virus – The Garden Professors™

Previously in this series I started with some plant disease basics and then covered some common fungal diseases and then bacterial diseases. Now let’s turn our attention to viruses. Just like with fungi and bacteria before, in this installment I’m going to talk about some of the most common viral plant diseases with some suggestions for treatment and prevention. This by no means will be an exhaustive list of diseases (there are so many!), but I hope to cover some of the most common ones that trouble gardeners.

Unlike fungi and bacteria and just like human viruses like the common cold, there typically aren’t treatments that you can use to “cure” or treat a viral infection. Therefore prevention is the only way to limit viral disease spread in plants. Also unlike fungi and bacteria, viruses are not living organisms. They don’t have cellular “machinery” and are typically a snippet of genetic material (DNA or RNA) encased in a protein coat or similar structure. Since they aren’t living, they don’t reproduce outside of a host organism and don’t typically have the ability to spread themselves around the environment, instead relying on hosts to carry them. For plant viral diseases this usually involves manual movement on humans, tools, or possibly animals or inside of a secondary host organism like an insect’s digestive tract. Aphids are a common vector, as they consume and secrete infected sap. Viruses can often spread through infected seed or vegetative propagules like seed potatoes or cuttings. And since they aren’t living entities and are microscopic there are no signs (visible presence of causal agent) of disease, only symptoms.

Mosaic Viruses

I’m lumping mosaic viruses together because there are lots and lots of them, each affecting a different range of host plants but with similar symptoms. Many viruses affect a specific species or genus of plants while others have a broad host range. Common symptoms include: yellowing, mottling, mosaic patterns on leaves, curling, stunted growth, reduced fruit quality and size, and necrosis.

Tobacco mosaic virus (TMV) is the most persistent and infectious virus and has a very wide host range, including tobacco and other members of the Solanaceae family like tomatoes, potatoes, and peppers, other vegetables, and ornamental plants. The host range is estimated to be up to 350 species.

Tobacco Mosaic Virus (TMV) on Tobacco, Source: UK Extension

TMV is spread through sap and infected seeds. Virus transmission through sap can be a result of physical contact (brushing against plants when moving through a field/garden), dirty tools, aphid feeding, or even from tobacco use. TMV is so pervasive and persistent that many nursery and greenhouse businesses have strict tobacco policies for employees and for employees who do use tobacco there is usually a hand sanitation requirement.

Tomato Mosaic Virus (ToMV) on Tomato, Source: UF IFAS

Other mosaic viruses include Tomato Mosaic Virus (ToMV), Cucumber Mosaic Virus (CMV), Zucchini Yellow Mosaic Virus (ZYMV), Cauliflower Mosaic Virus (CaMV), Squash Mosaic Virus (SqMV), Bean Common Mosaic Virus (BCMV), and Rose Mosaic Virus (RMV).

Zucchini Yellow Mosaic Virus (ZYMV), Source: Wikimedia Commons

Tomato Yellow Leaf Curl Virus (TYLCV)

Common symptoms: Yellowing and curling of leaves, stunted growth
Host Plants: Tomatoes, Peppers
Insect Vectors: Whitefly

Tomato Ringspot Virus (ToRSV)

Common symptoms: yellow rings on leaves, mottling, distortion, mosaic, rings on fruits, necrosis
Host Plants: Tomatoes, other Solanaceous crops, wide range of others
Vector: Nematodes

Potato Virus Y (PVY)

Common symptoms: Leaf discoloration, mosaic patterns, tuber deformation
Host Plants: Potatoes, Tomatoes, Peppers
Insect vector: aphids

Hosta Virus X (HVX)

Common Symptoms: irregular yellow or light green streaks, mottling, leaf distortion, feathering pattern (looks like colors painted on by brush), stunting, and reduced vigor
Host Plants: Hosta
Insect Vectors: None/unknown

Rose Rosette Disease (RRD)

Common symptoms: Rapid growth, witches brooming, excessive red coloration (in terminal bracts), deformed leaves, excessive thorns
Host Plants: Roses
Insect Vectors: Eriophyid mites

Plum Pox Virus (PPV)

Symptoms: Leaf distortion, fruit deformities
Host Plants: Plum, peach, apricot
Insect Vectors: Aphids

Control and Prevention

Unfortunately, since there is no treatment for viruses and plants don’t have immune responses that eliminate them like humans and animals do, the only “control” for viruses in the garden is by removal of infected plants. There is no way to “cure” an infected plant, but removal from the landscape or garden can reduce the viral load available to vectors in the garden and can help slow or eliminate the spread to other plants. Since symptoms may not appear right away, it is possible that viruses can spread to multiple plants before detection.

Since viral infection typically means a death-sentence for the plant, prevention through Integrated Pest Management is of utmost importance. Here are some common and effective IMP practices that can help reduce the spread of viruses in the garden.

Purchase certified disease-free seeds or plant cuttings. Unfortunately, viruses can spread easily through untested seeds and cuttings so take caution in sharing at places like plant and seed swaps.
Practice good sanitation: remove and destroy infected leaves and plants ASAP
Clean tools regularly: viruses spread through sap transfer, so cleaning and disinfecting tools is a must. Sometimes in high-value or susceptible plants, disinfection should be done between using tools on individual plants, especially pruners. Use a dilute 10% bleach solution, rubbing alcohol, or horticultural sanitizer for best results.
Quarantine new plants: If a plant appears suspicious, keep it potted in an out of the way place until you can determine possible infection. This is especially important for plants from discount retailers, plant swaps, etc but can hold true for plants from any source.
Purchase disease-resistant cultivars when possible.
Control vector insects, especially aphids.
Wash hands before gardening, ESPECIALLY if you are a tobacco user.
Do not use tobacco products while gardening.

