How to Diagnose Common Problems of Plants

Plants sometimes get problems because they all have diverse needs, and can be susceptible to different issues.

Plants need the right amount of sunlight, water, humidity, nutrients, soil conditions, temperature and protection. What might be ideal for one plant, is no good for another. So plant selection, position in the garden or home, soil preparation and plant care are all important to avoid plant problems.

Head to the How-to-Grow Guide to find out what your plant needs.

Plant problems can be divided into a few different categories including environmental issues, pests, diseases and natural events.

Head to our Garden Problem Solver for advice on specific pests, diseases, and other problems of plants.

Environmental issues of plants can be related to weather, soil and microclimate conditions, as well as chemical contamination.

Weather conditions include wind, humidity, frost, hail, snow, rain, drought, heat, cold, sunburn, and lack of light.

Soil conditions include drainage, soil type (i.e., clay or sandy soil), soil quality, soil structure, soil nutrient levels, soil pH, and soil pollution. Go to Beginner's Guide to Soil & Soil Improvement for more info.

Microclimate conditions include nearby structures (e.g. a brick wall creating radiant heat) and plants, the slope of the land, and aspect (i.e., north, south, east, west).

Chemical issues such as herbicide and pesticide spray drift, or contamination of soil or water. One common contamination issue occurs when new concrete is laid and lime leaches out from the concrete into the soil and raises the soil pH, starving plants of nutrients.

Plant natural events are just the way plants grow, reproduce, and die, as well as the timing and speed of these events.

Plant natural features are everything that makes up a plant such as leaves, flowers, and roots, etc.

Garden pests include any animal that attacks a plant to it's detriment. Pests can be large (e.g., Rats, Mice, Possums and Birds), small (e.g., Caterpillars and Aphids) or nearly or completely invisible to the naked eye (e.g., some Mites, and Nematodes).

Garden diseases can be bacterial (e.g. Bacterial Spot and Bacterial Wilt), fungal (e.g. Downy Mildew and Sooty Mould), viral (e.g. Rose Mosaic Virus) and various other diseases such as Phytophthora Root Rot (oomycetes).

You'll know if your plant has a problem if you can see a plant symptom or by the signs of pests and diseases.

Plant symptoms are the visible changes in a plant in reaction to a plant problem. Visible change might be in plant colour, death of tissue (e.g. brown spots, dieback of stems, or whole plant death), shape (e.g., twisted, shrivelled, stunted, etc.) or how the plant is functioning in growth and reproduction (e.g., flowering and fruiting).

Signs of pests and diseases are different to plant symptoms. Symptoms are visible changes within the plant, while signs are related to what the pest or disease is producing or the actual pest or disease itself.

Signs of diseases can be spores, mycelium, pustules, rotting, foul smell (especially for Root Rot diseases), or oozing.

Spores can be all different colours such as orange (e.g., Rusts), white (e.g., Powdery Mildew), or black (e,g, Sooty Mould). Some spores can be easily brushed off, while others are more like lesions inside the leaf (e.g., Rose Black Spot, and Early Blight).

Signs of pests include visible pests, dead pests, faeces, frass, webbing, silk, secretions and tracks. 

Visible pests at various stages of their lifecycle including adult, larva or instar, egg, and pupa.

Some pests are often found at the tips of stems and leaves (e.g., Aphids), on the underside of leaves (e.g., Thrips, Mites and Lacebugs), or hiding down in the stem junctions (e.g., Mealybugs).

Dead pests can be their cast skins (e.g., Mites), or mummified from predation (e.g. Aphids).

Faeces can be of different shapes and textures. For example, Snails and Slugs have long skinny poos, while Mites and Thrips have sticky black blobs found on the underside of leaves.

Frass can be a mixture of poo and other materials. Borers, and Webbing Caterpillars are often seen with frass.

Pests that produce webbing or silk include Caterpillars, some Mites, and Spiders.

The most commonly found secretion of pests is a clear, sticky and sweet substance known as honeydew. Pests that produce honeydew include Aphids, Mealybugs, Leafhoppers, Psyllids, soft Scales, and Whiteflies. Honeydew is often secreted to feed Ants in exchange for protection from predators. Excess honeydew also encourages the disease Sooty Mould.

Defence secretions can stinky and painful (e.g., Bronze Orange Bug).

