10 Common Lawn & Plant Diseases & How to Treat Them

With more than 20 years of professional horticultural experience under his belt, Rob Kerslake of Verduous Gardens shares 10 common lawn and plant diseases in Australia and how to treat and prevent them.

Fairy Ring

While magical to kids, fairy rings are rarely a welcome sight on most lawns. Fortunately, they rarely lead to widespread damage and are typically just an irritation for those who appreciate a uniform lawn.

"The main signs to look for are rings of mushrooms, dark green patches of grass or thinning or dying grass in the centre of a ring or arc."

While found in all lawn types, fairy rings tend to prefer lawns with thick thatch, low moisture levels, and low fertility soils. The easiest way to minimise their formation is to reduce thatch on your lawn with a dethatcher, boost air and water flow into the ground by aerating, and regularly irrigate. It's also good to avoid using fertilisers and lawn feeds with large amounts of organic undercomposed materials.

Grass Rust

Grass rust is a common fungal disease that affects many types of grasses. Lawn rust is most commonly seen in late summer and autumn when the weather is warm and dry.

"Rust appears as orange or brown patches on grass blades and can cause the leaves to turn yellow and die. If left untreated, rust can kill your entire lawn."

Rob also notes that the quickest way to treat grass rust is to "water your lawn deeply and regularly as this will help to reduce the number of spores in the air and stop the spread of the disease." Other ways to prevent lawn rust include mowing your lawn regularly to keep the grass short with a robot mower such as an Automower®. You should also do any watering in the morning so that the grass has time to dry out during the day and clear up any dead leaves from your lawn as they can harbour spores.

Armyworms

In Australia, Lawn Armyworm (Spodoptera mauritia) is the most prevalent species of caterpillar. These caterpillars strip leaves from plants, leaving behind a tell-tale "windowpane" effect. Rob notes that there's a preventative granular product you can buy from bunnings called RICHGRO Lawn, Beetle and Grub Killa, that slowly releases chemicals to rid your lawn of the pest.

"Moist soil makes it a lot easier for them to breed & spread, and the only way to really deal with an infestation is with chemical insecticides."

"Moist soil makes it a lot easier for them to breed & spread, and the only way to really deal with an infestation is with chemical insecticides."

However, there are preventative measures you can take to minimise the risk of Armyworm infestations, such as applying dolomite to your lawn and borders, which creates an environment that repels them. Keeping grass nicely trimmed with a grass trimmer in corners and along fences and borders will also help remove many of the places the moths like to lay their eggs.

Black Beetle

Another small yet destructive lawn pest that can wreak havoc on your grass is black beetles, which can often be found in the roots of Buffalo, Couch and Kikuyu varieties. A tell-tale sign your lawn has black beetles is when it starts to yellow and takes on a straw-like appearance.

According to Rob, "you'll be alerted of a potential infestation when you see little black beetles on concrete or stones near the lawn. To confirm black beetles, dig up a shovel of dirt and you should see lots of them on the shovel - only apply treatment when you see an infestation, not if you see just the odd black beetle."

Another way to check for them is to fill a bucket with soapy water and empty it on the lawn. If more than 20 beetles appear, you could have a problem. If you do have an infestation you need to treat, a wettable powder insecticide and a sprayer can make quick work of them.

Powdery Mildew

Powdery mildew is a lawn disease caused by Erysiphe cichoracearum, a fungi that thrives in warm, humid conditions.

"The fungus spreads easily through the air and can infect new plants quickly. The tell-tale signs of powdery mildew are white or grey powdery patches on the leaves and stems of infected plants."

While not all powdery mildews are damaging, they can cause significant damage to plants if left untreated.

To prevent powdery mildew from taking hold in your garden, Rob recommends watering early in the day so that the leaves have time to dry before nightfall. Avoid wetting the foliage when you water and remove any infected leaves or plant parts as soon as you see them. If powdery mildew does become a problem, there are a number of fungicides available from places like bunnings.

