Plant Diseases: Types, Identification & Tests

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson we'll discuss how to identify different types of plant diseases. We'll look at signs and symptoms of different diseases and learn what resources to use to identify the cause of the disease.

What Are Plant Diseases?

For many of us, summer involves backyard or container gardening. Big, fluffy tomato plants are popular on summer patios. Not only do these beauties grow incredibly tall, but they also yield delicious ripe tomatoes all summer long. However, imagine walking out onto your patio only to notice brown lesions all over your tomato plant leaves. The lesions are surrounded by tissue that is turning yellow and dying. What is happening to your plants?

These symptoms are one example of plant disease. Plant diseases are usually caused by fungi, as is the disease which causes the early blight in tomato plants described here. However, diseases can also be caused by bacterial or viral infections. In this lesson we will look at some common signs of these types of diseases and discuss how to test for the exact cause.

Fungal Diseases

Fungal diseases are plant infections caused by fungi. Fungi can be single or multicellular, but either way infect plants by stealing nutrients and breaking down tissue. Fungal diseases are the most common infection in plants.

Symptoms and Signs

There are some characteristic symptoms, or observable effects of the disease, in plants. Fungi infections can be recognized by symptoms like spots on plant leaves, yellowing of leaves, and birds-eye spots on berries. With some fungal diseases, the organism itself can actually be viewed on the leaves as a mold, mildew or spores. These may appear as growths or malformations on stems or the underside of leaves. These direct observations of the disease-causing organism are called signs of infection.

Leaf spots caused by a fungal infection
leaf spots

Bacterial Diseases

Bacteria are single-celled, prokaryotic organisms. Bacteria are everywhere and many can be beneficial, but some can cause disease both in humans and plants.

Signs and Symptoms

The signs of bacteria are often harder to detect than fungi, since bacteria are microscopic. Upon cutting an infected stem, a milky white substance may appear, called bacterial ooze. This is one sign of a bacterial infection. Other signs include water-soaked lesions, which are wet spots on leaves that ooze bacteria. Eventually, as the disease progresses, the lesions enlarge and form reddish-brown spots on the leaves.

A common symptom of bacterial infection is leaf spots or fruit spots. Unlike fungal spots, these are often contained by veins on the leaf.

Bacterial leaf spot on a hibiscus plant
bacterial leaf spot

If these spots advance rapidly, the disease can be considered a blight, where parts of the plant spot, wither and eventually die. Bacteria also produce cankers, which are malformed areas that appear to be sunken in on branches or stems. Bacterial cankers may ooze during the spring, whereas cankers caused by fungi may produce fruiting bodies.

Canker on an ash tree

Bacteria as well as fungi can also cause rot, where parts of the plant decompose and decay. Pathologists should look for soft, rotted patches on fruits, tubers, stems, and bulbs to diagnose a common type of bacterial rot called soft rot.

Viral Diseases

Viruses are infectious particles that are too small to be detected by a light microscope. They invade host cells and hijack host machinery to force the host to make millions of copies of the virus.

Signs and Symptoms

Viral diseases don't show any signs in plants since viruses themselves cannot be seen even with a light microscope. However, there are symptoms that the trained eye can observe. A mosaic leaf pattern, yellowed, or crinkled leaves are all characteristic of viral infection. This classic pattern of discoloration is where many plant viruses get their name, such as the tobacco mosaic virus. Also, decreased plant growth is also commonly seen in viral infections.

Leaf discoloration by Abutilon mosaic virus
Abutilon mosaic

The mosaic pattern of discoloration seen in leaves can also be seen in fruits. Fruits may have also have ringspot symptoms, where discoloration appears in rings, similar to leaves.

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