Gardening can be a deeply rewarding hobby, allowing you to reconnect with nature and find mindfulness in our often frantic lives. However, no matter how carefully we garden, our plants sometimes start to look sick. Plant diseases are a common challenge for all gardeners, no matter their experience level. They can be caused by fungi, bacteria, viruses, and environmental conditions, each affecting plants in distinct ways.
Rather than cry to Mother Nature, asking what you did to make her punish you so severely, I will help you understand what plant diseases are and how to avoid them.
Many plant diseases are caused by bad luck or poor plant hygiene. The good news is that most can be prevented if we garden with a bit more attention. Ramming plants into smaller spaces and wet, humid conditions are usually the culprits for many plant diseases.
Let me reassure you, though, dear gardener, that at some point, no matter how diligent you are, one of your plants, or many in some cases, will succumb to the occasional plant disease. Here’s a comprehensive guide to some of the most prevalent plant diseases, what causes them, and practical methods to treat or prevent them.
Powdery mildew appears as a white or greyish powder on leaves, stems, and flowers. It’s common in warm, dry environments but can also thrive in humid conditions with poor airflow.
Powdery mildew is a fungal disease caused by various species of fungi from the order Erysiphales, and it can quickly take hold in your garden if the conditions are right. These fungi spread through the air as spores, which land on plants and, under favourable conditions, begin to grow and infect vulnerable plants' leaves, stems, and flowers. The key to preventing powdery mildew is understanding what makes it thrive in the first place.
This fungal infection loves warm, dry environments with high humidity. Typically, powdery mildew appears when daytime temperatures range from 60-80°F (15-27°C), with moist mornings or evening dew. The fungus can take hold quickly in these conditions, especially if the air circulation is poor. Dense, overcrowded plants or areas with limited airflow trap moisture, creating the perfect breeding ground for the spores. Inadequate spacing between plants can increase the chances of an infection spreading, so be sure to give your garden room to breathe.
Certain plants are more susceptible to powdery mildew than others. Cucumbers, squash, roses, and many ornamental varieties often fall victim to this pesky fungus. Overhead watering, especially in the evening, is another contributor to mildew growth, as it leaves moisture on the leaves for long periods, increasing the chances of spore development. To avoid this, consider watering your plants at the base or in the morning to reduce excess moisture on the foliage.
Infected plant material is also a major factor in spreading powdery mildew. Spores can travel through the wind, insects, or even gardening tools, spreading the infection across your garden. To stay ahead of it, remove any infected leaves or plant debris immediately, and consider using mildew-resistant plant varieties or natural fungicides to keep powdery mildew at bay.
The first thing to do with all plants, whether seedlings or those in our flower beds, is to improve air circulation by spacing plants appropriately and pruning regularly. This prevents the fungus from taking a bold quite so quickly. Avoid overhead watering, as moisture on leaves can encourage fungal growth. Organic fungicides like neem oil or sulfur sprays can help control and prevent powdery mildew. Removing severely infected leaves or plants in extreme cases can help stop the spread.
Blight causes dark, water-soaked spots on leaves, stems, and fruit. Early blight affects older leaves first, while late blight, more devastating, affects younger leaves and spreads faster. Blight is a term that encompasses a range of plant diseases, with early blight and late blight being the most common forms, both of which can cause significant damage to crops like tomatoes and potatoes.
Early blight is caused by the Alternaria fungus, which primarily affects plant leaves, stems, and fruit. The fungus produces dark, concentric rings on the leaves, often starting at the lower parts of the plant and spreading upwards. Early blight thrives in warm, moist conditions and spreads through infected plant debris, wind, or rain. Once the fungus infects the plant, it can cause leaf tissue to die, leading to defoliation and, ultimately, reduced yields.
On the other hand, late blight is caused by Phytophthora infestans, a type of water mould. Unlike true fungi, water moulds are part of a different group of organisms that thrive in cool, wet conditions. Late blight typically begins with dark lesions on the leaves and stems, which can quickly expand and spread to the fruit. Infected fruit may rot rapidly, and whole plants can collapse in a matter of days.
Late blight is especially notorious for affecting tomatoes and potatoes, and it is known for its ability to spread quickly through wind, water, and infected plant material, making it a major threat in humid environments.
