Compared to growing plants outdoors, indoor plant cultivation circumvents many dangers that can occur under the open sky. Aside from being sheltered from the elements, indoor plants are also less exposed to pests and diseases. However, this doesn’t mean that indoor plant cultivation is impervious to these problems. Although the enemies are less numerous, some of them are very persistent and resilient creatures that can create serious problems.
Why Do Indoor Plant Diseases Occur?
The main source of disease in indoor cultivation systems is either the contaminated substrate or infected planting material. There are some reported cases of infections caused by contaminated irrigation water, but these are rather rare occurrences. Anyway, if you find plant pathogens in your indoor garden, you probably brought them along with the cuttings, seeds, or/and the substrate. Considering that the majority of common indoor diseases are caused by soil pathogens, it is easy to conclude that contaminated soil is the predominant source of pathogenic microorganisms. For this reason, it is very important to get your soil mixes from a trusted producer, as well as cuttings, seeds, and other planting materials. If you mix your own substrates, make sure to get clean, quality ingredients. Also, if you take the seeds and cuttings yourself, it is crucial to use healthy, vigorous plants and sterilized tools.
Hygiene is extremely important in indoor plant cultivation. When a plant disease occurs in a small area packed with plants, it is bound to spread quickly. This is especially true for hydroponic systems, as the pathogens easily spread from plant to plant with the help of water.
Many cultivated plants thrive in microbiologically active substrates, and it is hard to completely eliminate pathogenic species without affecting the beneficial ones. Sometimes you have to take the good with the bad. There is always going to be some level of contamination in any microbiologically active soil. However, this doesn’t mean that the infection is bound to occur. Pathogens need favorable conditions for growth, so depriving them of these conditions is an effective way to control their spread. Monitoring humidity levels, optimizing the watering regime and proper plant nutrition play a very important role in disease prevention. This especially applies to densely cultivated, sensitive plants like vegetables and herbs.
Who Attacks Indoor Plants?
There are many fungal spores and bacteria lying dormant in the soil, waiting for the right conditions to activate and reproduce. Some of these species are regular citizens of the soil micro-community, completely harmless or even beneficial, while others are notorious plant pathogens. You can even find some opportunistic species that can be good guys or bad guys depending on environmental conditions and the state of the plant’s health. Like all living beings, when plants are stressed out their immunity drops, making them more susceptible to disease. Opportunistic fungal and bacterial microorganisms use this chance cunningly. They sense the weakened plants and instead of feeding on the decaying organic matter in the soil, they opt for a juicier meal. This is another reason why proper nutrition and optimized environmental conditions are so important for a successful harvest.
Luckily, indoor plants are sheltered from the most harmful organisms that attack plants. However, lower variety doesn’t mean lower occurrence. Some species of pathogenic fungi and bacteria are very persistent, adaptable, and so widespread that it is almost impossible to avoid them. Even the most experienced growers battle with these notorious microscopic creatures from time to time.
In order to come up with a proper prevention strategy, we need to know how our enemy behaves. So, let’s meet the most common diseases in indoor plant cultivation.
Root Rot (Phytophthora, Pythium, Rhizoctonia, Fusarium)
Photo Credit: Scot Nelson, via Wikimedia Commons
Root rot is the most common outcome of overwatering and poor soil drainage. When the soil is waterlogged, the plant’s roots suffocate due to the lack of oxygen and start to rot. Heavy, rich, densely packed garden soil with poor drainage is especially prone to this issue. However, root rot can also be a result of a fungal infection. More often than not, this disease is caused by the combined effects of both waterlogging and pathogenic fungi. The four main fungal genera that cause root rot are Phytophthora, Pythium, Rhizoctonia, and Fusarium. All of them cause similar symptoms and are difficult to differentiate solely based on a visual inspection.
The main problem with root rot is that it is very hard to notice on time. The first visible symptoms usually appear when the root is already well damaged. This disease is hard to treat and can have a fatal outcome, which is why proper prevention should be the first line of defense. Fungal spores that cause root rot easily spread by water, so hydroponic systems are especially vulnerable.
