The Ultimate Guide to Plant Diseases and How To Treat Them

Powdery Mildew

• 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

• 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

• 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)

• 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

• 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

• 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

• 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 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

• 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

• 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

• 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

• 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.

Mosaic Viruses

• 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.

• 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.

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

• 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

• 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

• 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.