Both forms of blight are more likely to develop in wet and humid conditions, which create an ideal environment for the fungi and water moulds to spread. Excess moisture, such as from heavy rainfall, irrigation, or morning dew, increases the risk of blight, especially if plants are overcrowded or leaves remain wet for extended periods. Once the pathogen enters the plant, it spreads rapidly, infecting neighbouring plants and causing widespread damage.
Managing blight involves practising good garden hygiene, removing infected plant material, and ensuring proper spacing between plants to allow for adequate airflow. For the most part once a plant has bliught you need to destroy it by burning, so it does;t infect other nearby plants. There is little cure, and the risk of allowing them to continue with blight harms other nearby crops.
Plant resistant varieties and rotate crops each year to reduce the buildup of fungal spores in the soil. Remove infected plant material promptly and avoid overhead watering. Fungicides may help prevent blight if applied early, especially on plants known to be susceptible.
Root rot causes roots to become mushy, discoloured, and foul-smelling. Aboveground symptoms include wilting, yellowing, and stunted growth. The below image shows healthy white roots unlike the brown or yellow slimy roots that appear when root rot strikes.
Root rot is a common and destructive problem in gardens, primarily caused by soil-borne fungi such as Pythium and Phytophthora. These pathogens thrive in overly wet, waterlogged soils, where the conditions create an ideal breeding ground for the fungi. When soil drainage is poor, water accumulates around the plant’s roots, depriving them of oxygen and creating an environment where these fungi can flourish.
Both Pythium and Phytophthora are types of water molds, organisms that are different from true fungi but exhibit similar damaging effects on plants. Pythium is typically found in cooler soil conditions and can affect a wide range of plants, including vegetables, flowers, and shrubs. It causes the roots to rot, leading to yellowing leaves, stunted growth, and wilting, as the plant can no longer effectively take up water and nutrients.
Phytophthora, on the other hand, is most active in warmer, more humid conditions and can cause severe root, crown, and stem rot, often leading to rapid plant death, especially in susceptible crops like tomatoes, potatoes, and ornamental plants.
The fungi infect the roots, causing them to become soft, discoloured, and mushy. As the roots decay, the plant cannot take up essential nutrients and water, leading to symptoms like wilting, yellowing leaves, and poor growth. If the infection is severe enough, the plant may collapse or die. Phytophthora can also affect the crown and lower stems of the plant, causing lesions and leading to plant collapse from the base.
The best prevention for root rot is well-draining soil and proper watering practices. Avoid overwatering, and consider planting in raised beds or containers if your soil drains poorly. Compacted soils or heavy clay soils, which retain water and don’t allow it to drain freely, create perfect conditions for root rot to develop. In addition, planting in areas where water collects or near low-lying spots in the garden increases the likelihood of root rot. If you notice root rot, trim off the affected roots and replant in fresh, well-draining soil. In extreme cases, you may need to replace the soil or plant in a different location to avoid contamination.
Rust appears as small orange, brown, or reddish spots on the underside of leaves. Over time, it can lead to leaf drop and reduced plant vigour.
Rust is a common fungal disease caused by a group of fungal pathogens from the order Pucciniales, which thrive in moist, warm environments. The disease typically presents itself as orange, red, or yellow powdery lesions on infected plants' leaves, stems, and sometimes flowers. Rust fungi are highly specialized; each type generally targets specific host plants. The spores produced by rust fungi are carried by the wind or rain, making it easy for the disease to spread from one plant to another.
Rust thrives in humid, wet conditions, promoting its spores' development and dispersal. Warm temperatures between 60-75°F (15-24°C) provide an ideal environment for the rust fungi to grow and spread rapidly. Excessive moisture from dew, rain, or irrigation allows the spores to germinate and infect plant tissues. Rust is particularly problematic in regions with long periods of wet weather or where plants are crowded together, and air circulation is poor.
This fungal disease commonly affects ornamental and flowering plants like roses, snapdragons, and hollyhocks. On roses, rust typically appears as raised orange or yellow pustules on the underside of the leaves. As the disease progresses, the leaves may yellow, curl, and eventually fall off, weakening the plant and reducing its ability to photosynthesize.
Similarly, rust often affects snapdragons and hollyhocks, with yellow, orange, or brown pustules forming on their leaves and stems. If the disease isn't managed, these plants can suffer from stunted growth, poor flowering, and premature leaf drop. Rust is not a killer but does impact the look of the plant and its flowering potential.
