Guest Blogger Sharon Knight

The 3 categories of plant pathogens we are going to discuss are fungal, bacterial and viral.

I want to start by saying that there is a certain mindset when it comes to disease control that people need to change.  Currently people look at their plant, see that it doesn’t look healthy, then bring in a sample and ask: “what’s wrong with my plant, and what do I spray to correct it?”  What people should be asking is: “what are my plant’s cultural requirements, and how do I best meet them to keep my plant healthy?”

What makes a plant susceptible to pathogens?

-The wrong location. (sun loving plants in the shade, shade lovers in too much sun)

-The wrong soil type. (drainage issues, pH imbalances)

-Watering issues. (too much, too little)

-Wrong climate. (non-hardy plants out in the cold)

-Wrong fertilizer. (not enough, too much, wrong time)

-Injury from improper planting, pruning, severe weather, animals, mowers and weed whackers.


Three things need to be present for plant diseases to occur.  The host plant, the pathogen, and the correct environmental conditions.  Think of it as a triangle.  The pathogens are always there, so the only thing you can really control is the host plant and to some extent, the conditions.  If you start with a healthy plant, and give it the exact environment it needs you can go a long way to preventing diseases.  Remember the old saying; “an ounce of prevention is worth a pound of cure”? It is still true today.

In addition to knowing what the plant’s ideal environment is and trying to come as close as possible to matching it, it is important to know what diseases that particular plant is susceptible to.  It may be surprising to realize that most plants are immune to most diseases.  By that I mean, tomatoes don’t get black spot from roses, apples don’t get peach leaf curl, artichokes don’t get shot-hole, and raspberries don’t get black spot.  That being said, there are a number of diseases that seem to infect everything, like powdery mildew, rust and botrytis.  We’re going to discuss some of the more common diseases, what conditions favor them, and what can be done to prevent or discourage them.

First, realize that not every plant will work in every environment.  While most of us can’t match all of the ideal conditions for every plant we want, we should try to get as close as possible.  If you have a shady yard, stick to shade loving plants, if your yard is sunny, stick with sun lovers.  If your soil is heavy clay and you want plants that require better drainage, amend the soil with compost or soil conditioner to improve it.  If the area stays too wet in the winter, you might have to consider putting in some kind of drain system or plan to berm up your soil to raise the root zone out of the water. And while we’re on the topic of soils, most plants have a preferred pH level.  It’s good to know what that is and if your soil is in the correct zone.  If not, you may have to add something like lime to correct it.  The actual planting of the plant is important too.  Make sure the hole is the proper depth and width, the soil is amended if necessary, the roots should be loosened so they can spread out properly, and if the plant is tall or wind may be a problem, it should be staked.  Also the use of root stimulator and starter fertilizers to get your plants off to a good start will be a big help.

Knowing what fertilizer to use and when to use it and how much to use is important, as is proper pruning techniques and timing of pruning.  Plants that don’t get proper nutrients are more susceptible to diseases just like people are more susceptible to diseases when they aren’t eating right.  Pruning or mechanical injuries like those caused by weed whackers and animals can allow pathogens to enter the bark of a plant and cause disease.

Clean up is one of the most important things you can do to keep your plants disease free.  Keeping the area around the plant free of weeds and decaying plant tissue (like last fall’s leaves) will keep down the concentration of disease pathogens in the area, and may eliminate some alternate hosts for some diseases. Pruning out dead wood or any twigs or branches that have cankers is important.  Be sure to sterilize your pruning tools between cuts to prevent further spread of disease. A 10 percent bleach solution or use of isopropyl alcohol will clean your pruners, just remember to oil them when you are finished. Crop rotation can also be a useful tool to cut down on disease in a vegetable or annual flower garden. Also application of lime to the soil around your plants can sometimes kill the overwintering stages of fungi in the soil and reduce the amount of spores available in the spring,

When choosing your plant,you should consider planting disease resistant cultivars when you have a choice.  Seed packets and plant labels may list if a plant is resistant to certain diseases common to it’s species, if you aren’t sure, ask someone at the garden store, or research varieties on-line.  When a plant is listed as resistant to a disease, its not the same as immune.  For example, Frost Peach is leaf curl resistant, which means, in an environment where leaf curl is a problem, Frost will be less affected by it and show fewer symptoms.  Some plants may not be resistant, but may just be tolerant, which means they can get the disease just as bad as the next plant, but will still live and be productive (some vegetable varieties are more tolerant of some diseases than others).

