During our time as hunter-gatherers, the biggest threats to our food supply and safety were the elements and other animals. Trading this lifestyle for a life in organized units like villages increased food security and toned down the effect of environmental conditions on our food supply. By developing organized plant cultivation and sophisticated irrigation systems, we adapted large pieces of land to fit our needs and managed to make more consistent food sources. However, this doesn’t mean that getting food became easy. We just exchanged one threat for another. Our new main enemies became smaller, but their number and reproductive potential made up for their size.
Large fields full of plants of the same species turned out to be hotspots for various insects, nematodes, and microorganisms. Although we like to put the ‘germ’ label on all of them, actually very few species create problems. Many are neutral, some are even beneficial, like pollinators, symbiotic fungi, and beneficial soil bacteria. However, plant pathogens are completely different beasts. Even though there are only a few pathogenic species compared to the total number of species that live in a plant’s environment, their effect is often disproportionately more significant. Plant pathogens can cause severe yield loss, or make harvested crops less valuable or inconsumable due to contamination. The impact of pests and diseases on plant cultivation is second only to severe weather conditions and climate change.
How do plant diseases affect us?
Since the emergence of the first agricultural societies, our population, and our appetite only grew bigger. To feed all the hungry mouths, we needed to increase both the size of agricultural lands and the intensity of crop production. This increase resulted in a higher occurrence of plant pests and diseases, which necessitated the need for various management methods. The development of these methods had moved at a plodding pace until a couple of centuries ago, to reach almost an exponential rate in the 20th century. Large technological advances that happened after WWII sparked the Green Revolution in the late 1960s. This event, also known as the Third Agricultural Revolution, marked the beginning of a new era in agriculture, especially in the agrochemical industry.
The use of chemical fertilizers and pesticides, high-yielding varieties of various grains, improved mechanization, and better utilization of water resources have resulted in a significant yield increase. It seemed like we were a step away from eradicating world hunger. As we know today, this wasn’t the case. Instead of providing enough food for all, intensive agriculture and (over)use of agrochemicals have resulted in serious environmental and public health problems. Depleted soil fertility, polluted environment, poisoning of many non-target animal species, and pesticide residues on crops threaten not just the future of agriculture but also the future of natural ecosystems and our health.
Many pests and pathogens have developed resistance towards certain chemical groups to make matters worse, making a significant portion of available pesticides ineffective. So even with advances in technology and our vast knowledge of biology, we continue to lose a significant portion of our crops to various diseases, weeds, and pests.
A bit of history
Plant diseases have played a very important role in human history since the emergence of the first agricultural societies. Sometimes we benefit from them, like in the case of Botrytis cinerea, a type of fungus which can cause a “noble rot” of wine grapes. The noble rot occurs in dry weather conditions on ripe grapes infected by the fungus. Rotting changes the chemical composure of the grapes and the dry air dehydrates them. The wine made from such grapes is extraordinarily sweet and fine, highly valued amongst wine connoisseurs. Aside from enjoyment, certain plant pathogens can help us battle very deadly diseases. Researchers found that a fungus that infects the Pacific yew tree, Taxomyces andreanae, produces taxanes, providing a virtually inexhaustible source of these important anticancer agents.
However, these benefits are exceptions. The role of plant diseases is usually one of a villain, as their outbreaks often caused poisoning, famines, and massive migrations. These events have probably occurred countless times since the emergence of agriculture (about 12.000 years ago) all over the globe. Many historians like to focus on the impact of technology, wars, and politics on the development of modern societies, but let’s not forget that many significant historical events were triggered by a lack or abundance of basic resources like food and water.
The Great Hunger
History shows that famines do not have a single cause. They are rather a result of complex environmental, political, and technical factors within a particular territory or country. However, pathogens can sometimes be one of the main culprits for severe yield loss. One of the most memorable famines in recent human history was the Great Irish famine in 1845, for the large part induced by potato blight, a fungal disease caused by Phytophthora infestans. Also known as the Potato Famine, this event caused the death of about a million people and forced migrations of many more. Many Irish women and men fled Ireland to start a new life in North America and get away from poverty and a lack of basic resources that devastated their homeland. A simple fungus played a key role in the mass migration that changed the demographics and the later course of US history.