Wrapping it up

There are lots of bacterial diseases that can damage or kill plants in our gardens or landscapes. Prevention is key, as treatments only help slow the spread of disease. In the next (and final) installment, we’ll talk about diseases that are caused by things that aren’t fungi, bacteria, or viruses.

Throughout the garden season, extension professionals all across the country get to play detective when trying to diagnose plant diseases and recommend specific controls or preventative measures. We often have to put on our Sherlock Holmes-esque thinking caps and our standard issue detective’s magnifying glass (or microscope) to diagnose plant maladies.

Having a basic understanding of diseases, how they function, and what they look like is key. Gardeners who bring samples or pictures into our office often get exasperated when we play twenty questions trying to figure out if it is a fungus, bacteria, or virus (or something else) causing the issue. Knowing about placement, environment, planting, etc. can all be keys in discovering what might be causing the issue. Sometimes we can’t identify an exact disease at a glance and have to send things to the diagnostic lab on campus, but by looking at signs and symptoms and identifying factors about the plant we can often figure out the type of pathogen causing the issue, or whether it might be environmental, abiotic, or insect related.

What leads to plant diseases?

Of course, the thing that causes the disease is a pathogen or a causal agent such as a fungus, bacteria, or virus (or a few other odds and ends like phytoplasmas). But there are other factors at play to get a disease infection started and sustained. You need all of the factors in place for infection. This is often represented as a triangle, where a causal agent (pathogen) must be present with the right environmental conditions and a host plant that can actually be infected by the pathogen. I’ve also seen a plant disease “pyramid” where time is added as another factor (as in, the correct conditions must be present at the same time and for a long enough period for infection to start). And still yet in researching this article I found the PLANT DISEASE TETRAHEDRON, which adds human activity as another factor. What’s next, the plant disease fractal?

But I digress. When a sample comes in to our office, we play twenty questions with the gardener asking about these different factors. Like is it irrigated (many diseases need water to be spread or to develop), does it get shade or full sun, have you seen any insect activity, when do you usually work in the garden, etc. These questions can help us identify parts of the disease triangle/pyramid/tetrahedron that could inform the diagnosis.

Keeping these factors in mind can also help gardeners reduce the likelihood of disease through IPM. For example, viruses require a vector – usually an insect, animal, or human to spread the disease. Many viruses are spread through leafhoppers, bacterial wilt in cucurbits is spread through cucumber beetles, and mosaic viruses like tobacco mosaic virus has many vectors including humans and garden tools (which is why many green industry businesses have strict sanitation rules, including rules for tobacco users and hand sanitation). Knowing that fungi and bacteria can be airborne with spores or splashed by “wind splashed rain” or irrigation water can lead to improved practices like mulching, pruning for good air flow, and plant spacing.

How can you tell which diseases are which?

Let’s face it, many plant diseases look very similar. There are usually what we call spots and rots that can be very similar. But there are some identifying characteristics that help us at least determine what type of pathogen or causal agent is causing the issue.

The first thing to keep in mind is that plant diseases have both signs and symptoms. Signs are the presence of the actual disease causing organism, visible to the eye. Fungal diseases often have mycelia, or fungal threads, and reproductive structures like pycnidia present. You may also see the causal agent itself, such as leaf rust or powdery mildew. Bacteria will often have exudates that ooze out of plant parts, water soaked lesions, and bacteria that stream out of cut stems parts. You can actually diagnose some bacterial infections by suspending a cut stem in water and watching bacteria stream/ooze out of the cut. Viruses are not visible to the human eye, therefore do not have signs.

Powdery mildew on peony Source: Douglas/Sarpy Extension – Nebraska

Symptoms, on the other hand, are the effects of the pathogen on the plant. Common symptoms of fungal infections are leaf spots, spots or rots of fruits, chlorosis, and damping off in new seedlings. Bacterial symptoms include leaf spots (often with a yellow halo around them), crown gall, stem/trunk cankers, wilting, shepherd’s crook (like fire blight), and fruit rots. And since bacteria usually depend on streaming through liquids, they often leave definitive patterns for leaf spots that align with vein structures. For example, leaf spots will often be angular because they are “trapped” in between veins on the leaf. The most common symptom of viruses is a mosaic pattern on leaves and fruit, but also crinkling and yellowing of leaves, necrosis, and stunting.

Angular leaf spot on cucumber – the symptom pattern falls within veins, giving the angular appearance. Source: UMN Extension

Special mention- phytoplasma: Phytoplasmas are single-celled organisms that aren’t really bacteria, but are descended from them. They don’t have cell walls and are transmitted to plants through an insect vector like leaf hoppers. The most common type of phytoplasma diseases is called yellows (aster yellows, ash yellows, etc.) because plants often turn yellow. The symptoms are often interesting. Witches brooming, which is irregular growth that makes branches look like brooms is common. Aster yellows is a common disease affecting many plants in the aster family. Most commonly, flowers of these plants look distorted and may grow leafy structures instead of flower structures.

Unusual floral growth as a result of aster yellows Source: Douglas/Sarpy Extension – Nebraska

To wrap it up

It can be difficult to figure out what diseases are affecting plants, if it is a disease at all. Getting help in determining what disease might be affecting plants can help you treat or prevent the problem in the future. In my next installment of this series, I’ll talk about common diseases, their signs and symptoms, and treatments and preventative measures.

Sources:

https://www.canr.msu.edu/news/signs_and_symptoms_of_plant_disease_is_it_fungal_viral_or_bacterial

https://lms.su.edu.pk/lesson/1660/elements-of-an-epidemic

Tag: virus

Plant Disease Primer- Part 4: Going Viral

Plant Disease Primer: Part 1 – Shaping up the causes, signs, and symptoms of disease