Even if you did everything right, sometimes spraying may still be necessary.  It should be considered as a last resort when care and clean up is not enough.  It is important to always know what you are spraying for and with.  That seems obvious, but you’d be surprised how many phone calls we get that go something like this: “I have a plant that’s dying, and I sprayed it with some spray I have, and I want to know what else I need to do.” Often they don’t know what the plant is “dying” of, or even what kind of plant they have (I once got a call from a panicky woman whose maple tree must be dying because it was turning yellow and dropping leaves…in October!) When I ask what they sprayed it with, they often don’t know if it was a fungicide or and insecticide (or even a herbicide!).  The  best bet is to bring out a sample (in a sealed clear bag please) so we can diagnose the problem and steer you in the right direction.

When it comes to spraying for diseases it’s important to read, understand, and follow the directions on the label.  Not all fungicides are effective on all diseases.  Timing of the spray is also important. Spraying your apple tree once it is covered in scab won’t do any good.  Spraying should be done as a preventative before the infection occurs, or in some cases as a cure once the disease first starts.  Once the disease is well established, spraying is always ineffective. Spraying alone is never enough either.  Cultural practices like cleaning up fallen leaves, removing cankers and dead wood, cleaning up weeds to remove insects and alternate hosts is always important and should always be a part of disease management practices.

Now let’s talk about the 3 main pathogens.  Fungal diseases are the most common diseases we see here in the pacific northwest.  They usually overwinter on diseased plant tissue like fallen leaves and stem cankers.  Generally they produce spores in the spring when the weather starts to warm and the plants begin growing again.  Wet weather like rainy springs will splash spores onto the new leaves and start the infection.  Some fungi have a secondary infection after the initial spores grow and the fungus matures, it produces a different type of spore that is carried by wind and rain to infect more leaves. Generally wet or humid weather promotes fungal growth.

Bacterial infections are also a problem in the pacific northwest.  Bacteria usually infect the plant through an open wound.  Mechanical damage from pruning, animals and severe weather can leave open wounds for the infection to get in. Usually it is during a period of wet or humid weather.  Some bacteria can enter through normal openings in leaves like the stomata, or through flowers.  Bacterial infections often travel through the vascular system of the plant spreading the infection down the stems and to other parts of the plant.

Viral diseases, while less prevalent, are the most difficult.  There are no cures for viral diseases of plants.  Some viruses, like rose mosaic virus, don’t usually kill the plant, but in times of stress can weaken the plant and make it more susceptible to other infections. Other viruses, like tobacco ring spot in raspberries will weaken the plant and cause it to be non-productive.  Some viruses are spread by insects and are highly contagious and plants showing symptoms should be removed and destroyed before the virus can spread.  Others are less contagious and are usually spread by not sterilizing tools after pruning an infected plant.  Most viruses show up as mottled or streaked yellow markings in otherwise healthy looking leaves.  Some cause leaves to be distorted and yellow and look similar to herbicide damage.  Whatever the cause, the only treatment for viral diseases is to remove the plant and destroy it, usually by burning it. By removing infected plants, removing alternate hosts for certain plant viruses (wild blackberries can harbor raspberry diseases), and controlling insect vectors, you can cut down on chances of your plants getting viral infections.

If you want to learn how to deal with some of the more common diseases in our area, Garland nursery is offering a class on Saturday, February 21st at 11am.

About Brenda Powell

I'm one of the owners of a family-owned retail nursery. I have a degree in horticulture from Oregon State University. I love to garden and read. My technically savvy but horticulturally challenged husband, Mitch, spends most of his time as slave labor in the garden. Thank goodness he adores me! My goal in this blog is to share my enjoyment of gardening, my love of nature, and my addiction to books. Did I mention I like to cook, too?
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