Bumps, blemishes, spots, scabs, mosaics, growths – plant disease are nothing if not inventive in their forms. Gardeners worry when they see any kind of unusual symptom on their plants, but often don’t stop to think about exactly what kind of disease it is, or whether it actually needs to be treated. Microbes, beneficial or not, have been a constant companion since before plants even managed to creep up onto the land.
Why does a plant disease immediately warrant treatment? Much plant disease is not even fatal, and if it is the plants targeted are often in a poor location or are a subpar variety. It might be a bit radical, but I don’t think most plant disease in the garden needs any kind of treatment at all.
Diseases as Diversity
This might be a bit of a spicy take, but I think that pathogens that fulfil certain criteria should be valued and protected as a part of normal and healthy biodiversity and treated as having a right to live just like any other organism, provided they:
- do not cause animal/human suffering
- do not cause serious damage to crops in ways that would cause human/animal suffering
- are native to the area in question or at the very least do not cause harm to local ecosystems
Obviously I do not give them priority over animals (which can suffer) or crops (which can die and cause humans to suffer), nor do I think non-native diseases should be allowed to run wild over native ecosystems. But a disease is an amazing organism in its own right. Provided it is not directly causing suffering like a human or animal pathogen there is little that separates it from the rest of the microbiome: interesting and highly valuable as sources of genetic diversity.
Of course, this is all assuming that the disease is a result of a microbial pathogen. That seems like disease symptoms in the garden may also be the direct result of animal activity as well. There are a great many mites, flies, wasps, nematodes, and more that make their home inside of plant tissue, forming tunnels, galls, or growths that may appear to an untrained eye as disease. These creatures then feed a whole array of predators and parasites, serving as part of the assemblage of herbivores upon which other organisms feed.
I don’t like to place a value on these small creatures only based on their utility to others. They are fascinating organisms in their own right and the ways they use plant tissue to protect and feed themselves is a wonder of nature. They certainly have a right to exist and their mere existence in the garden should not be a call for extermination. Beyond that, many galls are wildly fascinating, with colors and shapes completely different than their host plant. Under close observation many of these insects themselves are beautiful, with glittering colorful carapaces and a diversity of form that illustrates the intricacies and quirks of evolution better than almost anything else.
That being said, if you can’t convince yourself of the value of these critters by themselves you can also think of the utility they provide to other animals. These tiny herbivores are fed upon by predaceous insects, birds, reptiles, and mammals, just the kinds of animals many want to see in their yards. They might worth a bit of tolerance if they can help make your yard a healthier, more diverse ecosystem.
Diseases as Drivers of Diversity
A disease is a world unto itself, a tiny drama on a cellular level. The life cycle of a pathogen can be simple or complex, and many pathogens toe the fine line between friend and foe, going from plant friend to dangerous disease with something as simple as a change in a weather. As a result these plant diseases often do not result in plant death, but either way they can cause serious alterations to plant growth and create large amounts of dead plant tissue.
This might sound like a bad thing to an avid gardener passionate about protecting their plants from harm, but I don’t think this reflex to treat and protect is correct. The dead parts of a leaf, the fruiting body of a mushroom, a tree fell by root rot – these are all new energy sources and habitat for other organisms.
Fallen logs comes back to life by the activity of saprotrophic fungi and insects, providing food and shelter for salamanders and snakes. Leaves killed by mildew fall and are quickly devoured by soil biota, enriching the soil. Fruit rotten by oomycetes and bacteria are dropped from the tree and quickly taken over by fly larvae, which birds harvest to feed their young. Even disease cankers themselves weep nutrient-rich liquid, providing food for a diversity of microbial life. Plant disease creates new opportunities for life to thrive and creates a more diverse and thus more resilient ecosystem.
Disease as a human problem
Respecting disease as promoters of diversity is all well and good, but as I mentioned above this is assuming they are not causing damage to edible plants. We rely on plants for food, thus it makes sense that we might keep a closer eye on any disease symptoms on our fruit trees or veggies. Our fruits and vegetables are still a part of our ecosystem, however, and disease control on the plants we eat can have unintended side effects on our garden ecosystem at large.
For those as small as a microbe, a host plant is a world unto itself. Even the smallest plant is home to a large community of microbes, ranging from beneficial to pathogenic. A plant is a very heterogeneous environment, with many different parts of the plant providing variable utility to a constantly changing assemblage of microbes. These microbes are often in cooperation, forming biofilms in the most habitable spots, but also compete with each other often quite brutally in a constant race for the best spot and the most nutrients.
This often happens through an effect called competitive exclusion. Microbes that exist on the plant first have the chance to colonize the habitable parts of the plant and keep others from invading. This effect tends to be stronger the more distantly related the microbes are, but even related and less harmful lineages of a pathogen can directly compete and might be able to reduce the risk of invasion by a closely related pathogen in addition to excluding unrelated diseases.
