Plant Disease Fundamentals | The Basics and Beyond

How are the plants in your garden doing so far this season? I don’t know about you, but here in my Atlanta-area GardenFarm™, disease is already rearing its ugly head. I had a conversation with extension plant pathologist, Dr. Janna Beckerman, of Purdue University to explore the world of plant disease, so she could share with all of us some of the essential plant disease fundamentals and management practices. As with all things gardening, understanding the basics and beyond will make us better equipped for success.

There are three primary types of disease in the home garden – fungal, bacterial, and viral. It’s challenging to diagnose which type is affecting a plant in your landscape, but more often than not, the culprit is fungal.

Well actually, Janna says the most common culprit is not a disease at all. Oftentimes, gardeners misdiagnose damage from drought, sunscald, leaf scorch, or other environmental conditions as a disease problem. If you make that mistake, you might be inclined to treat the plant with a fungicide, but that won’t correct the actual problem. This is a classic example of the need to really understand what you’re dealing with before you take action.

First things first – have you ever wondered where diseases come from in the first place? It often feels as though they strike out of nowhere. In a way, that’s true. Many diseases are endemic – they’re present in the garden at very low levels. They’re just lurking and waiting for the right conditions to attack.

disease management - Dr. Janna Beckerman)

Dr. Janna Beckerman works with commercial growers in the nursery, greenhouse & landscape industry. (photo: Courtesy of Dr. Janna Beckerman)

It’s also very easy for us to unwittingly import a disease. Whether it’s in or on something we purchase at the garden center or a seedling we bring back from a friend’s home, diseases can be difficult to detect. When it comes to plant diseases, there is typically a period of time when an infected plant looks perfectly healthy. By the time you realize something is wrong, it’s too late.

Plant diseases can be soil-borne, air-borne, and transmitted by insects. They are all around us and impossible to prevent altogether. That’s the bad news. The good news is that, when they do strike, their impact isn’t necessarily as disastrous as it might seem.

Fungal Diseases

When the symptom of damage on your plant is from disease, odds are very good that the cause is fungal. Spores of fungal diseases are everywhere. They float on the wind. They hitchhike on insects. They snuggle up in microscopic crevices in the soil.

If they are everywhere, why don’t all plants show symptoms? Diseases are host specific. They only impact certain species, families, or types of plant. Also, the spores must enter the plant in some way. That can be through a small area of damage or where an insect has pierced foliage to feed.

Some fungi can also create their own path into a plant by generating enough force to actually puncture the leaves. As if that’s not aggressive enough, they can also produce an enzyme or toxin which kills a small area of foliage, creating an opportunity for the fungus to feed on the plant.

There are even some types of fungal spores that enter through the plant’s flowers. It’s safe to say, fungi are a pretty determined bunch.

White mold is a good example. It’s a plant pathogen that commonly invades our garden through birdseed. In fact, it’s a ball of fungus that even looks a little bit like sunflower seed. White mold can be hosted by several hundred types of plant species. So wherever it might land in your landscape, it will germinate. Then, it produces oxalic acid that kills plant material, so the fungus can feed on it and spread.

Fungal diseases – like this bean rust – are the most common of the three primary types.

Bacterial Disease

Although they are less common, some of the worst diseases to take hold in the garden are bacterial. Fire blight is a common example of a bacterial plant pathogen.

These diseases also must infect through an opening of some kind. Again, the pathway in could be something environmental – like a puncture or pruning wound – but flowers are particularly susceptible to bacterial intrusion.

As pollinators move from flower to flower, they can spread bacterial pathogens. This is why fire blight can be so pervasive – spreading from flowering quince to crabapples to rose bushes. There are about 70 plant varieties which are susceptible to fire blight, and if the disease makes its way into the plant’s rootstock, it can be game over.

Viral Disease

Like bacteria and fungi, viral diseases can also be soil-borne or transmitted by insects or even human contact. Are you a smoker? You could transmit Tobacco Mosaic Virus (TMV) just by touching your plants. It moves from a cigarette or other tobacco product onto your fingertips and – from there – onto plant foliage. The virus is so tough that scientists have found it can still create infection from tobacco that is several hundred years old.

None of us are fans of aphids or thrips in the garden, but the fact that they are common culprits for transmitting many viral pathogens makes them even more loathsome.