Symptoms
- Stunted growth for no apparent reason
- Leaves and stems turning yellow or brown
- Wilting
- Discoloration of the stem
- Branch dieback
Typically, older growth displays symptoms first. However, as the disease progresses the whole plant becomes wilted and dry. The biggest problem with root rot is that it’s very unpredictable. Sometimes a single overwatering can trigger the infection, while sometimes it takes prolonged waterlogging for spores to germinate and attack. The easiest way to know whether your plants are suffering from root rot is to take a spade and remove a portion of the soil to expose the roots. If they have turned brown or black and became soft and squishy, your plant is probably suffering from root rot.
Phytophthora root rot
Perhaps the most famous member of the genus Phytophthora is P. infestans. This species of fungi is the causative agent of potato blight – a devastating disease that caused the Great Famine in the 19th century. Considering that Phytophthora species are soil pathogens, it is very difficult to detect their presence early. Seedlings and young trees can succumb in a matter of weeks, while mature woody plants can sometimes take months to develop symptoms. Either way, Phytophthora infections usually have a highly detrimental effect on plant health. They often occur in cool environments and waterlogged soils, affecting vegetative parts of the plant (root, stem, leaves), as well as tubers and fleshy fruits.
The first visible symptoms usually include slowed or stunted growth, yellowing and wilting, followed by dark spots at the base of the stem. The spots start to girdle the stem and spread upwards as the disease progresses. If the fungus has infected the fruits or tubers, you will find patches of brown, rotten tissue that go deep into the fruit’s flesh.
Pythium root rot
Pythium species are cosmopolitan fungi that are the most common causative agents of root rot in both indoor and outdoor growing environments. Due to their omnipresence, you can often find them as contaminants in rich garden soil mixes. Pythium readily infects the plant when the substrate is dense and overwatered. Overfertilizing can also trigger this fungus to attack.
Pythium root rot is especially harmful to seedlings, due to their sensitivity and constant need for moisture. It is a very big problem in greenhouses, where it commonly causes damping-off in seedlings.
This pathogenic fungus likes to feed on young tissues, which is why root tips and young root hairs are usually affected first. As the disease progresses, the whole root gets infected and rots. You will recognize this disease by rotten spots on the root that are easily peeled away to expose the fibrous tissues beneath. Like all fungi that cause root rot, Pythium stunts the growth of plants, causing wilting and dark spots at the stem base.
Rhizoctonia root rot
Rhizoctonia has a very wide range of hosts, and it can infect almost all species of vegetables and flowers. It causes significant economic losses in greenhouses all over the world. The name of this fungus indicates its menacing behavior – rhizoctonia directly translates to root-killer. This soil-borne pathogen is often a problem in cold, moist soils with poor drainage. Rhizoctonia is especially devastating to seedlings and young plants. It produces dark, watery lesions on roots and the base of the stem, turning the tissues soft. The girdled stem is unable to support the plant, and the seedling drops.
Mature plants that are infected with Rhizoctonia lose vigor and turn yellow. If you move the top layer of the soil to expose the base of the stem, you may find patches of rotten, mushy tissue. If you give them more time, the rotten areas will dry and create sunken spots full of mycelium and fungal resting spores (sclerotia).
Fusarium root rot
Fusarium species are common soil pathogens that are very harmful in soybean, corn, and vegetable production. These fungi are a bit easier to differentiate from the previously mentioned species, as they cause typical purplish-red lesions on roots and at the base of the stem. As the disease progresses, these lesions spread and merge to form large areas that gradually turn necrotic. Growth is stunted, leaves turn yellow and the whole plant starts to wilt. Even though it usually doesn’t kill plants, Fusarium root rot can cause severe yield loss in the right conditions. Dense planting, cool temperatures, high soil moisture, and physiological stress play significant roles in infection and dictate the severity of the disease.
Stem Rot (Sclerotinia)
Photo Credit: Rasbak, via Wikimedia Commons
Stem rot is caused by Sclerotinia sclerotiorum, a pathogenic fungus that can infect many annual species of vegetables, field crops, succulents, and ornamental plants. Sometimes it can even infect young woody plants. S. sclerotiorum is a problem in both outdoor and indoor growing environments. It can infect the plant in any phase of growth, and cause the rotting of stored crops. Considering its wide array of hosts, high pathogenicity, destructiveness, and persistence, it is safe to say that S. sclerotiorum is among the most economically damaging pathogenic fungi in agriculture.
