Viral Diseases

Many plant diseases are caused by viruses.


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

Pepper mild mottle virus (Pamela Roberts) CC BY 2.5

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


​Most viruses are made up of half a DNA strand, called RNA, and are protected by a coat made out of protein. A small handful of plant viruses contain full DNA strands. There is another group, called viroids, which contain an RNA strand but do not have a protein coat. In a recent article, The Scientist reported that new research shows different segments of a virus’ genetic information are used to infect separate cells, creating a domino effect of plant disease. I swear, the more I learn, the more amazing the world gets! But I digress…

Leaf curl virus (Philarium) CC BY-SA 3.0

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The science of viruses

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


There are several families of viruses that cause plant disease. We won’t go into that now. What’s important to know is that the common names of most viruses start with the plant most likely to be infected, followed by the most characteristic symptom. For example, bean yellow mosaic is commonly seen in beans and a yellow mosaic pattern is the most common symptom.

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Symptoms of viral infection

While there are more viral diseases than I can count, many of them share similar symptoms. The most common symptoms of viral disease in plants include:

• blistering - leaf puckering
• bronzing
• chlorosis - yellowing
• chlorotic lesions
• curly top
• dead zones
• galls
• leaf crinkling, cupping, curling, or rolling
• leaves that become thicker than normal and leathery
• light and dark mosaic patterns on leaves
• mottling
• ringspots on leaves or fruit
• rosettes - abnormal growth in the shape of a radial cluster
• shoestringing
• smaller than normal leaves
• stunting
• variegation - flecks or streaks of abnormal color
• vein banding
• vein clearing
• vein outgrowths (enations)
• wilting

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Viral vectors

Viruses are generally spread to plants through insect feeding. Common disease-carrying insects include:

• aphids
• artichoke plume moths
• leafhoppers
• mealybugs
• psyllids
• thrips
• whiteflies​

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Dagger nematodes and some fungi and single-celled organisms also carry viruses. Viruses can also be moved 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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California’s viral diseases

There are dozens of viral plant diseases found in California. The most commonly seen include:

• aphid borne yellow virus
• 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, Y and X
• 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 other areas by contacting your local County Extension Office.


Controlling viral diseases in the garden

Healthy plants are better able to ward of viral infections. This means proper feeding, irrigation, and pruning. It also means selecting resistant plants that are suitable to your microclimate, buying only certified disease-free plants and seeds, planting at the proper depth, and avoiding mechanical injuries from rubbing branches and weed wackers, among other things.


Use an integrated pest management (IPM) program to control viral diseases. This means encouraging natural predators and parasites of viral diseases, sanitizing garden tools regularly, and using cover crops and crop rotation to interrupt disease triangles. Yellow sticky sheets can be used to trap many disease-carrying pests. Pesticides and insecticides used to kill disease carriers are not effective. Reflective mulches have been used successfully to confuse some disease-carrying pests. 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, largely 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.