What is Plant Disease?

When our plants are stressed, we are stressed. Yellowing leaves, brown spots, withering stems, and funny smells are only a few signs that tell us that something is off with our beloved plants. While our minds tend to jump straight to disease, there are a number of other causes for leaf issues. Once you have eliminated water, humidity, pests, and lighting as potential causes, you can then turn to plant diseases.

As humans, we are constantly in contact with good and bad germs that can become a problem when our immune systems or natural defenses are down. Plants function in a similar way. The world is full of microscopic bacteria, fungi, and viruses that only need an excuse to enter the plant and cause issues. 

Root Diseases

Soil-borne diseases can lie in wait for an issue like damage from overwatering and poor drainage to enter into a plant’s roots, contributing to what is called root rot. Once the root cells are damaged and collapse from too much water, fungus can move in, feeding on the decaying root tissue and further spreading the rot. Fungi need moist environments to live, so oversaturated soil is the perfect environment.

We often do not realize that our roots are having issues until other parts of the plant are showing symptoms. These symptoms can include wilting leaves or flowers, yellow or brown spots, and smells coming from the soil. The reason why a plant might look like it is thirsty when it is overwatered and struggling with root rot is because it technically is thirsty. Decaying root tissue is not able to function properly, and the plant may no longer be able to drink enough water to support itself.

It is difficult to bring a plant back that has been seriously affected by root rot, but it can be done with care and patience. To treat root rot, you need to remove a plant from its potting media and trim off any dead or decaying roots with sterile scissors. There are some home remedies that can be used to treat the roots, but each of these should be thoroughly researched, as some can do further damage to healthy plant cells. Be sure to repot your plant in a pot that is not much bigger than the root ball, using fresh, sterilized soil. Restart a watering routine with care, after giving the roots some time to dry out.

Foliar Diseases

Foliar diseases are those that originate on the leaves of the plant. There are three basic causes of foliar diseases: bacterial, fungal, and viral. Molds and mildews are also found on leaves, though technically mildew is a type of mold and mold is a type of fungus. Pathogens that infect a plant’s leaves are usually transported by water or air and enter through a wound or vulnerable structure. For this reason, it is important to always use sterile scissors when trimming your plants. Insects also have the ability to spread diseases, so stay ahead of them by scouting your plants for signs of pests regularly. While plants have the ability to scab and seal off wounds, you can assist in this process by applying cinnamon to the opening, as it has natural antibacterial and -fungal properties and can dry out the wound. Use cinnamon sparingly and only on open wounds as it can dry out healthy plant tissues.

Bacterial

There are approximately 200 species of bacteria that will infect plants, most thriving in warm, humid conditions. Bacterial leaf spots will often be “wet” in appearance, showing up as black or brown spots, sometimes surrounded by a yellow halo. The spots will slowly grow in between the leaf’s veins, looking moist and eventually collapsing to create a crater or hole in the leaf. Interestingly, this ‘shot hole’ is actually a combination of the bacteria’s damage and the plant’s immune response that isolated the infection! Other bacterial infection symptoms include wilting, cankers, soft rot, and even abnormal growths called ‘galls.’

Wetting the leaves of your plants and poor air circulation will exacerbate the spread of bacterial diseases. Since a plant cannot heal damaged tissue, the best treatment is to remove the affected area early to prevent further spread. If the disease becomes systemic (spreads through the entire plant), the plant will, unfortunately, need to be disposed of.

Fungal

Thousands of types of fungi are responsible for diseases above and below the soil. Leaf spots caused by fungi can be similar in appearance to those caused by bacteria, the most distinct difference being that fungal leaf spots often have a ‘target’ pattern to them. Fungal spores create a center point within a brown to black necrotic spot. Fungal leaf spots can occur in clusters, eventually growing together. Like bacteria, fungal spores are spread through water droplets, and poor air circulation can contribute to their spread. 

As there is no repairing already infected tissue, leaves infected with fungal spots can be removed to prevent its spread. As fungal spores can survive on dead plant tissue, keep the area around your plants clean and regularly remove fallen leaves from the soil. 

Molds and Mildews

Molds and mildews can appear on flower, leaf, stem, and soil surfaces alike. The presence of visible molds is an indication that something is off about your plant’s environment, regardless of whether they are harming the plant or not. White mold on your soil may be an indication that your soil is staying too wet. Gray mold may infect your flowering plants when they are in stagnant air, ruining the blooms before their prime. Sooty mold is indicative of sap-sucking insects like aphids. Each of these has similar treatments: remove the infected tissue, isolate infected plants, treat with fungicide, and change their environment by providing better air flow and reevaluating your watering methods.

Powdery mildew is one of the most recognizable types of fungal leaf diseases. While more common at certain times of the year, it can spread rapidly if left unchecked. Early on in infection, perfectly round, raised, dusty white spots can be found on the leaf surface, especially where there is stagnant, humid air. An infected leaf should be completely removed as the infection will be deeper than just a singular spot. Early detection, treatment, and follow-up treatments are key.

Viral

Viral infections tend to have more signature symptoms like mottling and growth distortions. Viruses are extremely small, highly infectious pathogens that have no cure. If your plant suddenly looks like it is becoming variegated and its new growth is coming out distorted, it might be a viral infection. To prevent spreading the virus to other plants, it is best to destroy the host plant. Viral infections can be prevented with clean indoor plant practices.

Your number one defense against plant diseases is keeping your plants healthy, to begin with. Plants have immune responses to isolate and fight off diseases, some of which are still being discovered today. It is when a plant’s health is compromised that diseases can really take off. Always research how to care for your plant - what kind of light they need, how to water them, whether they are naturally prone to certain pests or diseases, and how to prevent them. When you are doing regular maintenance with your plants, like watering, pruning, repotting, and fertilizing, be sure you are using clean materials. Sterilize your scissors, avoid reusing soil, sterilize pots prior to transplanting, and check for anything funky like mold growing in your fertilizers. An important note about many chemical treatments for plant diseases is not all are suited for indoor use. Be sure to always read labels prior to using pesticides, or opt for more natural options like Neem or Horticultural oils. Keep in mind that there are still good bacteria and fungi that actually help our plants, so indiscriminate use of chemicals is discouraged.

When trying to identify specific plant diseases, you may need to seek the help of microbiologists and plant pathogen specialists. Some state schools partner with the United States Department of Agriculture to provide such services to their residents. You can find your state’s land-grant university extension service by checking out this website!