Sometimes S. sclerotiorum is referred to as white mold due to its white, cottony mycelium that is usually seen at the base of the stem in humid conditions. If you take an infected stem and cut it open, you will notice that the insides have become pithy, spongy and dry. You may also find tiny black specks in the white mycelium. These are called sclerotia, and they are compact masses of hardened mycelium that are packed with nutrients, ready to withstand harsh conditions. Sclerotia can lay dormant in the soil for years, and start to germinate once the conditions are favorable – increased humidity and cool temperatures.
Sclerotinia root rot (SSR) is often very hard to detect early, as the symptoms can take a while to manifest. When you see visible signs of SSR, it is highly likely that the plant has been battling with the infection for some time already.
Symptoms
- Watery lesions on leaves and stem base
- Leaf discoloration
- Bleached stems
- White mycelia on the base of the stem
- Wilted, curled leaves
As mentioned earlier, this pathogenic fungus can also attack stored crops, especially fleshy ones like cucumbers, carrots, leafy greens, eggplants, and squash. The initial sign of infection is watery lesions on fruits. They become larger and darker as the disease progresses, and the entire fruit can become mushy and rotten. White, cottony mycelium covers the spots, and if the conditions were suitable for their production, you may also notice tiny black sclerotia.
Powdery Mildew (Erysiphales)
Photo Credit: I, Rude, via Wikimedia Commons
Powdery mildew is another cosmopolitan, common plant disease, and probably the most widely recognized one. As the name indicates, you can easily recognize this disease by its typical symptom – powdery, white mycelial growth on foliage. It all begins with small white or yellowish spots on the upper side of the leaves. In favorable conditions, the fungus spreads rapidly and occupies the entire leaf surface. Even though it first occurs on older foliage, powdery mildew can also successfully grow on buds, flowers, and young shoots. Spreading mycelium causes the photosynthetic surface to shrink, and the fungus also impairs the plant’s growth by stealing its nutrients.
Powdery mildew often occurs when the air humidity level is high, and the air circulation around the plants very poor. These fungi need moisture to initiate the infection, but once they make contact with the plant, they can thrive even in dry conditions. Although it rarely kills plants, powdery mildew can decrease the quality and quantity of yields. It can also progressively deteriorate the health of perennial species with repeated infections. As it gives plants a weakened, unsightly look, the presence of this disease is especially unfavorable in ornamental gardens and landscaping.
Symptoms
- Pale yellowish spots and blotches on leaves
- Powdery spots on the upper or both sides of the leaf
- Premature fruit ripening
- Leaves dry and start to curl towards the upper side
As the disease progresses, the whole surface of the leaf can be covered with powdery white mycelia. The affected foliage eventually turns from yellow to brown and dries completely. This decreases plant vigor and drains it of resources that would have otherwise gone to fruit or bud production.
Gray Mold (Botrytis cinerea)
Symptoms on a tomato stem. Photo Credit: Goldlocki, via Wikimedia Commons
Symptoms on tomato fruit Photo Credit: Goldlocki, via Wikimedia Commons
Gray mold is among the most common diseases in plant production, affecting more than 200 plant species. It is caused by a very persistent and widespread fungus called Botrytis cinerea. The name of this fungus directly translates to “grapes like ashes” which describes the shape of its reproductive organs and the gray color of the spore masses. Besides directly affecting growing plants, this pathogen can remain latent until after the harvest. Seemingly healthy stored fruits infected with B. cinerea will develop brown blotches that soon become covered in the grayish mycelium. The tight packing of the fruits enables the fungus to easily spread from infected fruits to healthy ones. This can result in severe losses of stored goods, so it is not a surprise that gray mold is considered one of the most important diseases in agriculture.
Gray mold usually develops on older growth first, causing little to no damage to the plant. However, if the conditions become favorable for its development (high air humidity, moisture, temperature between 65-75℉) the fungus will grow rapidly. It can spread onto younger growth and ripe fruits, deteriorating the plant’s health and causing yield loss. Botrytis cinerea is perhaps most famous for its detrimental effect on wine grape production, but it is also “popular” in fruit, vegetable, and cannabis production.
