Viruses, Bacteria, Fungi

Just like people, plants are susceptible to illness from bacteria, viruses, and most of all, fungi. Learning to recognize the signs and symptoms of these issues will help you treat and prevent them. 


Almost all (approximately 85%) of plant diseases are caused by fungal pathogens, which can be spread by spores through the air and water, in soil, and even on the unwashed blades of your pruners. Some of the most common ones are:

      • Black spot appears as dark blotches on the top sides of leaves, expanding until the leaf is mostly yellow and dotted with black. It requires the presence of water on plant surfaces to thrive and reproduce, and so does very well in crowded gardens especially during periods of heavy rain.
      • Rust derives its name from the orange pustules that form on the undersides of leaves. As the fungus spreads, the leaves discolor and eventually fall off. The spores spread with the help of wind, water droplets, and insects, and thrive in cool, damp conditions. 
      • Botrytis blight, also known as gray mold, is a fungal disease which primarily affects the buds, flowers, leaves, and bulbs of many plants like African violet, begonia, chrysanthemum, cyclamen, dahlia, geranium, lily, peony, rose, and tulip. Flowers will develop abnormally and have irregular brown flecks or turn completely brown. Affected plant parts may also be covered with a gray mold following damp, cool weather.
      • Powdery mildew actually refers to a number of different fungi, each of which affects a specific type of plant. Manifesting as a powdery growth on leaves, new shoots and other plant parts, powdery mildew needs only the wind to spread, and stays active even in dry, warm weather, making it one of the most persistent garden pests.
      • Root Rot is a fungal attack caused by various fungi below the soil surface. These fungi thrive in moist soil and are often fueled by increased watering when the plant owner sees a wilted plant. Root rot causes decaying roots, which in turn prevent the plant from processing water or nutrients. Signs of the disease can be wilted or yellow leaves, stunted growth, or leaf drop. These fungi can kill a plant quickly (cacti/succulents) or gradually (trees and shrubs) depending on the size of the plant and severity of the fungus.

To avoid fungal infections, use sterile, well draining soil, avoid crowding your plants, check soil for moisture before watering, and make sure the plant has adequate light. It can be difficult to remove a fungus, so prevention is the best tool, but if an infection does develop, remove any infected areas of the plant and consider applying a fungicide such as baking soda, copper, or sulfur. Washing or removing the infected roots and using fresh soil is also recommended for severe cases.


There are millions of bacteria in the world, and only a small percentage of them are harmful to plants, and these dangerous varieties tend to be mainly active in warm humid environments. Unlike fungi, bacteria are invisible to the naked eye, so what we see are their symptoms, not the bacteria themselves.

Common bacteria symptoms include leaf spot– as the bacteria attacks the plant it releases toxic chemicals into the surrounding tissue, and the plant will defend itself by killing off its own tissue surrounding the infected area, basically quarantining that section of the leaf. Sometimes this dead zone will fall out, creating small holes in the leaf. Bacteria can also quickly disrupt a plant’s ability to move water and nutrients through its body, causing severe wilting. An additional opposite sort of symptom is a rapid, misshapen growth called a “gall,” an overabundance of cellular growth in response to the bacteria’s presence. 

Bacteria can spread in several ways: insects, water, proximity to other diseased plants, or contaminated tools. And once they’ve gotten established they can be difficult to control, so it’s best to practice preventative measures such as immediately removing infected plant parts and destroying them (do not compost), always using clean tools, and maintaining a clean garden, free of debris.


Unlike bacteria and fungi, viruses cannot be spread by wind or water. Instead, they must physically enter the plant. One of the most common vectors of viruses are insects, which may feed on infected plants and transmit the viruses to healthy plants when they feed again. Other ways a virus can  spread are through plant propagation and infected seeds.

A virus can survive in your garden for years, lying in wait before any symptoms appear on your plants. These symptoms can include yellow foliage, or mosaic-like patterns of yellow, pale green, or white. Plants may also appear to be stunted or misshapen. Leaves may also be rolled up, puckered, swollen, or abnormally narrow. 

Unfortunately, there are no chemical treatments for eliminating a virus. Though it might seem extreme, you should remove all infected plants once a virus has been detected, in order to prevent further spreading. 

How can I protect my plants from disease?

  • Choose plant varieties with proven disease resistance, and match your planting site to the plant's requirements. Poor matches set plants up for stress and disease.
  • Water wisely. Blindly spraying your garden’s foliage with the hose encourages water-spread pathogens. Get as close to the soil as possible with your hose nozzle or with your watering can to reduce wet leaves, and water early in the day so excess moisture dries by nightfall. Always check the soil moisture before watering. Overwatering is the leading cause of plant death.
  • Maintain good air circulation and light penetration in and around plants through strategic pruning and proper placement. For houseplants, place your plant as close to a window as possible.
  • Prune infected plant parts promptly and dispose of the debris — don't compost it. Always cut back into healthy tissue, so no disease remains.
  • Sterilize your pruning implements by wiping them with any common household disinfectant. If you suspect disease, wipe before and after each cut, lest your well-intentioned pruning unwittingly spreads the problem.