Become a pest and disease detective!

After putting in so much work to get your plants established, it is important to inspect them regularly for signs of pests and diseases.  When diagnosing plant damage, first you need to gather information and clues.    

To narrow down and diagnose the most logical cause of your plant damage, combine careful observation and inspection.  Treat the problem when you are sure you have an accurate diagnosis. 

Steps to diagnosing your plant problems 

Step 1. Define the problem   

• Identify the plant and its “normal” characteristics so you know what it should look like at this time of year.  

• Examine the entire plant and the plants around it.  What is the main problem? For example, the leaves are being eaten, or the leaves have powdery white spots. Which parts of the plant are affected?  Describe the symptoms and signs of the "abnormality". 

Step 2. Look for patterns  

Is the problem on more than one plant or on more than one plant species?  What type of damage is it? 

• Non-uniform damage pattern (i.e. scattered damage on one or only a few plant species). is suggests LIVING FACTORS (insects or bacteria, virus, or other microorganism that can cause disease).  

• Uniform pattern on the individual plant and plant parts or uniform damage pattern over a large area (i.e. damage patterns on several plant species) suggests NON-LIVING FACTORS (mechanical, physical, or chemical factors).  

Step 3. Describe the time-development of the damage pattern  

• Gradual spread of the damage on a plant, onto other plants, or over an area over time indicates damage caused by LIVING FACTORS  

• Damage does not spread to other plants or parts of the affected plant. Clear line of separation between damage and no damage. These clues indicate NON-LIVING FACTORS. 

Step 4. Decide the cause of the plant damage.  

• Decide which LIVING FACTOR is causing the problem - signs and symptoms 

• Decide which NON-LIVING FACTOR is causing the problem – signs and symptoms 

USE REFERENCE BOOKS OR THE INTERNET TO HELP YOU WITH IDENTIFICATION. 

Non-Living factors 

• Mechanical factors e.g. the stems or roots have been broken or the leaves have been bruised, punctured, or broken 

• Physical factors e.g. environmental factors such as extremes of temperature, light, moisture, oxygen 

• Chemical factors e.g.  phytotoxic chemicals (such as trace metals or pesticides) or nutritional disorders 

Living factors 

• Pathogens – Fungal, Bacteria, Virus, nematodes 

• Insects – Chewing, sucking 

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Some common garden pests and diseases  

Aphids 

Small sap-sucking insects (common names include greenfly and blackfly), which use their mouthpart to pierce stems, leaves and other tender parts of a plant and suck out fluids.  Aphids cause a plant’s growth to be stunted or distorted (they are often found on newest plant growth) and often excrete a sticky substance on which moulds can grow. 

Treatment and prevention  

• Ladybirds eat aphids so attract ladybirds into your garden. 

• African marigold has a strong scent and masks the smell of plants especially tomatoes well, making it harder for aphids to find them! 

• Plants from the allium family (including onion and garlic) have a strong smell which aphids don’t like.  Grow these near plants you want to protect.  

• Mild solution of water mixed with a little washing up liquid sprayed directly onto leaves acts as an irritant to the aphids.  

Cabbage white caterpillar 

There are two cabbage-white butterflies that do serious damage: the large white (Pieris brassicae), and the small white (Pieris rapae). Damage can also be caused by moth caterpillars. The cabbage moth (Mamestra brassicae) is the most common.  Cabbage white caterpillars feed mainly on cabbage family plants. 

Treatment and prevention  

• Rub off any eggs you discover and pick off and destroy any caterpillars you find. 

• Treat plants as early as possible, before the caterpillar has a chance to get inside.  

• To find the caterpillars and eggs you will have to search the plants, especially in the centre. 

• You could try an organic spray based on nematodes, such as Nemasys Caterpillar Killer. 

• Cover plants with insect proof netting when planted. Make sure there are no gaps. Use hoops or canes to support the netting above the plants. 

Slugs and snails  

One of the most common garden pests, they can quickly eat and destroy a wide range of plants, especially those with young and soft leaves. Their silvery trail can often be seen across affected leaves or along the ground near to affected plants. 

Treatment and prevention 

You will never be free of slugs and snails! Priority is to protect vulnerable plants. But, if you come across a slug or a snail you would be well advised to pick it up and dispose of it. Don’t throw them over the fence – they have a ‘homing’ instinct and will come back! There are numerous species of slugs, not all eat and damage plants. Some are carnivorous and actually eat other slugs. Many of the damaging slugs spend most of their time living in the soil.  

• Surface and underground control: 

• There are many slug repellent controls that are placed on the surface and act as a barrier to the slugs and snails getting at the plant. These include eggshells, grit and even coal fire ash. These have limited success and are no control for those slugs living in the soil. 

• Some slug controls are applied as a liquid, watered around vulnerable plants. These can will deal with all those slugs that can’t be seen, as well as those slugs and snails that can. Being a liquid, they are invisible to pets and children. 

