedlings before purchase: check for swellings near the crown of seedlings as this is a good indicator of whether the new plant is healthy or not.
  • Control weeds: to avoid the recurrence and spread of the disease, remove weeds from your garden because some are susceptible to the crown gall.
  • Replace the soil in contaminated areas: the bacteria that causes crown gal can live in soil for up to 2 years; therefore, replace the old soil with healthy soil to prevent reinfection.
  • Plant resistant cultivars: to eradicate the disease, you can either grass over the area for up to 2 years or plant resistant varieties of susceptible ornamentals.

Viral Plant Diseases

Smaller than both fungi and bacteria, viruses are primarily spread by insects, fungi, seeds, and soil. They depend on living hosts for food and reproduction and so they can’t survive very long once the organism dies.

Mosaic Viruses

Mosaic Viruses Plant Example
Image by Howard F. Schwartz -, CC BY 3.0,

Named after the discoloured patterns that develop on leaf surfaces, mosaic viruses have, strangely enough, actually been known to increase the value and desirability of some plants! One of the most well-known examples is that of tulips in the 17th Century. The tulip mosaic virus caused such stunning streaks of colour on tulip bulbs that it led to the notorious tulip mania period where the price of certain tulips exceeded even that of an extravagant, furnished house.

However, mosaic viruses aren’t all fun and games. Despite not causing severe immediate damage, they do result in a gradual weakening of plants, production of fewer flowers, and shortening of stems. Plants also become more vulnerable to winter or frost death. The virus infection may also appear as yellow ring patterns or dead spots on leaves. Symptoms usually appear during spring and, once infected, plants remain diseased throughout their lifespan.

There are hundreds of different mosaic viruses. Most are spread by insects, such as aphids, and a few are transmitted through contact with contaminated plants, tools, and some pollinators, such as bees. This virus is known to affect a wide variety of ornamental plants, including petunias, chrysanthemums, roses, and marigolds. Vegetables like cucumber, tomatoes, spinach, and cauliflower are also regularly infected by the virus.

How to Treat Mosaic Virus
The only way to treat mosaic virus is to avoid it entirely because it’s incurable.

  • Purchase certified disease-free seeds and seedlings: the best way to keep the disease out of your garden is to make sure that all seeds and plants that you purchase bring into your garden have been verified as disease-free.
  • Remove infected plants: uproot all infected plants and either burn or dispose of them in the rubbish bin.
  • Sterilise gardening tools: disinfect all gardening equipment and gear that comes in contact with contaminated plants.

Impatiens Necrotic Spot Virus

Known to principally afflict the vibrant and beautiful impatiens species of plants, impatiens necrotic spot virus has also been observed in a wide variety of other ornamentals including asters, begonias, dahlias, calla lilies, chrysanthemums, and petunias. It’s spread by tiny insects called thrips, specifically western flower thrips, that are most active in dry, hot temperatures and are particularly drawn to blue and yellow flowers.

Thrips infect plants within 15 to 30 minutes of feeding on them and, once infected, the virus spreads quickly throughout the rest of the plant. Symptoms of the virus include plant wilt, stunted growth, yellowing of foliage, and browning of stems. Dead spots that some botanists liken to chickenpox in appearance also develop on leaves.

One of the best indicators of a possible infection of impatiens necrotic spot virus is simply the presence of thrips. They are easy to spot and multiply rapidly. Due to their temperatures, greenhouses provide favourable conditions for these insects to spread the virus, and even a small number of them can quickly infect an entire greenhouse of plants.

How to Treat Impatiens Necrotic Spot Virus

  • Inspect newly purchased plants: check for any symptoms of infection and the presence of thrips when buying new plants, and if possible purchase certified disease-free plants. Additionally, just to be safe, place new plants in isolation for at least 3 weeks to make sure they’re healthy.
  • Space plants: distance between plants limits disease contamination.
  • Control thrips in your garden: there are several ways to manage thrips in your garden, including using insect screens as well as blue-coloured sticky traps. Western flower trips are particularly attracted to the colour blue. Pyrethrins, which are natural pesticides found in some chrysanthemums flowers, are also an effective option.
  • Eliminate weeds: this virus also affects a variety of weeds, including chickweed, jewelweed, oxalis, and gill-over-the-ground. Therefore, remove them as soon as you spot them.
  • Plant resistant species: one of the best ways to prevent impatiens necrotic spot virus is to plant cultivars that are not susceptible to the disease.

