How to Stop Plant Diseases Before They Grab Hold

How to Stop Plant Diseases Before They Grab Hold

Disease needs three things to flourish: the right environment, the host, and the pathogen. It’s called the disease triangle. Think of it as a three-legged stool: kick out one leg and the disease epidemic collapses.


That sounds simple, and frankly, it can be. The secret to being successful in containing greenhouse vegetable diseases is to know your battlefield, understand how each part of the disease triangle contributes to the disease taking hold, and take practical steps to take down at least one leg of that metaphorical stool.

Get to Know Your Greenhouse

When thinking of disease management, the biggest steps to reduce pressure are to:

  1. Observe
  2. Record and map it
  3. Think about the patterns of spread within your greenhouse and within the plant, and be open and creative about solving problems.

Observe. You want to spend a lot of time with your crop. Depending on the size of your operation, you may or may not have an official scout. If you don’t, make sure you scout at least once a week. I personally like scouting to be systematic, because if you wait until you have a problem like an epidemic, it will be difficult to solve that problem economically. So you want to be there all the time.

Record and map it. Create a sketch or program that shows what you have in the greenhouse. I’ve used Excel, making the horizontal lines or X axis my rows of crop, and the Y axis or vertical lines the posts.

nclude the various features like fans, heaters, irrigation lines, and so on. Not only does it help you get oriented on which part of the greenhouse you’re looking at, it helps you pinpoint factors that can create or influence microclimates. That way, you can tell where your heat is, or where the cooling or the vents are, and even the edges. These features are locations you want to always include in your scouting.

Once you get familiar with what a healthy plant looks like, it’s easier to know when a plant is beginning to have symptoms of disease. Any number of pathogens can be difficult to spot before they’re very symptomatic (really strong).

Look for patterns of disease. This kind of systematic scouting and recording will help you identify where problems start in the greenhouse. If you are taking notice only when a disease takes hold and has spread, you are likely working with a larger area that doesn’t give you the information you need to address what allows the disease to thrive in the first place.

So now, pair your observations with this knowledge of the disease triangle, and try to see what is going on. Let’s take a look at how to combat the three sides of the disease triangle. The steps you’ll take for controlling the pathogen and the host plant are straightforward. It’s in addressing the third leg — making sure you disrupt the environment pathogens need to flourish — that you’ll have the most options.

The First Leg: The Pathogen

To minimize the pathogen spores, an annual clean-out of your greenhouse is recommended, especially in large monocultures. And of course, always use clean seed from a reputable company. When appropriate, remove diseased plants from the property.

The Second Leg: The Host

Disease management is another reason to always test several varieties each season. You may be able to see some clear winners and losers in terms of disease management.

An often overlooked issue is how injuries to the host plant give pathogens a chance to take hold. Regular crop work like pruning or people and equipment bumping plants and causing leaves to break creates open wounds.

Could adjusting the crop work schedule minimize the spread of disease? Can it make the host a less suitable host? When are the most open wounds created in your crop? Think about timing that task to when it is warm enough that the wounds will dry quickly to minimize disease spread.

Changing labor schedules can be a real challenge. But I encourage you to think this through. Something as simple as not disturbing plants first thing in the morning when it’s too cool for the wound to heal quickly can have a significant impact.

The Third Leg: The Right Environment

Creating the wrong environment for disease is where your regular scouting will pay off. You can have the most sanitary greenhouse in the country, and still have diseases. For example, Botrytis spores are ubiquitous, they are probably even in our homes.

Every greenhouse has microclimates that are out of sync with the rest of the range. And this is often where disease can take hold.

Take a look at several examples of what you can do. Climate management is the key to botrytis management. If you have the right tools — the ability to heat sufficiently and vent — to get the plant to the right temperature and to cook off the humidity before daylight, you’ve gone a long way, and have done better than you can with fungicides.

Take counter-intuitive steps with Levillula. But if you are dealing with Levillula (a type of powdery mildew quite common in pepper and tomato that grows through the leaf), air movement actually helps it thrive, unlike other fungal diseases. It tends to be more common along the pathways. This pathogen is very destructive in some varieties, but not all. Plant the less susceptible varieties along the pathway. If you can afford automatic doors, you’ll find the doors shut quickly, minimizing blasts of air that happen when the doors are opened.

Visit the range at night. Be willing to go to your greenhouse in the quiet of the night and listen for dripping condensation. By the same logic, go in and look around for leaks and condensation when it rains. Any plant that regularly is dripped on by cold water will get stressed, and become more susceptible to disease (gray mold and leaf mold in particular).

Watch the shade cloths and heat blankets open and close (keep your shade cloths and heat blankets tidy). Another place I often see stressed plants is where the shade cloth/heat blanket is pulled back. Again, if the heat is not managed properly, this can create a shower on the plant. Be there sometime to see if this happens.

Consider changing up your irrigation schedule. One grower I worked with thought it was good to get the tomato plants started with a wake-up shot before sunrise. This was Arizona after all, so as soon as it is light, the plants are taking up a lot of water. We discovered, however, that Botrytis was increasing. It turns out the plant was taking up water it was not able to transpire yet. The water the plant took up pushed against the wounds from harvesting and leafing. This kept the wounds from healing as quickly and left them more susceptible to Botrytis.

Address air circulation in micro-climate problem areas. When you have sufficient air movement within the plant canopy, wounds dry up more quickly. Consider adding tubing from the fans that directs air down closer to the crop. (to the floor, so air is circulating up from there, underneath the plant canopy, not onto them from above).

In most cases, it is important to focus on disease management, not eradication (which just isn’t possible in most cases). Be aware that pathogen spores may always be there, so you need to knock out one of the legs of the disease triangle by removing the host or adjusting the environment.

Karin Tifft ([email protected]) is a consultant specializing in pest management at Greenhouse Vegetables Consultants (