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.

The first slide you see is of malanga infected with dasheen mosaic virus. 

Fig. 52. Infected malanga.

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.

What is wrong with this yellow squash leaf?

Fig. 53.  Symptom type?

Fig. 54.  Symptom type?

What would you describe as the major symptom of the diseased tomato plant in the middle of the picture?

Fig. 55. Major symptom?

What is abnormal about these papaya fruit?

Fig. 56. Symptom type?

Fig. 57.  The disease triangle.  

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.

Next: Module 7 -- Plant Disease Control