Plant Diseases Commonly Found in Australia

Over the years, Australia has done a remarkable job of keeping many plant diseases out of the country. However, due to weather and other environmental conditions, some diseases simply can’t be avoided and occur all over the world, and a few others are native to the country.

Phytophthora Dieback

Once thought to be a fungus, Phytophthora cinnamomi, which causes Phytophthora dieback, was later reclassified as a water mould. Water moulds, also known as oomycete, are extremely similar to fungi in most ways except that water moulds have cellulose in their cell walls, while fungi have chitin.

Phytophthora dieback is one of the more deadlier plant diseases, so much so that the word Phytophthora in Greek literally means ‘plant destroyer’. Commonly affecting banksias and rhododendrons, this disease attacks plants from their roots, stealing their nutrients, and causing root rot. In dry, hot conditions, Phytophthora dieback can lead to death in susceptible plants.

Found all across Australia, symptoms of this mould include plant wilt, dark stem discolouration, and yellowing of leaves. It’s spread through contact with infected roots and movement of infected soil, and the mould can actually swim short distances in water to infect healthy plants.

How to Treat Phytophthora Dieback

  • Remove infected leaves: promptly remove leaves exhibiting any symptoms and if the disease continues to spread, then remove the whole plant.
  • Use mulch: water moulds can live in both water and wet soil; therefore, apply mulch on plants to soak up water after rains or watering to prevent transmission of the virus to healthy plants.
  • Keep leaves dry: avoid overhead watering if possible and water your plants earlier in the day to allow leaves more time to dry.
  • Clean all gardening equipment: disinfect garden tools and thoroughly clean gloves, shoes, and any other equipment that come into contact with the disease to prevent or limit the spread of the infection
  • Use fungicide: phosphite is currently the only known fungicide that is effective in managing this disease. Maintained to be safe and biodegradable, this fungicide works by inhibiting the fungus and enabling the plant’s defence system to fight the disease.

Fusarium Wilt

Known to have been attacking banana plants in Australia since 1874, Fusarium wilt disease is also a menace to carnations, gladiolus, begonias, and asters, among many other ornamental plants. It causes stunted growth, leaf wilt leaves, yellowing of leaves, and death to the entire part of the plant that is visible above ground.

The symptoms of Fusarium wilt are quite similar to those of Verticillium wilt, but there’s an easy way to determine which of the two has infected your plants. Cut off a piece of the stem, slice it open lengthways, and check whether it has dark streaks inside. If so, that’s a sure indicator of Fusarium wilt.

The fungus can live in soil for decades, and can easily be spread through contact with contaminated soil, tools, and shoes and by water splash. The spores can also infect seeds, and seedlings that are contaminated often die. Ideal conditions for the development of Fusarium wilt are soil temperatures of 24°C and above. However, it can survive even in arctic or desert environments while waiting for the right conditions in which to become active again.

How to Treat Fusarium Wilt

  • Uproot infected plants: since the disease attacks plant tissue, pruning isn’t effective. The only way to treat a diseased plant is to remove it entirely.
  • Maintain plant health: stressors such as overwatering or underwatering, and poor nutrition make plants vulnerable to attacks by the fungus that causes fusarium wilt.
  • Clean tools and gear: the fungus is spread via soil, water splash, and contact with contaminated equipment. Therefore, make sure to clean your gloves, shoes, and clothing and to regularly disinfect your garden tools.
  • Remove weeds: fusarium wilt can survive on weeds, such as pigweed and others. So regularly inspect for, and eliminate, weeds in your garden.
  • Plant resistant species: although chrysanthemums, gerberas, and marguerite daisies are susceptible to Fusarium wilt, many varieties of these plants have been bred to be resistant to the fungus.