The leaf spot on this sedum is caused by a potyvirus pathogen. Some potyvirus strains are serious enough to warrant removal of the infected plant. (photo: courtesy of Dr. Janna Beckerman)

Nematodes

Are these microscopic worms a pest or a disease? Actually, there are many species of nematode. There are nematodes categorized as beneficial, as pest and as disease. So if you’re a gardener (or know a gardener) who likes answers to be simple, remember the nematode and resolve to learn rather than look for the quick fix. Nature is elegant and challenging, but it certainly is not easy.

Soil-borne nematodes create microscopic wounds in plants which open the door for soil-borne diseases. Foliar nematodes are actually considered a disease. They can survive in soil, but they move through water – like splashing irrigation or rain. They feed on broadleaf plant foliage, creating angular lesions that are primarily cosmetic.

Now what?

By this point, you might feel like plant diseases are a foe that can’t be defeated, and you’d be right. We can’t win against disease in the garden, but we can take steps to keep them at bay and – when they do rear their ugly heads – fend them off through the growing season.

Have you ever heard of the Disease Triangle? A disease pathogen won’t become a problem until all three components of the triangle are present:

The pathogen
A host plant
A conducive environment

1. The Pathogen

Yes, they are everywhere, but don’t invite more problems into your landscape. If you’re purchasing a new plant, don’t select something already showing signs of disease. By the time symptoms are visible, the disease isn’t just established – it’s reproducing.

That one small spot of disease damage represents thousands to millions of bacteria or fungal spores primed to attack other plants in your garden. So, purchase only the healthiest plants. It’s no guarantee of disease avoidance, but it sure is better than the alternative.

The dark areas between the leaf veins on this impatiens foliage have been caused by foliar nematodes, but it’s not serious enough to impact the overall health of the plant. (photo: courtesy of Dr. Janna Beckerman)

2. The Host

Since disease pathogens are host-specific, learn which are most common in your area and the types of plants most at risk to that disease. Armed with that knowledge, you can avoid those at-risk plants – or you can look for varieties with resistance to that disease.

For example, fusarium wilt is a soil-borne bacterial disease which is most likely to strike tomato plants. If fusarium wilt is common in your area – like it is in mine – consider planting resistant tomato plants. Plants with an “F” after their name have been bred for – or have a natural resistance to – this disease.

I’ll be honest. I prefer tomato varieties which aren’t resistant to fusarium wilt. If the disease strikes a plant in my garden, I avoid planting tomatoes in that bed for at least a few years. It’s possible that I might be able to “starve” out the bacteria in the soil. That said, I know that – every year – I’m rolling the dice when it comes to this disease.

It will probably affect the heirloom tomatoes in my garden, and it will require me to stay vigilant to keep ahead of the pathogen (more on that in a minute). However, it’s a price I’m willing to pay in hopes of enjoying a crop of my favorite tomatoes before the disease does a plant in.

Janna gave the example of verticillium wilt. If you lose a maple tree to this disease, it would be wise not to replace it with another tree variety susceptible to this pathogen. Verticillium wilt is soil-borne, so any susceptible plant in the same location could fall victim. You could eliminate the “host” aspect of the disease triangle by planting a conifer (which has a very low risk of verticillium susceptibility) instead.

3. The Environment

When you opt to grow a plant variety which is susceptible to disease, ask yourself what you can do to make the environment of your garden less hospitable to the pathogen.

For example, fungal diseases love stagnant, moist, and dark conditions. You might just be able to prevent fungal pathogens from gaining a foothold by making sure to space plants far enough apart or pruning out enough foliage to allow plenty of light and air movement through susceptible plants.

Although the damage here was caused by sunscald, it’s not uncommon for gardeners to misdiagnose this as a disease issue. The plant will recover from this temporary environmental issue.

In other words, when you are faced with a disease issue – consider the disease triangle and ask yourself which of the three elements you can change. The pathogen won’t take hold unless it has a host and a conducive environment.

Another environmental impact is stress. Disease problems can sometimes be dodged simply by avoiding conditions which put plants into a state of distress. If you set warm-climate edible crops out when temperatures are still too cool, you’re dialing disease risk up a notch. When you plant a new tree or shrub during the hot, dry months of summer; you’re broadcasting a “Welcome” for disease to strike while the victim is weak and trying to establish.

You’ve probably heard me say it a hundred times: The right plant in the right place. Provide the right conditions at planting time – and ongoing – because it will help to reduce that ever-present “conducive environment” aspect of the disease triangle.

In spite of your best efforts, you’ll probably see disease damage on some of your landscape plants at some time or another. That’s when the next phase of management begins.

