Viral Diseases

 If you can get beyond their disease-carrying behavior, viruses are amazing things.

Pepper mild mottle virus (Pamela Roberts) CC BY 2.5

What are viruses?

First off, they are obligate, which means they cannot reproduce on their own. Instead, viruses need living plant tissue to survive and reproduce. The jury is still out on whether or not viruses are alive at all.

Most viruses consist of half a DNA strand, called RNA, protected by a coat made out of protein. A small handful of plant viruses contain full DNA strands. Another group, called viroids, contains an RNA strand but does not have a protein coat. In a recent article, The Scientist reported that new research shows different segments of a virus’ genetic information infect separate cells, creating a domino effect of plant disease.

Leaf curl virus (Philarium) CC BY-SA 3.0

The science of viruses

Viruses enter a plant cell and use their RNA strand to reprogram that cell’s genetic instructions, causing the cell to start producing more of the virus’ RNA. These new strands then infect neighboring cells, and so on.

Several families of viruses cause plant disease. And you can use their names to determine which plant is most likely to become infected and the most characteristic symptom. For example, bean yellow mosaic commonly occurs in beans, and a yellow mosaic pattern is the most common symptom.

Symptoms of viral infection

Many viral diseases share similar symptoms. The most common symptoms include the following:

• Blistering - leaf puckering
• Bronzing
• Chlorosis - yellowing
• Chlorotic lesions
• Curly top
• Dead zones
• Galls
• Leaf crinkling, cupping, curling, or rolling
• Leaves that become thicker and leathery
• Light and dark mosaic patterns on leaves
• Mottling​

• Ringspots on leaves or fruit
• Rosettes
• Shoestringing
• Smaller leaves
• Stunting
• Variegation - flecks or streaks
• Vein banding
• Vein clearing
• Vein outgrowths (enations)
• Wilting
• Witches’ broom

Viral vectors

Viruses often infect plants through insect feeding. Common disease-carrying insects include aphids, artichoke plume moths, leafhoppers, mealybugs, psyllids, thrips, and whiteflies. Dagger nematodes and some fungi and single-celled organisms also carry viruses. Viruses also move around the garden on pollen, clothing, tools, and plant debris. Many viruses overwinter in seeds, flowers, perennial weeds, and crop root systems, where they can lie dormant for years.
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Common viral diseases

There are an estimated 1,000 viral plant diseases. But you may only face a handful of those problems, and many are avoidable. The most commonly seen viral diseases include the following:

• Aphid borne yellows
• Artichoke curly dwarf
• Banana bunchy top
• Barley yellow dwarf
• Bean common mosaic
• Bean yellow mosaic
• Beet curly top
• Calico
• Celery mosaic
• Cherry leaf mottle
• Cucumber mosaic​​
• Cucurbit yellow stunting disorder
• Curly top

• Dwarf virus
• Maize streak virus
• Pea enation mosaic
• Pea streak
• Potato viruses A, S, X, and Y
• Squash mosaic
• Strawberry crinkle
• Strawberry mottle
• Tobacco mosaic
• Tobacco ringspot
• Tomato bushy stunt
• Tomato spotted wilt
• Tomato yellow leaf curl

You can find lists of viral diseases common to your area by contacting your local County Extension Office.

Controlling viral diseases in the garden

Healthy plants are better able to ward off viral infections. Proper feeding, irrigation, and pruning help keep plants healthy. Select resistant plants suitable to your microclimate, buy certified disease-free plants and seeds, plant at the proper depth, and avoid mechanical injuries from rubbing branches and weedwackers, among other things.

Integrated pest management (IPM) programs can help control viral diseases. An IPM lures natural predators and parasites with insectary plants and water. Sanitizing garden tools can slow the spread of disease. Cover crops  and crop rotation can interrupt any potential disease triangles. 

Yellow sticky sheets can trap many disease-carrying pests. Pesticides and insecticides used to kill disease carriers are generally not effective. Reflective mulches can confuse some disease-carrying insects, but they can burn sensitive plants during the peak of summer. Diseased plants should be removed and thrown in the trash to prevent healthy plants from becoming infected.

Viral diseases of plants are on the rise, mainly due to monoculture, mass production, climate change, global shipping, and other human activities. You can reduce the likelihood of viral diseases affecting your plants by putting new plants into quarantine and knowing what to look for.

Now you know.