Pest tracks are more easily seen for larger pests such as Rats and Mice where you might find foot and tail marks, but it can also be the damage to the plant caused by tunnelling (e.g. the squiggly lines on leaves by Citrus Leaf Miner or on the trunk by Scribbly Gum Moth).

If your plant isn't growing but is otherwise healthy, then the problem could be due to lack of feeding, incorrect soil pH, lack of light, temperature including heat stress or cold weather, wind, plant dormancy, or transplant shock. If your plant is flowering or fruiting, this might also impact plant leaf and stem growth.

If your plant isn't growing and seems stunted and unhealthy, look all over the plant for other signs and symptoms of pests and diseases.

There are so many causes for a dying plant and you'll first need to ask yourself some broad questions to narrow down the cause, like... did you put it in the right spot? did you give the plant everything that it needed to survive? is there a pest or disease on the plant? and did something happen to your plant?

Every plant needs different things, what works with one plant, may not work with another.

Head to the How-to-Grow-Guide for information on where to position your plant; light, watering and feeding requirements; and common problems that are specific to your plant.

Curling or wilting leaves and stems can be due to pests, diseases, watering or light issues, incorrect nutrients, soil pH issue, transplant shock, or chemicals.

Sap-sucking pests such as Aphids, Mealybugs, and Mites suck out the plant juices causing the leaves to curl and stems to wilt. Curling leaves on citrus is most likely Citrus Leaf Miner.

Various plant diseases cause leaves to curl, such as Peach Leaf Curl of stone fruits like Peaches and Nectarines; and Root Rot - this causes all the leaves to wilt and mostly happens after prolonged and heavy rain.

Transplant shock can cause the whole plant to wilt after being transplanted. 

Overwatering causes the tip of the leaf to curl downwards.

Sunburn or heat stress can cause leaf curl.

Chemicals, either applied directly to plant or sprayed nearby, can cause leaf curl. For some plant protection products, such as horticultural oils, always use the product according to label instructions. And, if you, or your neighbour, are spraying with herbicides be sure to use the sprayer as close to the weed as possible to prevent any spray drifting on to plants.

Take a look all over the plant, as well as into the soil, to see if you can find more plant symptoms as curling leaves curling is usually accompanied by other plant symptoms.

Yellow leaves can be caused by a lack of sunlight, sunburn, a watering issue, natural leaf death, or that the plant is moving into the deciduous phase.

Lack of sunlight is often associated with stunted growth, stretched growth where there is more distance between each growth point, leaf drop, or poor flowering and fruiting.

Sunburn will happen suddenly after heatwave conditions, or if you have planted in the wrong positions, or if you have moved the plant into a sunny position too soon. It's likely to be seen on the sides of the plant that gets the most sunlight, especially the hot afternoon sun.

Watering issues can have very similar symptoms. Underwatering usually starts with wilting foliage, dry crispy leaves, brown leaf tips, dry soil or shrunken potting mix. Overwatering will usually be found with rotting roots, foul-smelling soil/potting mix, 

Garden pests, especially sap-sucking pests like Aphids, Mealybugs, and Mites, and plant diseases (e.g., Rose Mosaic Virus) can cause leaves to go yellow and are mostly found sporadically on new leaves where most of the sugars are being produced.

Nutrient issues either from lack of or excess fertiliser, and/or a soil pH issue, can cause yellowing leaves. Leaves usually develop a consistent pattern, and will either be found on most of the new leaves or most of the old leaves.

If you notice there is only one branch with a yellow or white pattern on the leaves, this could be a mutation that has caused leaf variegation. You can simply prune these off if they are unsightly, or prune them off and try to propagate it.

A watering or humidity issue, commonly seen in indoor plants, causes brown leaf tips. Root Rot diseases will also cause brown leaf tips, but this is also associated with drooping leaves, leaf drop, and dieback.

Brown and crispy leaves soonafter extreme heat and/or sunlight is heat and drought stress, as well as Sunburn.

Some deciduous plant leaves will change colour and then go brown.

Nutrient deficiencies or toxicities as well as soil pH issues can also cause brown leaves. Initial symptoms will start with a pattern of yellowing first.

Some leaves are naturally purple, and some may turn purple, or more purple during winter in preparation for the deciduous phase.

Silvery leaves are usually caused by Mites or Thrips.

Most dark spots on leaves are caused by diseases such as Leaf Spot and Rose Black Spot.

Some pests can chew at the surface of leaves causing circular spots.