Ergotism and witch hunts
Plant diseases can have some quite unpredictable effects. It is speculated that the notorious Salem witch trials in the 17th century had a lot to do with a fungus that infects rye (Claviceps purpurea), along with religious dogmas. Widely known as ergot, Claviceps fungus forms black claw-like structures on infected rye, called sclerotia. The sclerotia contain alkaloids and lysergic acid that cause various physiological, as well as hallucinogenic and psychoactive effects.
In years with high ergot infestation, the rye flour was heavily contaminated with sclerotia, as they were ground together with the grains. The concentration of alkaloids and lysergic acid in the flour was high enough to cause toxic and hallucinogenic effects. Since women generally have a lower body mass, they were affected by the toxins more strongly than men. Without any knowledge of the cause, and in the God-fearing atmosphere of the day, the erratic and often hysterical behavior of the poisoned women was seen as the work of the devil.
Ergotism, as we know it today, can cause dysfunction of the nervous system that is followed by convulsions and muscle spasms. The abrupt body movements, hallucinations, and delusions of the intoxicated victims made them look possessed. Exorcism was an ineffective treatment in such circumstances, and the affected women got labeled as witches, often sentenced to end their life in flames.
It is indisputable that ergot caused a lot of deaths directly and indirectly, but we shouldn’t forget that it also caused a certain type of revolution. More than two centuries after the Salem witch trials, the effects of ergot alkaloids attracted the attention of Swiss chemists. They were trying to develop a drug that would constrict muscles and blood vessels. In 1938, Albert Hoffmann, a Swiss chemist working at Sandoz was looking for a way to synthetically produce ergot’s active ingredients and improve their structure. He playfully experimented with a particular compound – lysergic acid. He was mixing it with other compounds to see if the product will achieve desired effects in lab mice. The 25th blend, labeled LSD-25, produced very interesting results. Tested mice displayed great excitement, but with no relevant effects for the research topic, LSD-25 was discarded from the study.
However, LSD-25 continued to occupy Hoffmann’s thoughts in the following years. He felt that the compound was special and that he needed to find out how and why. In 1943 he managed to synthesize tiny LSD crystals and decided to start experimenting on himself to see what would happen. Knowing the dangers of ergotism, he started with a seemingly minuscule amount of 250 micrograms, expecting it to have no effect. On the 19th of April at 4:20 P.M. Hoffmann drank the dissolved LSD crystals and went on to have one of the most memorable bike rides in human history. His discovery revolutionized the development of psychedelics and changed psychiatric medicine. LSD marked an entire era, brought hellish nightmares and the most beautiful epiphanies, and inspired many artists and scientists. Such is the effect of a simple fungus that infects rye.
Where we are today
The advancement of science and technology revealed the causes of plant diseases and gave us tools to manage their negative effects. However, the battle continues. According to data provided by FAO, global yield losses resulting from fungal and bacterial diseases cost us more than $220 B every year, so we cannot say that we’re close to winning. The majority of the most significant pathogens are highly adaptable organisms that are quick to become tolerant or resistant to our efforts to eradicate them. Pharmaceutical companies are constantly developing new pesticides to replace the ones that are no longer viable due to resistance (and toxicity), which seems to be a never-ending battle where pathogens are always a step ahead.
Indoor gardens offer protection from many common pathogens that roam in the great outdoors. However, some persistent, adaptable species can find their way in and thrive in these environments. Knowing the risks, we can take steps to decrease the incidence of disease in our gardens and protect our crops from harmful microorganisms. To find the best way to prevent pathogenic fungi and bacteria from infecting your plants, let’s first look into how they do this in the first place.
What are plant diseases?
According to the general definition, plant disease is a state where the plant or some of its parts are functioning abnormally due to the effect of some abiotic and/or biotic factors. In more simple terms – disease includes anything that prevents the plant from achieving its full genetic potential. When faced with adverse environmental conditions or a pathogen attack, plant cells change their activity and stop functioning normally. Successfully preventing plant diseases includes being aware of the risks, so let’s dive a little deeper into how different factors affect plant health.
