University of Florida

Plant Pathology Guidelines for Master Gardeners

Contact: Dr. Richard Raid

Symptoms Of Plant Diseases

Symptoms are abnormal states that indicate a bodily disorder. It is important that all concerned, Master Gardeners, Master Gardener coordinators, county agents, and UF plant pathologists use the same terminology when describing disease symptoms. Fortunately, the terminology used for description of plant symptoms is really quite simple and straightforward, not at all like that of, say, human medicine.


Figure 13

Observe Figure 13 (below) carefully. It is a schematic representation of the basic functions in a plant (left) and of the interference with these functions (right) caused by some common types of plant diseases.

Figure 13
(Modified from Agrios, G.N. 1997. Plant Pathology (4th ed.). Academic Press, NY, NY.)

Many of the symptom classes are illustrated here. A "spot" is a relatively small, distinct lesion, with definite borders. Most times, we indicate the plant organ affected when describing a plant disease symptom. For example, if the spot is on leaves, it is called a "leaf spot". If the spots are on fruit, it is naturally a "fruit spot".

As spots grow and coalesce, the symptoms may well be described as a "blight". There are gradations from spots to blights and the better term to use may not always be clear. Galls or tumors are masses of undifferentiated growth (similar to cancerous growths in humans). They are usually associated with the woody growth of stems and branches. Cankers, again, found mostly on stems and branches, are sunken lesions. Wilts occur when plants droop, indicating problems with water uptake. Rots occur when tissue breaks down. Often rots lead to a slimy, wet "mush". However, dry rots can occur. It is important that you get used to using these terms, so that everyone is on the same page when describing symptoms.

Examine the following set of pictures. Name the symptom type (e.g., leaf spot, wilt) that best fits each malady.


Figure 14

Figure 14
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Figure 15

Figure 15
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Figure 16

Figure 16
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Figure 17

Figure 17
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Figure 18

Figure 18
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Figure 19

Figure 19
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Figure 20

Figure 20
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Figure 21

Figure 21
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Figure 22

Figure 22
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Figure 23

Figure 23
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Figure 24

Figure 24
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Figure 25

Figure 25
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Figure 26

Figure 26
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Figure 27

Figure 27
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Figure 28

Figure 28
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Figure 29

What is the symptom seen here on mango?

Figure 29
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Figure 30

What is the symptom seen here on strawberry?

Figure 30
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Figure 31

What type of symptom is seen on this papaya fruit? Where did the causal agent come from?

Figure 31
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Figure 32

What is the symptom seen on these tomato plants?

Figure 32
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Figure 33

What is wrong with this watermelon plant?

Figure 33
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Figure 34

What is abnormal in the internal tissues of this tomato stem?

Figure 34
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Figure 35

Figure 35
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Figure 36

Figure 36
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Figure 37

Figure 37
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Figure 38

What would you call this fungus-caused problem on snap bean?

Figure 38
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Answers

Figure 14

Leaf spot is correct. This is bacterial spot of tomato.

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Figure 15

Leaf spot is the right answer. These are older lesions of bacterial leaf spot of malanga (Xanthosoma), an edible aroid.

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Figure 16

Another leaf spot. This is rust on snap bean.

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Figure 17

This is a transition state from leaf spot to leaf blight, as the individual lesions begin to coalesce. Remember that gradations across symptom classes occur all the time. The name of the disease is bacterial spot of pepper, irrespective of the symptoms seen.

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Figure 18

Symptoms best described as both leaf spot (upper portion of image) and leaf blight (lower portion of image) are both plainly evident. This combination of symptoms occurs all the time in nature. This example is downy mildew of yellow squash.

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Figure 19

Blight best fits this symptom on tomato leaves. This is the famous late blight of tomato, the causal fungus of which also attacks potato.

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Figure 20

Leaf blight is the most reasonable choice. This is common bacterial blight of southern (blackeye) pea.