Rust spores are highly contagious and can easily spread from plant to plant through wind, water splash, and even through contaminated tools or clothing. The disease cycles throughout the growing season, and if left unchecked, it can significantly damage or even kill vulnerable plants. Rust also has a multi-stage life cycle involving different spore types, which allows the fungus to overwinter in infected plant debris and re-infect plants the following year.
Preventing rust involves ensuring adequate spacing between plants for improved air circulation, which reduces humidity around the foliage. Watering at the base of the plants rather than overhead can help minimize moisture on the leaves, where the spores are most likely to land. Regularly cleaning up fallen leaves and plant debris, which can harbour rust spores, is another key step in rust prevention. Additionally, rust-resistant varieties of roses, snapdragons, and hollyhocks are available, making them a great choice for gardeners looking to reduce the risk of rust.
If rust does take hold, fungicides can help manage the disease, but it’s essential to treat plants early before the disease spreads too widely. Removing and disposing of infected plant material is crucial to stopping the disease from cycling and spreading further throughout the garden. Early detection and consistent maintenance will give your plants the best chance of surviving and thriving despite the threat of rust.
Anthracnose causes dark, sunken lesions on leaves, stems, flowers, and fruits. Infected areas may become dry and brittle.
Anthracnose is a fungal disease caused by various species of the Colletotrichum fungus. This pathogen is particularly prevalent in warm, wet conditions, which provide the ideal environment for its spores to spread and infect plants. The disease infects many plants, from deciduous trees like ash, maple, and sycamore to vegetables such as tomatoes, beans, and peppers. Anthracnose attacks a plant’s leaves stems, flowers, and fruit, causing various symptoms, including dark, sunken lesions, wilting, and premature leaf drop.
The fungus spreads rapidly through rain splash, wind, and contaminated tools, making it highly contagious in areas with persistent moisture. Spores can overwinter in fallen leaves and infected plant debris, reactivating in the spring when the conditions are right for infection. Anthracnose thrives in high humidity and periods of heavy rainfall, but it can also develop during long periods of wet, damp weather. In trees, it often results in noticeable leaf spots or large, irregular lesions, which can weaken the tree, reduce its vigour, and stunt growth.
The disease can cause fruit rot in vegetables, often affecting crops like tomatoes and peppers, leading to disfigured or inedible fruit. The fungus can also cause stem lesions that impair nutrient and water flow, leading to plant decline. Anthracnose is a significant threat to gardeners and farmers alike, as it can rapidly defoliate plants, reduce yields, and leave crops more vulnerable to secondary infections.
To prevent anthracnose, avoid overhead watering and prune plants to improve air circulation. Copper-based fungicides can help manage the disease if applied at the first sign of infection. Remove and discard infected plant parts, and consider crop rotation to prevent soil-borne spores from spreading.
Botrytis blight causes grey, fuzzy mould to form on flowers, stems, and leaves. It’s particularly common on plants with dense foliage, like strawberries, grapevines, and peonies.
This disease is caused by the Botrytis cinerea fungus, which thrives in cool, damp conditions. Overcrowded planting and prolonged leaf moisture can worsen its spread, as the fungus requires a humid environment to grow and infect plants. Botrytis blight typically affects a wide range of plants, including flowers, vegetables, and fruit crops like grapes and strawberries.
The fungus often begins by causing greyish mould to form on infected leaves, flowers, or fruit and can quickly spread to other parts of the plant. Prolonged exposure to moisture from rain, dew, or overhead watering increases the likelihood of infection. Additionally, poor air circulation and excess shading can create the stagnant conditions that Botrytis favours, making it crucial to ensure plants are spaced properly and that water is directed to the base rather than the foliage.
Water at the base to ensure good air circulation and avoid wetting leaves. Immediately remove any dead or infected plant material to prevent the spread. Fungicides, such as copper or sulfur-based options, can help if applied early.
Leaf spots appear as small, round, brown or black spots on leaves. As the disease progresses, leaves may become yellow and fall off.
Leaf spot diseases, caused by fungi like Cercospora and bacteria such as Xanthomonas, result in spots or lesions on plant leaves. Cercospora causes dark-bordered spots, while Xanthomonas creates water-soaked lesions that turn yellow or brown. These pathogens thrive in high humidity and wet conditions, with rain, dew, or overhead irrigation helping spread the disease.
