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.
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.
(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.
What is the symptom seen here on mango?
What is the symptom seen here on strawberry?
What type of symptom is seen on this papaya fruit? Where did the causal agent come from?
What is the symptom seen on these tomato plants?
What is wrong with this watermelon plant?
What is abnormal in the internal tissues of this tomato stem?
What would you call this fungus-caused problem on snap bean?
Leaf spot is correct. This is bacterial spot of tomato.
Leaf spot is the right answer. These are older lesions of bacterial leaf spot of malanga (Xanthosoma), an edible aroid.
Another leaf spot. This is rust on snap bean.
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.
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.
Blight best fits this symptom on tomato leaves. This is the famous late blight of tomato, the causal fungus of which also attacks potato.
Leaf blight is the most reasonable choice. This is common bacterial blight of southern (blackeye) pea.
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.
These impressive growths are galls on oleander. The disease is called Sphaeropsis gall and is common on bottlebrush trees as well.
If you look carefully, you can see a gall on the leaf of this croton. This is caused by a fungus.
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.
This mushy, soft stem is best labeled as a stem rot. This is bacterial soft rot or hollow stem of potato.
This is a dry stem rot of celery, known as brown stem.
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.
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.
Fruit spot is the symptom seen. As might be expected, this is another manifestation of anthracnose disease.
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.
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.
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.
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).
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.
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.
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.
This is a soft, mushy root rot of cassava caused by a bacterium, Erwinia carotovora. This followed borer insect injury.
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.
- Next: Signs Of The Pathogen