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Figure 21

This is an example of canker on tomato stem. The lesions are sunken and may eventually girdle the stem, leading to wilt and death. The symptoms are part of the disease known as early blight.

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Figure 22

These impressive growths are galls on oleander. The disease is called Sphaeropsis gall and is common on bottlebrush trees as well.

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Figure 23

If you look carefully, you can see a gall on the leaf of this croton. This is caused by a fungus.

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Figure 24

This symptom on mango is best described as leaf blight. The disease is anthracnose, by far the most common disease of mango in Florida. The causal fungus attacks all parts of the mango tree, damaging leaves, fruit, and flowers.

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Figure 25

This mushy, soft stem is best labeled as a stem rot. This is bacterial soft rot or hollow stem of potato.

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Figure 26

This is a dry stem rot of celery, known as brown stem.

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Figure 27

The symptom here is fruit spot. The lesions are described as "scabby"; indeed, in Israel, the common name of the disease is bacterial scab. The disease is actually bacterial spot of pepper. You have seen pictures previously of this disease on pepper leaves.

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Figure 28

These cucumber fruit are soft and almost watery. The causal fungus can be seen as a white growth covering portions of the rotted fruit. The symptom is obviously fruit rot, and the disease is known as Pythium fruit rot of cucumber. Local growers call it "leak", because as the fruit break down, they literally leak fluid out of packing boxes.

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Figure 29

Fruit spot is the symptom seen. As might be expected, this is another manifestation of anthracnose disease.

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Figure 30

This is a fruit rot. Even though the tissue is dry, fruit rot is still the best term for describing the symptoms. This disease is anthracnose of strawberry, although the fungus that causes the disease is not the same one that causes anthracnose of mango.

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Figure 31

The symptom is fruit rot. This is Phytophthora fruit rot of papaya. The pathogen came from the soil, probably splashed up onto the lowest fruit during a rain event.

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Figure 32

The plants exhibit wilt. How do you know if the problem is truly caused by a pathogen and not by a general lack of watering? If you noticed that surrounding plants appear healthy, you're a good detective! A problem with underwatering would be manifested throughout the planting. A disease, such as the bacterial wilt seen in this picture, attacks in "hot spots" more-or-less randomly distributed about the field.

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Figure 33

Obviously, it is showing symptoms of wilt. This is an isolated plant among many healthy-looking ones in a large watermelon patch. Therefore, it is likely to be a disease rather than merely a lack of water. This is one of the most important diseases of watermelon in Florida - Fusarium wilt (a fungus disease).

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Figure 34

This view of the interior of a tomato stem illustrates quite nicely a sure-bet diagnostic feature of true, pathogen-induced wilts. A longitudinal cut has been made in the lower stem. Note that the vascular (water-conducting) tissue at the bottom of the picture is brown ( the vascular tissue is toward the outside of the stem, parallel to the outer surface). This is abnormal. The tissue should be white to light-green as seen in the top of the picture. This appearance of brown vascular tissue when stems are cut in this manner, is definitive proof of a wilt disease.

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Figure 35

This is a special symptom known as damping-off. Sometimes seeds planted in soil will rot before emergence, or young seedlings such as those pictured here will rot at the soil line and topple over. These are examples of pre- and post-emergence damping-off, respectively. All damping-off problems are caused by pathogens, soilborne fungi such as Pythium and Rhizoctonia.

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Figure 36

This is an example of damping-off of watermelon seedlings caused by Rhizoctonia. The thinning and twisting of the young stems leads to the common name of this problem among growers - wirestem.

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Figure 37

This is a soft, mushy root rot of cassava caused by a bacterium, Erwinia carotovora. This followed borer insect injury.

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Figure 38

If your botany background is up to speed, you may very well have described this as a fruit rot, knowing that the bean pods are the fruit of this plant. Pod rot would also be a workable description, telling your extension colleagues and clientele exactly what you're observing.

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