Overcrowded plants with poor air circulation are more susceptible, as they create a damp environment ideal for infection. To prevent leaf spots, ensure good spacing between plants, avoid overhead watering, and remove infected leaves. In severe cases, fungicides or bactericides may be necessary.
Properly space plants and prune to improve airflow and water at the base. Remove and discard infected leaves to stop the spread of spores. Copper-based fungicides may help, but prevention through garden hygiene is the most effective method.
Verticillium wilt causes wilting, yellowing, and dieback on one side of the plant. The symptoms are often mistaken for drought stress, and the plant just looks dehydrated.
Verticillium wilt is caused by the Verticillium fungus, which belongs to the soil-borne genus. Like Fusarium wilt, this pathogen infects a plant’s vascular system and blocks the flow of water and nutrients.
Verticillium dahliae and Verticillium alboatrum are the primary species responsible for the disease. The fungus enters the plant through the roots, typically through wounds or areas of the root system that are weakened by environmental stress or injury. Once inside, the fungus spreads through the plant’s xylem (water-transporting tissue), causing it to wilt, turn yellow, and eventually die.
Verticillium wilt thrives in cooler soil conditions than Fusarium wilt, with temperatures between 60-80°F (15-27°C) most conducive to infection. The fungus can survive in the soil for several years, even without a host plant, by forming dormant chlamydospores. These spores can be carried by wind, water, or contaminated soil, making the disease difficult to control once introduced into the garden.
Sadly, there is no cure for verticillium wilt. Therefore, prevention is crucial. Avoid planting susceptible plants in infected soil, consider crop rotation to help reduce fungal buildup, and remove and destroy infected plants to prevent the spread of the disease.
Downy mildew causes yellow or light green spots on leaves, often accompanied by a fuzzy white or greyish growth on the undersides.
Downy mildew is a disease caused by Peronospora fungi, which thrive in humid, wet conditions. This fungal infection typically affects plants' leaves, stems, and sometimes flowers. It leads to characteristic yellowish spots on the upper surface of leaves and a fuzzy, greyish-purple mould appearing on the undersides.
The disease thrives in cool, moist environments, with high humidity, rainfall, or excessive dew being the perfect breeding grounds for spreading the spores. Downy mildew can spread rapidly, particularly when the plants are overcrowded or when air circulation is poor, as these factors trap moisture around the leaves and encourage fungal growth. I often find I get this when growing squashes at Garden Ninja HQ, but it rarely causes them too much harm.
Cucumbers and lettuce are especially vulnerable to downy mildew, though many other plants, including beans, grapes, tomatoes, and brassicas, can also be affected. In cucumbers, the disease causes angular yellow spots that eventually turn brown, leading to defoliation and declining fruit production. For lettuce, downy mildew can result in leaf curling and premature bolting, ruining the crop before it reaches maturity. The disease often causes leaf drop, which weakens the plant and makes it more susceptible to other secondary infections.
The Peronospora fungus spreads through the release of spores that are carried by wind, water, or insects.
Yet again, improving air circulation and avoiding overhead watering are key. Fungicides are rarely effective, so I never use them. Removing and discarding infected plant parts will help reduce the spread.
Clubroot causes swollen, distorted roots, which in turn lead to stunted growth, wilting, and yellowing leaves. See the twisted and misshapen brassica root below.
Clubroot is a soil-borne disease caused by the pathogen Plasmodiophora brassicae, which primarily affects plants in the brassica family, including cabbage, broccoli, cauliflower, kale, and Brussels sprouts. The pathogen infects the plant’s roots, causing them to become swollen, distorted, and misshapen, restricting their ability to absorb water and nutrients effectively. This results in stunted growth, yellowing leaves, and, in severe cases, the death of the plant. The infected roots are often characterized by swollen, club-like structures, where the disease gets its name.
The disease thrives in acidic, poorly-drained soils with a low pH (typically below 6) and is most prevalent in cool, wet conditions. The pathogen can survive in soil for many years in the form of resting spores, which are incredibly resilient and can persist even in the absence of a host plant. When brassica plants are grown in infected soil, the spores infect the roots and begin to proliferate, causing the characteristic root galls and hindering the plant’s ability to take up water and nutrients.
Clubroot is spread through the movement of contaminated soil, water, tools, and even on the feet of gardeners, making it a challenging pathogen to control once it’s introduced to an area. The disease often leads to wilting, as plants struggle to maintain hydration, especially during warm weather. It can also make plants more susceptible to other secondary infections, weakening their health.