Symptoms
- Elliptical, watery lesions on stems
- V-shaped or circular yellow to brown spots on leaves
- Dry, curled leaves
- Tan or brown soft blotches on fruits that quickly become covered in gray mycelium
Gray mold spores are everywhere, and they move around easily with the help of air currents, water and insects. High levels of moisture, warm temperatures, and poor air circulation stimulate their germination and mycelial growth. The first symptoms usually appear on older growth in the form of large brown lesions on leaves and stems. If there was a lot of moisture during budding and fruit production, grey mold can develop in the crevices of buds and fruits, and decimate the yields early. On fleshy fruits, it forms dark blotches of softened tissue that is soon covered by a layer of grayish mycelium. In humid conditions, ashy mycelial growth can cover the fruits and buds entirely.
Leaf Spot (various fungi and bacteria)
Photo Credit: Dr. Parthasarathy Seethapathy, Tamil Nadu Agricultural University, via Wikimedia Commons
Leaf spot is quite a wide term, and it encompasses many different plant diseases that share a common symptom. As the name indicates, the typical symptom includes spotting on leaves. The spots are small, initially yellow, but soon they turn yellowish-brown and have clear edges with a yellow halo. It is usually very hard to know which species of fungi or bacteria caused the disease, but you can at least differentiate between the two. If the spots are watery, the disease is probably a result of a bacterial infection. In both cases, the spots will increase their numbers as the disease progresses, grow larger and turn necrotic.
Leaf spot is mostly caused by fungal and bacterial pathogens that enter the plant through stomata and hydathodes on its foliage. This is why it manifests itself in such a way – each spot is a place where the pathogen has entered the plant and established itself. This is also the reason why it appears on older leaves first. Older foliage has stomata big enough for the pathogen to get in.
Causative agents of leaf spot only feed locally, and rarely affect other plant organs. Even though leaf spot is usually not lethal, it can reduce the photosynthetic surface of the plant and spoil the looks of ornamental species. By reducing its vigor and photosynthetic activity, it also makes the plant more susceptible to other diseases.
Sooty Mold (Ascomycetes)
Photo Credit: Bidgee, via Wikimedia Commons
Members of genera Cladosporium and Alternaria are the main causal agents of sooty mold. These fungi usually invade after aphids, whiteflies, mealybugs or other honeydew-producing insects have already occupied the plant. The sticky, sugary substance these insects leave on foliage and leaf axils is very attractive to sooty molds. All that sugar is a very rich source of energy, and few can resist such a luxurious meal.
It is very easy to identify sooty molds, as they cause typical symptoms on leaves – black, powdery spots of mycelium that make the leaves look charred. These fungi are a nuisance if present sparsely, as they do little to no harm to plants. However, if the plants are heavily infested with honeydew-producing insects, sooty molds can cover the entire leaf surface and reduce its photosynthetic activity. Along with the sucking activity of the pests, it contributes to the further decline of plants.
Viral Diseases
Raspberry Mosaic Virus Photo Credit: Jerzy Opioła, via Wikimedia Commons
Zucchini Yellow Mosaic Virus Photo Credit: Downtowngal, via Wikimedia Commons
Considering viruses are not active outside a living host, they are usually brought into the grow room with the contaminated planting material. Another way plants can get infected with viruses is with the help of their insect vectors. Some species of sucking insects help these simple organisms to reach their primary host by carrying them inside their mouth or saliva. When these insects suck the juices of an infected plant, they also consume the viral particles. When they move onto the next victim, they will release a bit of their saliva along with the viral particles before beginning to feast, thus infecting the plant.
Symptoms
Different plant viruses can cause different symptoms, but the most common ones include:
- Leaf yellowing
- Mosaic-like or stripey spotting on leaves or/and fruits
- Leaf curling
- Abnormal growth of leaves, flowers, and fruits
- Stunted growth
How severe can the symptoms be? Well, it depends on multiple factors. These include the strain of the virus(es), environmental conditions, age, stage of development, and specific species of the host plant. Symptomatic plants usually carry a couple of different viral strains, because these microscopic troublemakers often occur in mixed infections. The most tell-tale signs of a viral infection are mosaic-like spots on leaves and abnormal growth of buds and fruits.
Viral diseases cause very severe yield loss (up to 100%) when the infection occurs early, in young plants. Unfortunately, there are still no viable remedies for plant viruses. If you find any symptomatic plants in your garden, dispose of them and the substrate immediately.
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Ana Mileusnic
Ana is a scientific writer and researcher passionate about sustainable agriculture and environmental protection. As a speleologist in training and a member of the Bird Protection and Study Society of Serbia, she is involved in field research and various projects related to ornithology and biodiversity conservation in her home country.
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