Common Causes of Plant Diseases

Instructor: Yuanxin (Amy) Yang Alcocer

Amy has a master's degree in secondary education and has taught math at a public charter high school.

Read this lesson and you'll learn that plants can get sick just like we do. You'll see how viral, bacterial, and fungal infections can affect plants and even kill them.

Plant Diseases

Just like we can get sick, plants can get sick too! And sometimes, the same types of things that make us sick can affect our green companions. A sick plant can even look 'under the weather'; it's leaves may wilt or have holes in it. If not treated, these plants may succumb to the disease and perish.

Pathogens are any organism that makes plants ill, whether it be another animal such as an insect, or smaller organisms such as bacteria and viruses.


When a virus, a microorganism that invades living cells, infects a plant, you can see its effects in the leaves and fruit. You might see yellow spots or yellow streaking, or notice stunted growth or deformed leaves.

Plants infected with a virus may show yellow spots or streaking, like in this cucumber mosaic.
cuc mosaic

When a cucumber has this viral infection, you can see it in the skin, known as 'cucumber mosaic'.

When a virus infects a plant, the virus lodges itself inside the cells of the plant and hijacks the machinery to produce even more viruses. This process prevents the plant's cells from operating as they should, weakening it.

There is no one common virus that affects plants. Viral infections can spread from plant to plant through touch or insects such as aphids that end up feeding from a sick plant and then a healthy one. Quick disposal is often the best way for farmers to cut their losses.


When a plant has a bacterial infection, it means that it is being infected by microscopic living organisms that are usually one-celled. You'll see the effects as spots, wilts, and scabs on the leaves, fruit, and roots.

This plant is exhibiting spots and scabs from a bacterial infection.
plant diseases

'Blight' refers to a bacterial infection that spreads rapidly. You'll see the spots on the plant spread rapidly. If not treated in time, the bacteria will spread throughout the plant and weaken it the point where the plant has no energy to grow anymore.

Another common bacterial infection is simply called 'wilts'. Wilts usually affects vegetables, crops, and tropical plants. The bacterium prevents the plant from absorbing water as it should, causing it to wilt and die.

Specific wilt disease can also be caused by fungi and viruses, but bacterial wilts is caused by species in the genera Corynebacterium, Pseudomonas, and Xanthomonas. Younger leaves succumb first, but stems too can shrivel, and show an off-color interior.


In addition to viruses and bacteria, plants can also be affected by fungi, organisms that produce spores and feed on other organic matter. Fungal infections often result in mold or mildew that grows on the plant.

Mushrooms growing on wood.
plant diseases

This fungi ends up eating the plant from the inside out. Fungi can attack plants through any openings such as its own stomata or through wounds caused by pruning, insects, or diseases. Fungal infections can be spread through the air and water splash. Plants can also get infected if they are put into soil that is already contaminated with the fungus.

Downy mildews, for example, is a fungal infection that affects a wide variety of crop plants including peas, lettuce, kale, celery, herbs, and spinach. This particular fungal infection produce yellow leaf spots at first which then turn brown. The underside of leaves will have a downy growth.

Some fungal infections are specific to the plant. For example, carrots get leaf blight and beans get red root complex.

Not all fungal growth is bad though. Mushrooms that break down dead wood and other dead organic matter can be beneficial to plants as they return valuable nutrients to the soil. But, if the fungal growth begins to cover the plant, then it is definitely harmful.

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