Diseases of Vegetable, Ornamental and Spice Crops

Exercise 14: SYMPTOMS OF PLANT VIRUS DISEASES

Symptoms are defined as the macroscopic or microscopic modifications or variations (form, colour, size) appearing in plants as a result of pathogenic actions, nutrient deficiencies, adverse environmental conditions, etc.

The study of viral disease symptoms requires careful periodic observation of infected plants (indicator plants) to determine precisely the type, intensity, and prevalence of any abnormality. The most evident symptoms are those appearing in the leaves, although some viruses may cause symptoms in the stems, fruits, roots, or other parts of the plant. The most common symptom of viral diseases is slower development resulting in plant dwarfism. Almost all viral diseases reduce yield, and the effect may range from severe and easily perceivable to negligible.

In most viral diseases of plants growing in the field, the virus affects the whole plant (systemic infection). The symptoms are called "systemic." In artificially inoculated plants, certain viruses cause small, usually necrotic, lesions at the point of entry (local infection). The symptoms are called "local lesions."

Many viruses can infect certain hosts without the development of evident symptoms. They are called "latent viruses" and their hosts, "asymptomatic." In both cases, plants that usually develop symptoms when infected by a virus may remain asymptomatic for a certain period under certain environmental conditions, such as high or low temperature. This is known as "masking."

Lastly, artificially virus-infected plants may show severe symptoms immediately after inoculation, and eventually die. If the plant survives the initial shock, the symptoms can be mild ("chronic symptoms") with subsequent partial or total plant development. This is called "recovery." Alternatively, symptoms may gradually become more severe and result in slow or rapid plant degradation.

The symptoms of these types of viral infections are:

a) Local symptoms

Local symptoms usually appear on the leaves of inoculated plants. They can be discrete isometric lesions, which can be counted and, in some cases, used for quantitative studies. These symptoms are of no economic importance, but are very useful for diagnostic work.

Local symptoms include:

• Chlorotic spots
• Chlorotic rings
• Necrotic spots
• Necrotic rings

b) Systemic symptoms

Systemic symptoms appear in non-inoculated plant tissues. They are the most important symptoms because they can affect any part of the plant (flower, fruit, petiole, etc.). Systemic symptoms include:

Mosaic

This symptom consists of alternating normal, green areas and chlorotic (light green) areas on the leaves. They appear irregularly and do not follow a fixed pattern. Occasionally, the symptoms start with the veins losing their color (vein clearing). We can include mottling, vein banding, and chlorotic strips in mosaic-type symptoms.

Yellowing

This symptom consists of a partial or generalized chlorosis. Yellowing-type symptoms include chlorosis, calico, aucuba, vein yellowing, interveinal chlorosis, etc.

Necrosis

This symptom appears in systemic infections as apical necrosis, vein or stem necrosis, systemic necrotic rings, etc.

c) Growth abnormalities

These symptoms may appear in any of the plant parts. They include leaf rolling, epinasty, blistering, sprout proliferation, rugosity, curling, coriaceous leaves, dwarfisms, etc.