Downy Mildew

Downy mildew is a disease caused by a fungus-like organism called an oomycete, which thrives in cool, damp conditions. Downy mildew can cause leaves to turn yellow or brown, and they may eventually drop off the plant. The disease can also cause stunted growth, and in severe cases, it can kill the plant. To prevent downy mildew, water your lawn and plants in the early morning so that the leaves have time to dry off before evening.

If you live in an area where downy mildew is common, avoid planting susceptible plants next to each other. Rob recommends removing any affected leaves and disposing of them immediately. Incineration is the best way to dispose of organic material carrying downy mildew. Composting should be avoided as the spores can take hold wherever the compost is used at a later date.

Red Thread

As a professional lawn care expert, Red Thread disease is a common lawn and plant disease Rob has seen many times. Caused by Laetisaria fuciformis fungi, the disease gets its name from the distinct reddish-pink colour of the affected leaves. He follows on that tell-tale signs include small, reddish-pink lesions on the leaves of affected plants.

"These lesions will eventually coalesce to form large, pinkish-red patches. In severe cases, the entire leaf may turn reddish-pink and die."

Red thread can also affect grass, causing it to take on a reddish-pink hue in patches. The disease is most commonly seen in cool-season grasses and wet weather conditions, usually around the end of autumn to the start of spring and can affect both lawns and gardens. To treat Red Thread, Rob recommends removing affected leaves and applying a fungicide containing chlorothalonil or mancozeb.

Fusarium Wilt

Fusarium wilt is a soil-borne plant disease that can affect both turfgrass and plants. The pathogen enters the plant through its roots and then moves up into the stem, where it blocks the flow of water and nutrients. This can cause the leaves to turn yellow or brown and eventually die. Fusarium wilt is most common in warm, humid climates and has recently been found in the Northern Territories and far North Queensland.

There are several different strains of Fusarium, each of which can affect different types of plants. For example, one strain may infect only tomatoes while another may affect only roses. Treatment for fusarium wilt depends on the type of plant affected. For turfgrass, fungicides may be applied to the soil to prevent the disease from spreading. For ornamentals, affected plants may need to be removed and destroyed. However, for now, the disease is being contained in small areas within Australia and is not currently a widespread issue.

Anthracnose

Anthracnose is a common plant disease that affects a wide range of trees, shrubs, and turfgrasses. Rob mentions that symptoms typically include leaf spots, blighted leaves and stems, as well as defoliation. This disease is most often caused by the fungi Colletotrichum spp and Gloeosporium spp and thrives in wet, humid conditions. He also notes that it can be difficult to control once it takes hold, but luckily,

"There are a few things you can do to treat this disease and keep it from coming back."

To treat anthracnose, remove and destroy infected plant parts as soon as you see them. Incineration is best as it will kill the spores. If you cannot burn the infected plant material, dispose of it in a sealed bag in the trash.

Rob also recommends to "Clean up any fallen leaves or debris from around the affected plant as these can harbour spores and lead to reinfection. Regular pruning and mowing will ensure your plants are getting enough air circulation, reducing the levels of moisture and humidity that anthracnose thrives in."

Botrytis

Caused by the Botrytis cinerea fungus, it thrives in warm, moist conditions causing leaf spots, stem rot, and flower death, it is most commonly seen on roses, annual flowers, and vegetables. Signs of the fungus include small, brown or grey spots on leaves that may merge together and cause the leaves to turn yellow or brown and eventually drop off. Botrytis can also cause stem rot, and flower buds may turn brown and die before they open.

If you suspect Botrytis in your garden, Rob suggests removing any affected leaves, stems, and flowers from the plant. Destroy them by burning them or placing them in a sealed bag in the trash. Do not compost them as Botrytis spores can overwinter in plant debris. It's also recommended to water your plants at ground level so that the foliage does not stay wet for long periods of time. If possible, increase air circulation around your plants by trimming back dense ground-level foliage, especially with dense hedges and shrubs.

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