Preventing and managing clubroot involves improving soil drainage and adjusting soil pH, as more alkaline soils (pH 7-8) can help suppress the pathogen. Crop rotation is essential, as planting brassicas in the same soil year after year allows the pathogen to build up in the soil. Practising garden hygiene by cleaning tools, removing infected plant debris, and not working in the garden during wet conditions can also reduce the spread of the disease.
Sometimes, lime treatments can help raise soil pH, making it less hospitable for the pathogen. Resistant varieties of brassicas are available and should be prioritized in areas where clubroot is a known issue. Once infected, plants should be removed and destroyed to prevent further contamination.
Please don't panic, dear gardener, if your precious plants and beloved crops get diseases at some point. It's all part of the process; even we experts must contend with them. Most of the time, it's just fluke conditions that cause them. While each plant disease requires specific treatments, good gardening practices can prevent many issues. So, let's look at some plant hygiene practices that can lower the risk of your plants becoming infected with fungus, viruses, or diseases.
The first step is probably the most common sense: remove dead plant material, especially from annual crops (not so much from herbaceous perennials), and sanitize garden tools regularly to maintain a healthy garden and reduce the chances of plant problems.
Dead or decaying plant matter can harbour pests, fungal spores, and bacteria, spreading diseases to healthy plants. Inspect your garden regularly for withered leaves, stems, and flowers and carefully remove them to prevent contamination. Use clean pruning shears or scissors to cut the debris and dispose of it away from your garden to minimize the risk of reinfection.
To sanitize garden tools, scrub them with soap and water to remove dirt and sap, then disinfect them using a solution of one part household bleach to nine parts water. Allow the tools to air dry before use. Regular cleaning prevents the transfer of pathogens from plant to plant and ensures your gardening efforts yield healthy, thriving plants. It's just good old-fashioned common sense!
Many plants have disease-resistant cultivars that can save time and effort by reducing the need for intensive pest and disease management. Disease-resistant cultivars are varieties specifically bred or selected for their ability to withstand common plant diseases such as powdery mildew, blight, rust, or fusarium wilt. These cultivars are ideal for beginner gardeners who want to minimize the use of chemical treatments and have a higher chance of success!
When choosing plants for your garden, look for labels or descriptions that indicate resistance to specific diseases, such as 'tomato resistant to verticillium wilt' or 'rose resistant to black spot.'
Remember that while these cultivars offer enhanced protection, they are not entirely immune to all problems and still benefit from proper care, such as adequate spacing, watering, and pruning. Selecting disease-resistant varieties can help ensure a productive, low-maintenance garden with fewer challenges from pests and pathogens.
Rotating crops annually reduces the buildup of disease-causing organisms in the soil. This involves following a yearly plan for each of your planting beds, moving through the different crop types. You can read more on crop rotation here.
Watering at the base rather than overhead helps keep leaves dry, reducing fungal and bacterial growth. Avoid using sprinklers that indiscriminately spray water all over your plants, as this increases humidity, and water splash can quickly transfer fungal diseases around your plants. That and it's a complete waste of water, too!
Mulching can prevent soil-borne diseases from splashing onto plants, but avoid piling mulch too close to the stems. A layer of mulch, such as straw, wood chips, shredded bark, or compost, absorbs the impact of water and reduces soil disturbance, keeping disease-causing organisms contained in the soil.
However, it’s crucial to avoid piling mulch too close to the stems or trunks of plants. Excess mulch near the base can trap moisture, creating a damp environment that promotes rot, encourages fungal growth, and attracts pests like rodents or insects. To prevent these issues, leave a 2–3-inch gap around the stems of smaller plants and a slightly larger gap for trees or shrubs. Maintain a consistent mulch layer of 2–4 inches deep, spreading it evenly to ensure optimal coverage without suffocating the roots.
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Understanding common plant diseases and taking proactive steps can create a healthier garden and have less drama by following some simple gardening housekeeping. The main rules are to avoid overwatering your plants, particularly crops, or using sprinklers, which can spread fungus and create conditions for pests and diseases to take over.
Consider your plant spacing to ensure adequate airflow so these nasties can't take hold, so always sanitise and clean your secateurs and tools.
If you have any questions, please comment below or add one to my YouTube channel. I'm happy to help. You can also check out my Tweet, Facebook, or Instagram for more guides and tips.
Check back soon for more garden guides. Happy gardening!
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