Plant Pathology Guidelines for Master Gardeners
Contact: Dr. Richard Raid
Symptoms Of Viral Diseases
Of all the diseases, those caused by viruses are the most difficult to diagnose. This should not be surprising - viruses produce no telltale signs that can be readily observed. What’s more, symptoms are often quite subtle, often easily confused with nutrient deficiencies and herbicide injury. In many cases, as a Master Gardener, you will need assistance from county agents and the regional Plant Disease Clinics in properly diagnosing suspected virus problems.
Figure 52
The first slide you see is of malanga infected with dasheen mosaic virus.
The disease is called dasheen mosaic. You can see that the leaf is a pattern of off colors, some darker, some lighter than normal. These alternating darker, lighter patterns are referred to as mottling. This is a quite common symptom associated with virus diseases. Obviously, you must be familiar with what a healthy malanga leaf looks like in order to detect this subtle symptom pattern in the dasheen mosaic virus-infected malanga.
Figure 53
What is wrong with this yellow squash leaf?
Figure 54
Symptom type?
Figure 55
What would you describe as the major symptom of the diseased tomato plant in the middle of the picture?
Figure 56
What is abnormal about these papaya fruit?
Figure 57
Most of us realize that disease development requires a susceptible host plant and a virulent pathogen. This image shows the disease triangle where the third essential factor, a favorable environment, is included. Diseases usually have relatively specific conditions of temperature, relative humidity, free moisture, etc. for the symptoms to be expressed or the severity to increase. Knowing what time of the year a specific disease likely occurs can affect the choice of probable causes of a plant malady.
Answers
Figure 53
You should have readily noted the marked mottling (dark green/yellow) of the leaf. In addition, if you are familiar with the shape of normal squash leaves, you can see that the one in the picture is narrowed and distorted. All these symptoms are associated with papaya ringspot virus infection of squash.
Figure 54
This tomato plant has a disease caused by tobacco etch virus (TEV). Note that the internodes (space between branches on the stems) are shortened, giving the plant a bushy appearance. Several of the leaflets show browning (necrosis). This virus was transmitted by aphids.
Figure 55
The obvious symptom here is stunting. The infected plant is considerably shorter than adjacent healthy plants. Stunting occurs frequently in virus-infected plants. This disease is tomato mottle, caused by the tomato mottle virus (TmoV), vectored by whiteflies.
Figure 56
These fruit are covered by circular and C-shaped markings. These marks give the virus that causes them its name - papaya ring spot virus. Mottling and distortion of leaves also occurs. Fruit sometimes have reduced sugar levels, so flavor can be affected. This is the most common and serious disease of papaya in Florida.
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