Soil pH issues, and nutrient deficiency and toxicity, such as molybdenum, are rare but can cause leaf spots.

Spray drift of contact herbicides can cause spots.

Weather conditions such as wind, heat, frost, as well as a watering or light issue can cause premature leaf drop, as well as Root Rot diseases.

Holes in plant leaves are most likely caused diseases such as Shot Hole; or by chewing pests such as Caterpillars, Beetles, Earwigs, Grasshoppers, Snails or Slugs. Physical injury from hail and wind can also cause holes in plant leaves.

If the lumps and bumps are small and can be scared off, or squished, then it's probably the pest Scale.

If the lumps and bumps are part of the stem, this is most likely a Gall. Galls can be found on leaves, stems and branches. Galls can be caused by a variety of pests and diseases. For example, swollen stems and branches of Citrus can be the pest Citrus Gall Wasp; while the stem tips of Azaleas with strange growth are Azalea Leaf Galls.

Injury to stems and branches can cause lumps or bumps as the plant grows and tries to seal off (compartmentalise) the area.

The most common cause for circling tree roots is due to poor plant selection. If a potted tree runs out of root space it will grow roots in a circular direction. These roots will continue to circle even when planted in the ground. This can make the plant unstable and likely to fall over, and the plant may be starved of water and nutrients because all the roots are so close to together.

When purchasing or growing trees in pots, make sure the plant is not pot-bound with circling roots.

When planting in the ground, dig a hole at least two times the width of the pot, and avoid planting in clay or compacted soils. This will ensure the roots can spread into the soil.

Bumps on roots can be caused by microbes, pests, diseases, injury, or specialised root storage organs.

Some plants, such as Chick Peas, have a beneficial relationship with microbes (symbiosis) that cause nodules on roots. As can pests, such as the Root Knot Nematode which are detrimental to plant growth and health.

Rotting roots can be caused by diseases such as Root Rot and Damping Off, a watering or drainage issue especially in clay or compacted soil, and planting too deep.

Plant establishment - after planting, some plants need time to settle into their new environment before they can invest energy in flowering.

Plant age - some plants need time to mature before flowering.

Time of year - not all plants flower all year round, most are seasonal, and some might only flower every other year.

Environment - heat, frost, drought, lack of light or water.

Fertiliser - make sure you are not using high-nitrogen all-purpose fertilisers as this will promote only leafy growth and prevent flowering and fruiting.

Pruning - some plants (e.g., Roses) need pruning to help promote new growth and better flowering.

A plant that flowers but is not setting fruit may be caused by a lack of pollinators such as Bees and Butterflies; a lack of cross-pollinators (e.g., Apples and Pears); lack of care such as feeding and watering; incorrect soil pH; poor weather conditions such as hot or windy weather; plant age - some plants need time to fully mature and bear fruit; and pests and diseases.

An unhealthy-looking plant that is flowering really well is a sign of plant stress or that the plant is near death - either as a natural part of aging or from stress.

Make sure the plant is getting everything that it needs like watering, feeding, and check for pests and diseases.

For seeds to germinate, each plant has unique and varying requirements in temperature, moisture, darkness, light, and sowing depth. Some native plants need ash to germinate. Some seeds of plants may be dormant and need to be sacrificed or soaked. Some pests and diseases can also stop seeds from germinating. For more info on how to grow plants from seed click here.

Tree trunks and branches oozing sap could indicate a pest, disease, injury or for natural reasons.

Pests, such as Borers and White Louse Scale, and diseases, such Root & Collar Rots and Cankers. Look for other signs and symptoms of pests and diseases.

When the deeper layers of a tree are injured, this can cause oozing sap, and are also a site for infection by pests and diseases. Injury can be the result of environmental stress such as drought; or from physical damage such as wind, animal activity, pruning, ringbarking, or grafting.

Deciduous trees naturally ooze sap in spring at the end of dormancy.

Tree trunk covered in a white substance could be from a pest, disease, or is naturally occurring.

White Louse Scale is a tiny hard and white Scale that covers the trunk and branches of trees.

Lichens are a symbiotic relationship between fungi and cyanobacteria. These come in an array of colours and types, some of which are white. These are not detrimental to plant health and do not necessarily need control.

Some plants, including Bamboo, Eucalyptus, and Poplars, exude a wax-like substance (glaucous coating) on their trunks and branches that can be rubbed off. This is naturally produced by the plant.