Disease Management Options

When it comes to disease treatment, the greatest misconception out there is that the product fixes everything. In our spray-happy society, gardeners often believe they can reach for something in a bottle and cure the problem, but there are no cures when it comes to plant diseases.

Plant disease treatments do not cure. They can prevent infection from happening, but once the disease has entered a plant, treatments only slow down the spread of the pathogen. Think of disease treatment as a vaccine that must be re-applied in order to be effective.

Before you apply anything, take the time to actually diagnose what’s going on. There is so much misinformation on the internet – especially when it comes to plant problems. Check out Purdue University’s new smartphone apps for their Tree Doctor, Tomato Doctor, Perennial Doctor, Annual Doctor, or Shrub Doctor apps. They’re easy to use and provide reliable information to help you diagnose a problem.

Insects are often vectors for transmitting disease pathogens to plants through the damage they cause when feeding.

Fair warning – diagnosing plant disease isn’t easy. It’s a skill that takes time and practice, but your efforts will pay off in seasons to come. Don’t be fooled by the name of a disease either. Its name may simply have come from the first plant host on which it was discovered. Tomato ringspot virus, for example, can infect a number of ornamental plants – not just tomatoes.

Once you know what the issue is, Janna recommends the Midwest Vegetable Production Guide for treatment recommendations. This resource is designed for commercial growers, so not all the products listed will be available to you as a home gardener.

Whatever you use, know that it won’t be effective unless you apply it correctly. Good coverage on all areas of the affected plant is crucial. On the other hand, be careful not to over-apply.

Janna recommends Daconil as effective for many diseases. However, it can be harmful to some beneficial creatures, so if you do opt to use it, bear that in mind. Bacillus subtilis is an organic option which can be effective in treating a number of foliar diseases, and it’s less detrimental to wildlife.

As with any treatment, there are always trade-offs, and careful application is key to minimizing unintended consequences. Follow the instructions on the packaging.

It’s important to remember that perfection should not be our goal in the garden. Learn to tolerate some spots. They usually don’t mean the death of your plant or the loss of your crop. With many fungal and bacterial diseases, you can stay ahead of the pathogen by pruning out damaged foliage.

Take early blight as an example. This is a fungal disease which is common in the South. I can pretty much count on this pathogen affecting my tomato plants. However, I also know that I can prune to allow time for the plants to produce a good crop in spite of the disease.

Early blight tends to affect the lowest foliage first. As leaf branches begin to show symptoms, I prune the entire branch out of the plant. This doesn’t remove the pathogen, but it does slow its spread through the plant. Photosynthesis continues in the upper canopy of foliage, so the plant can continue to set fruit – even as more and more lower branches succumb to disease and are removed.

When I see this type of damage on the lower leaves of my tomato plants, I cut the leaf branch all the way back to the main stem. I know this plant is infected, so I can’t cure it of this fungal pathogen. However, removing these branches will slow its spread throughout the rest of my tomato plants.

Since I know bacterial diseases spread more quickly through wet foliage, I do two things to try to prevent and stay ahead of those problems:

I avoid handling the tomato plants in my garden in the morning when they are wet from dew – or after a rain.
I proactively apply a copper fungicide before a forecasted heavy rain.

I don’t always use fungicide treatment in my garden. I would rather tolerate some damage and prune out what I can, and I encourage other gardeners to increase their tolerance as well. That said, I know that some years bring more severe occurrence of disease than others. So, I only reach for a treatment when I feel it’s my last option to tip the balance a little less in favor of the disease.

Some diseases are so detrimental or virulent that it’s worth considering removal of the entire affected plant. So, when should you choose complete removal versus pruning out only what’s showing signs of damage? First, diagnose the disease to learn how it spreads and determine the risk to the other plants in your landscape. Another key consideration is the affected plant itself.

What’s the cost of a tomato plant? Losing one of those is a small price to pay to prevent a more serious bacterial disease from spreading to other plants. On the other hand, you’ll probably be more inclined to prune out damage on a prized rose bush in hopes that it will outgrow the effects of the pathogen.

Avoid touching your plants while they are wet from rain or morning dew (or guttation in this case). The moisture causes disease to spread more quickly through your contact.

Disease management takes some effort. There’s just no getting around that. Hopefully, you will look at these challenges as learning opportunities. Every year will bring different circumstances and a fresh chance to improve your skill at diagnosis and containment.

I hope you’ll be sure to listen in to my conversation with Janna by scrolling to the top of the page and clicking the Play icon in the green bar under the page title. In spite of challenges of the subject matter, this was a fun conversation packed with plenty of nuggets of good gardening advice.