The plant’s environment is crucial for its healthy development and vitality. The physical properties of the growing area are the abiotic factors that affect plant growth, and they include:
- carbon dioxide
- the pH level of the substrate
- nutrient content of the substrate
- structure of the substrate
Every plant species has its specific growing requirements and optimal range for all these factors. Plants suffer if any factor is above or below the optimal range. We consider such disorders as abiotic diseases. The intensity of a plant’s reaction depends on the severity and the number of unfavorable conditions it is exposed to.
Besides directly affecting the normal functioning of the plant, unfavorable abiotic conditions weaken its immunity, making it more susceptible to pathogens and pests. When the plant is already putting a lot of effort into dealing with environmental adversity, it is unlikely that it can also readily defend itself from pathogens. This is why ensuring optimal abiotic conditions is the bedrock of disease prevention.
Biotic factors are influenced by living organisms that inhabit the substrate and plant’s surface. Some microbes are highly beneficial, like the ones that degrade organic and inorganic matter in the soil and make nutrients plant-available. However, some can cause serious problems. Biotic diseases are caused by plant pathogens, and different types of organisms fall into this category including:
- bacteria and mollicutes (phytoplasmas)
- viruses and viroids
- parasitic plants
The most common harmful microorganisms in indoor plant cultivation are fungi and bacteria. Plants have a slightly acidic pH that is favorable for fungi, which is why fungal diseases are more prevalent. Most bacteria thrive in neutral or alkaline pH, so it’s not a great surprise they are less represented in the ‘plant pathogen’ group.
Know thy enemy – the mechanism of infection
In order to successfully prevent pathogenic diseases in the long term, we need to know how the infection occurs in the first place. The pathogenic organism may be present in the plant’s environment and never actually harm it. This is because infection doesn’t occur unless all of the following requirements are met:
- An active, virulent pathogen is present in the environment
- Environmental conditions are suitable for infection
- The plant is sensitive to the specific pathogen
According to these requirements, we can conclude that the three basic prevention strategies include:
- preventing pathogens from getting into your garden
- making environmental conditions favorable for plants and, if possible, unfavorable for potential pathogens
- growing plants or varieties that are disease-resistant
How to prevent plant diseases
Although general concepts of disease prevention are the same for both indoor and outdoor plants, the approach and focus points are a bit different. Plants grown indoors aren’t as exposed to the uncertainties and many potential threats as plants living in the great outdoors. However, this doesn’t mean that you should let your guard down. It may be easier to protect plants from diseases in an indoor setting, but if the pathogens manage to get in and spread, you’re in for a bad time.
Indoor plant cultivation is all about prevention since treatment can be very costly. And we really don’t want toxic pesticides near us or the plants we eat. While it is impossible to grow plants in a completely sterile environment, there are a few basic ways to keep harmful microorganisms at bay and prevent them from overtaking your indoor garden.
Bottom-up approach – start from the substrate
You can look at soil or any other type of plant substrate as the plant’s home. It is the place where it should feel most comfortable, and where the right food is always available. For this reason, the soil must have a favorable structure, healthy microbial community, as well as proper moisture and nutrient content. Plants living in healthy substrates are more resistant to disease, and ensuring strong plant immunity is a key element of prevention.
Soil contaminated with pathogenic fungi and bacteria is likely to foster problems like root rot and stem rot. When purchasing commercial substrates, make sure to get them from a trusted, certified producer. Be especially careful with substrates with high organic matter content, as the possibility of contamination is very high. If you make your own substrates, make sure to use raw materials from clean sources to reduce the risks of harmful microorganisms.
Substrates for propagation require extra care, as the seedlings are very sensitive to pathogens. Their gentle tissues are easily attacked by fungi and bacteria that cause rotting. Since the plants are affected so early, the effects of contamination are often devastating.
Healthy planting material
Once we have a nice, healthy substrate, the next thing we want to look into is the planting material. Not all seeds, cuttings, bulbs, and tubers are equal in terms of health and growth potential. Not even the ones that come from the same plant. You want to introduce strong, resilient plants to your garden, so it’s best to separate the wheat from the chaff before you invest your efforts and resources. The general advice is to:
- take cuttings/tubers/bulbs from healthy, vigorous plants
- use seeds from mature fruits
- discard any planting material that is showing symptoms of disease, abnormalities, or slow growth
If you are getting your planting material from someone else, make sure it’s coming from reliable, trusted sources and producers.