Myrtle Rust

Example of Myrtle Rust on Plant
Image by User:SmallBiologie - Own work, CC BY-SA 4.0,

Initially detected in New South Wales in 2010, myrtle rust is a fast-spreading fungal disease that is now found across Victoria, Queensland, Tasmania and some places in the Northern Territory. It is known to attack a wide range of Australia’s beloved native plant species, including eucalyptus, tea tree, willow myrtle, and bottlebrush.

Although myrtle rust only attacks members of the Myrtaceae family, it’s very unique in that it infects a wide range of species within the family. This is because rust fungi are usually host-specific, which means they can attack only one kind of plant in a family. However, myrtle rust is known to infect approximately 400 species of plants in the Myrtaceae family.

The first typical symptom of this fungal disease is the appearance of dark raised spots with a purplish edge on new leaves and stems. This is followed by the appearance of powdery yellow pustules that in severe infections can grow larger, leading to leaf deformation, defoliation, and, in a few cases, plant death. The fungus is spread via wind and contaminated tools, and it favours wet, dark conditions of between 15°C and 25°C.

How to Treat Myrtle Rust
Fungicides aren’t the best course of action, as they don’t kill the virus but rather only help to reduce spore production. Therefore, re-infection is common after fungicide application. Below are the best ways to control myrtle rust:

  • Remove infected plant: once you discover myrtle rust on a plant, uproot the entire plant immediately. To prevent fungal spores from spreading, cover the plant with a plastic bag before removing it. If the plant was potted, get rid of the pot and soil as well.
  • Clean garden tools: disinfect and wash any equipment and gear that comes into contact with infected plants.
  • Plant resistant species: since this disease only attacks plants in the Myrtaceae species, you may want to avoid them altogether or plant varieties that are resistant to the fungus.

Clubroot Disease

As its name implies, clubroot causes the roots of plants to swell and become misshapen or club-like. Hindering the uptake of water and nutrients, this fungal disease causes wilting, yellowing, and stunted growth in infected plants. Susceptible plants include stocks, alyssum, wallflowers, broccoli, cauliflower, cabbage, and other plants in the Cruciferae family.

Some plants may not exhibit any signs above ground until the infection is very serious, but the roots will always show symptoms. Therefore, the best way to diagnose the disease is to carefully pull up the plant without damaging the root system and examine the state of the roots.

Once infected, the plant’s roots start to rot and disintegrate, leading to the fungal spores being released into the soil where they can survive for up to 20 years. The disease favours wet acidic soils and it can spread through contaminated tools and gear, infected cuttings and transplants, and even soil on shoes.

How to Treat Clubroot Disease

  • Remove infected plants: once plants have become infected, remove and dispose of them immediately.
  • Apply calcium and boron: calcium and boron are effective in protecting seedlings from being attacked by the fungus that causes clubroot and should be applied early in the season.
  • Use well-draining soil: ensure good soil drainage by either planting susceptible species in raised beds or using a well-draining soil mixture.
  • Adding lime to soil: alkaline soil inhibits the growth of the fungus that causes clubroot. Therefore, check your soil’s pH before planting any susceptible species and if it’s acidic, add lime at least 2 months before planting to ensure that the alkalising process has time to take effect. Be careful not to alkalise it too much as that will cause other problems. A pH of 7.0 – 7.5 is sufficient.


Bacteria, fungi, and viruses have existed since the beginning of time and, although it may not initially seem like it, we actually need them in our ecosystems because they drive many important processes. They help to breakdown and recycle waste, they support the health of various organisms, and the maintenance of environmental balance.

However, these pathogens also do cause disease and one of the best ways to keep most plant diseases out of your garden is by maintaining sanitation. Sterilise all garden tools that come into contact with plants and soil, and wash your gloves, shoes, clothing, carts, and benches. Replace pots that have held diseased plants because it can be difficult to completely eliminate the infection from plant containers.

Additionally, make sure to inspect your plants for any signs of disease before purchasing them and continue to do so regularly after planting them. Check out Flowers Across Brisbane for healthy plants, cut flowers, and bouquets. They also offer the superb advantage of same-day delivery.