• Biological control 

• Not effective at surface level, but an option for underground control. Nematodes (minute parasitic worms) can be watered into the soil and can help control slugs. They penetrate the slug and kill it. They are only effective against slugs and have no effect on snails. They are available from numerous mail order biological control suppliers, such as Green Gardener. 

Powdery mildew (Fungal disease) 

Starts as white powdery spots.  The spots will spread to cover most of the leaves or affected areas (usually covers the upper part of the leaves and affects the older leaves first) which turn yellow and dry out. The mildew that forms is made up of many spores. These spores carry the infection to other plants through the wind.  Powdery mildew can drastically reduce the productivity of plants. 

Treatment and prevention 

• Remove all the infected plant parts and destroy them. (Remember the disease can still be spread by the wind). 

• Choose plants that are resistant or tolerant to powdery mildew. 

• Reduce humidity. Powdery mildew thrives in hot and humid weather, so avoid overhead watering to reduce humidity.  

• Increase air circulation. 

• Spray infected plants with:  

• Fungicides such as sulphur or lime-sulphur. 

• If you don’t want to use fungicides use a solution of neem oil, vinegar and water or baking soda  

Blossom end rot 

Caused by a low concentration of calcium in the fruit which usually occurs when there are wide fluctuations of moisture available to the plant.  The fluctuations reduce uptake and movement of calcium into the plant. Usually blossom-end rot occurs when the fruit is green or ripening. It starts with a small, depressed, water-soaked area on the blossom end of the fruit. As the spot enlarges, it becomes sunken and turns black or dark leathery brown in colour. 

Treatment and prevention 

• Remove the affected fruit. The plant might blossom again and produce normal fruit. 

• Apply a liquid calcium fertilizer after removing the affected fruit. 

• Good soil preparation can prevent the problem occurring 

• Maintain a soil pH around 6.5.  

• Lime the soil to increase the ratio of calcium  

• Add crushed eggshells to the transplant hole to fortify calcium intake.  

• Maintain a more uniform moisture supply. Water little and often   

• Use mulches and/or irrigation to avoid drought stress.  

• If it’s rainy, ensure plants have good drainage and soil dries out (don’t stop watering).  

• Avoid cultivating, or hoeing, near the roots of tomato plants.  

White mould (fungal disease)  

Affects over 300 different plants (including beans, peas, lettuce, and members of the cabbage family).  The virus can appear on flowers, stems, leaves, and pods. Plants are most susceptible during flowering, but young seedlings are also very vulnerable. White mould typically infects the plants early in the spring or summer and can develop unnoticed for a while. White mould releases spores when the weather is cool, and these spores infect other plants.  

Symptoms vary depending on the environment and type of plant:  

• The stem might first appear to have a water-soaked part. At this point in the infection, the plant will look healthy from above. 

• Leaves will wilt, turn yellow, and die; stems and pods may rot. 

• Wilting of individual stems, especially at the base with light brown discolouration/light brown to dark brown lesions.  

• A dense, cotton-like growth will form from these lesions (If conditions are humid). 

Treatment and prevention 

• Destroy diseased plants immediately. It is important to catch white mould and destroy infected plants quickly. 

• Remove as much soil as possible in case it is also infected. Replace it with clean soil (You can also use a barrier, such as plastic or mulch, to cover the infected ground). 

• Use well-drained soil.   

• Space your plants properly.  

• Try not to water the tops of your plants. Or water the plants early in the day so they have the chance to dry.  

• Control your weeds. Weeds can host this disease and spread it to your plants. 

• You can spray your plants with an approved fungicide as a preventative.  

• Spores are long-lasting, so they will survive the winter if given the chance. 

Mosaic virus 

Can infect over 100 plants including many fruits, vegetables and flowers. Leaves become mottled with yellow, white, light and dark green spots or streaks. Cucumber mosaic virus is one of the most common types and it is usually spread by aphids. Tobacco mosaic virus spreads through seeds and direct contact. The best way to avoid it is to grow resistant varieties. 

Treatment and prevention 

• Remove all the infected plants and destroy them. 

• Once plants are infected, there are no controls.  Also disinfect your gardening tools. 

• Plant resistant plants when available in your garden 

• Control aphids to prevent them spreading the virus. 

Rust (fungal disease) 

Affects a wide range of woody and herbaceous plants. It reduces health and flower production but rarely kills the plant as it is caused by a parasite that needs living plants to survive. Rust is spread by spores that can be transferred either by the wind or water (Rust disease can often spread after watering). 

Treatment and prevention 

• There is no easy treatment for rust.  

• Remove all infected parts and destroy them.  

• Replant the area with resistant varieties. 

• Remove all debris in between plants to prevent virus spreading. 

• Avoid splashing water onto the leaves, as this can spread rust. 

• Space your plants properly to encourage good air circulation. 

• Local nurseries may be able to give advice on effective rust fungicides. 

Good luck out there detectives, if you have any pests you can’t identify feel free to comment below or post on the Growing Together Facebook page.

Suzanne