Keep the stress low
Stress in plants is a broad term that includes a variety of conditions that adversely affect plant health. Stress occurs when any abiotic or biotic conditions in a plant’s environment are unfavorable. While a bit of stress can improve resilience and keep the immune system active, intensive or long-term stress can have the opposite effect. As fallible human beings, we make gardening mistakes here and there – it is an essential part of the process. However, we can avoid making key mistakes by following a few basic rules.
1. Pick the right plants
Organize your indoor garden by adding plants that are suitable for the conditions within it. Light exposure and space are usually the most limiting factors in indoor cultivation. Picking the plants with adequate robustness and light requirements is likely to cause minimal problems in the future. Take into account the capacity of your indoor garden, the potency and the number of your grow lights, and the conditions you can offer with your resources. The right plan and the right plants can really make the growing easier and more rewarding.
2. Pick the right substrate
Make sure that the pH, nutrient content, and structure of the substrate fit with the requirements of your plants. Dense and heavy substrates are likely to foster problems, especially in seedlings and young plants. Most plant species like light, aerated soils with moderate levels of organic matter. Overly acidic and overly alkaline soils can cause nutrient deficiencies and destabilize the activity of the microbial community.
3. Do not overwater or overfertilize
These two are very common mistakes in gardening that can have quite devastating outcomes. Too much fertilizer can have a toxic effect on plants or cause deficiency of certain elements, while the soaked soil can quite literally choke the roots and create favorable conditions for root rot pathogens. Remember that too much care can have worse effects than too little care. Most plants have mechanisms that help them deal with short-term droughts and nutritionally-poor environments. Very few species can tolerate toxicity or anaerobic conditions.
Establishing a proper watering and fertilizing regimen, and keeping track of those activities can help you prevent these problems. Do not worry or try to overcompensate if you miss the schedule by a few days. If the plants are displaying symptoms, try to correct the conditions and provide extra care until the plant restores its vitality.
4. Check the thermometer
Heat stress and cold stress can be extremely harmful in key phases of a plant’s development – germination, budding and flowering. If your indoor garden is badly isolated or it tends to heat up or cool down due to other reasons, it is highly advised to add a heating or cooling device. This is especially important if you are growing temperature-sensitive plants.
5. Keep track of air humidity levels
High air humidity can invite a variety of molds to the party, both the type that can harm your plants and the type that can harm you. However, if the air humidity gets too low, you get other kinds of problems. Low humidity is generally unfavorable for plants that are not adapted to such conditions, causing leaf curling, browning, and wilting. Dry conditions are also favorable for a spider mite infestation, so you really don’t want air humidity levels below 40% in your indoor garden.
Most plants thrive at relative air humidity levels between 40 and 60%, while the average home is somewhere around 30%. This often necessitates the need for misting or getting a humidifier, especially if the cultivated plants come from the tropics.
6. Cleanliness is next to godliness
When you are handling your plants, replanting, taking cuttings, or similar, make sure that your hands and tools are clean. Otherwise, you might spread the pathogens onto other plants in your garden. Washing your hands and gloves, disinfecting pruning scissors and other tools with rubbing alcohol or other antimicrobial products before working with each plant (pruning, taking cuttings, removing wilted parts, etc.) will prevent unintentional pathogen spread and keep your indoor garden healthy.
In a nutshell: Keep it clean, ensure that the substrates and planting materials are healthy and that you can provide your plants with optimal conditions for growth.
Hope this general guide to plant diseases helps you grow a greener thumb! Send us a comment in the section below if you’d like to share your experiences or ask any questions.
- Caporael, L. R. (1976). Ergotism: The Satan Loosed in Salem? Convulsive ergotism may have been a physiological basis for the Salem witchcraft crisis in 1692. Science, Vol 192, Issue 4234, pp. 21-26.
- Katz, S. (2002). Beneficial uses of plant pathogens: anticancer and drug agents derived from plant pathogens. The Canadian Journal of Plant Pathology, Vol 24, Issue 1.
- Shroder, T. (2014). ‘Apparently Useless’: The Accidental Discovery of LSD. The Atlantic.
- Agrios, G.N. (2004). Plant pathology 5th edition.