Of course, microbes also directly attack and prey on each other. Many of our antibiotics and antifungals come from microbes themselves: they make these chemicals as weapons to attack each other directly. Whether it is for defense or offense, whether it is for finding a dinner or avoiding becoming one, these microbes can be applied to plants as a form of biocontrol to attack plant pathogens. Microbes are already being used today for this exact purpose, and there are multiple products on the market all purporting to use the principle of competitive exclusion to control plant disease.
The suite of chemicals used by microbes to fight each other has also been the inspiration for the creation of pesticides, including fungicides and bactericides. These products are often marketed for the control of a wide variety of relatively minor plant diseases, promising control of practically every leaf spot and fruit blemish you could ever think of. They operate off the common assumption that every disease needs to be cured, an idea they gleefully use because it allows them to make more money off of often unnecessary and possibly dangerous products.
We should not assume that plant diseases actually require control. Some of the microbes we think of as disease are not usually very harmful. A great many disease-causing microbes are really only pathogenic in the eyes of humans: causing scabs, dots, or blemishes that otherwise would not cause much harm to the plant. As long as the infections are not massive, the plant will not suffer too much from their continued presence. They are considered undesirable for human use, however, as end consumers have been trained to reject any and all imperfection in what is ultimately an imperfect product (produce). This is done not because of biology but ultimately for profit.
The push for perfect, unblemished fruits and vegetables has many undesirable side effects, from pesticide overuse to food waste. More relevant to us here is the collateral damage that comes with the control of minor aesthetic issues. Treating with pesticide will seriously change the makeup of the plant microbiome, possibly leaving it much depleted and less diverse. Stripping off the microbial community with fungicides or bactericides leaves a massive unexploited ecological niche, possibly giving much worse pathogens a chance to invade the now-unprotected plant.
Chemical disease control, even on edible crops, should be our absolute last line of defense. In a low-stakes environment like a home garden, the negative impacts of chemical disease control will usually massively outweigh the positive. When pathogens do become a problem and cause massive infections that kill and weaken plants it is often because the plants are not well-suited for the local environment and are particularly susceptible to disease.
Apple scab, a very common disease of apple, is normally not a life-threatening disease unless an apple variety is particularly susceptible. Varieties that are almost completely resistant exist, and yet susceptible varieties are still sold in garden centers and planted in orchards. Apple scab is simply taking the opportunity to colonize a particularly weak plant – susceptible varieties should be weeded out and replaced with robust cultivars that do not require treatment in the first place.
In some cases even resistant varieties of plants can suffer infections in poor conditions and susceptible cultivars can resist infection if they are healthy. Choosing resistant, robust cultivars and maintaining good growing conditions should be enough to prevent the majority of plant disease. If it isn’t, the genetics in use are not good enough. Using chemical control to make up for a lack of good genetics is simply creating a time bomb: the minute resistance develops, all your control efforts have basically been for nothing and the disease will rampage through the susceptible population.
Minor pathogens themselves occupy a niche on the plant, often are not terribly harmful, and can help exclude more serious plant diseases, but they can also create new opportunities for other microbes to invade a plant and have unintended effects on the course of a plant epidemic. The formation of cankers and wounds on a plant can give an easy point of access to other organisms, creating diversity but also risk. A gardener needs to weigh these risks before allowing a pathogen to thrive in their garden. In the reverse, however, a gardener also needs to assess the actual risk posed by a pathogen before attempting control efforts. The gardener also needs to carefully assess the effects of any control methods on the ecosystem of the garden as a whole, including plant microbiomes, or else they will risk reducing the diversity of their garden and causing unintended consequences (pest or disease problems worse than the original).
Assessing the risk
Disease management in the garden needs to be more complicated than just seeing a disease, buying a potentially dangerous product/concocting some ineffective homemade remedy, and spraying. Diseases themselves are organisms with their own place in the garden ecology and often increase biodiversity in the garden. Thus, any disease that meets the following criteria should probably be left to their own devices.
- do not cause animal/human suffering
- do not cause serious damage to crops in ways that would cause human/animal suffering
- are native to the area in question or at the very least do not cause harm to local ecosystems
If control is necessary, measures need to be taken to prevent negative impacts of the control measure on biodiversity. Limiting the scope of the application to only disease-afflicted plants is important, and using plants with genetic resistance can help avoid the problem in the first place. Biocontrol products, while usually relatively safe to humans and pets, should not be the first line of defense because their effects on local ecology are not well understood. They are valid backups, however, and almost certainly better than using either natural or synthetic fungicides/bactericides. The best defense is not always a great offense, and in the case of plant disease even defense is probably unnecessary.