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Past Articles Library | Plant Diseases & Control | Stopping Plant Diseases Before They Overwhelm You

Diseases in plants need three things to get established:  pathogen, host, and environment.  Think of this as a three legged stool. If you knock any of the legs out, the stool falls and the problem disappears.  This article focuses mainly on greenhouse care, but the same three legged stool approach works on outdoor plants and indoor plants.

Before you can eradicate your problem, you need some baseline data.  At least once a week, you need to scout your greenhouse and make sure there are no pests in it.  The more days you scout the better chance you have of catching a pest or disease before it becomes an epidemic.

In the course of your scouting, use a map of your greenhouse and the plants in it.  On the map, include the vents, blowers, heaters, and other fixed equipment.  This helps you mark where pests are hiding and if something needs to be fixed because it allows diseases in.  These objects should be scouted, too.  Once you become familiar with what a healthy plant looks like, you can tell if a plant has an early problem more easily.

Now you look at your greenhouse map and try to figure out where the disease is coming from.  Is there a pattern to the infestation?  If you wait until the problem gets wide spread in your greenhouse, not only does it cost you a lot more money, but you have no idea where it is entering your greenhouse.  You want to be able to find patient zero and see what conditions made it vulnerable to the disease.  Now you are ready to use the three legged stool and knock out the pathogen before it gets going good.

Pathogens.  Each year, greenhouses should receive a major cleaning to remove any spores or other problems that are lurking.  Sick plants should be removed and put in the trash.  The greenhouse should be sterilized to kill any lingering pathogens.  All tools used to work on the plants need to be sterilized in one part bleach to nine parts water before moving on to the next plant you intend to work on.

It is also important to use clean seed from a reputable dealer.  If the seeds are infested with disease, you are unlikely to ever have a successful crop using them.  Check your seed orders carefully as soon as you receive them for any problems.

The Host. You should always trial more than one cultivar of a plant so if a problem develops, you still get some plants that year.  Trials also let you clearly show the ability of the plant to resist the disease and grow like they are supposed to.  Plants that do not stand up to the disease can be removed.  Planting more of the survivors next year makes sense.

The Environment.  This is the aspect you have the most control over. Every greenhouse has microclimates that are often the place the pathogen first got hold.  Look carefully at these microclimates when scouting for diseases or pests. 

If you have the correct tools to control the heat in the greenhouse you can bring the temperatures up at dawn, when the temperature is the coldest.  This will help control fungal infections such as Botrytis spores.  While they are all over the place, you can control them to some extent with temperature.

Levillula, oddly enough, likes heat and air blowing through the greenhouse.  It uses the wind to spread all over the greenhouse, infecting plants everywhere it goes. In addition, this type of powdery mildew likes warm, dry places to grow.  In effects peppers and tomatoes and can kill the plants it is found on. Even if it does not kill the plants, they will not have much of a crop with this problem.

When it is peaceful at night, you need to walk through your greenhouses and listen for dripping water and condensation.  You need to do the same thing on rainy days.  Plants that are dripped on by water tend to get fungal infections. Fix any leaks you find as soon as possible.

You should watch the shade clothes and heat blankets open and close.  If the shade cloth is folded back on a hot day, the plants in the sun may be burned.  Make sure plants get the shade they need.  In the winter, the heat blankets are important and should be checked to see that they are functioning properly.

Changing your irrigation schedule can help your plants, too.  If you water them first thing in the morning, they may take up more water than they are able to transpire.  This swells the plant and pushes on wounds from grafting and leaf buds, making those wounds more susceptible to Botrytis.  Wait until the plant is transpiring well before watering it for the day to prevent this from happening to your plants.

Greenhouses have microclimates in them just like any enclosed space.  The best way to address this is to direct the fans so that the air comes from the floor, thorough the plants, and then upward toward the roof.  You can use pipes that run from the fan to the floor to achieve this.  It will help defeat fungal diseases by drying plants out quickly after watering.  It also dries up any wounds to the plant more quickly.  This helps prevent fungal infections from infesting any plants who have just been grafted or cut to be grown from stems.

There will always be pathogens in your greenhouse, however careful you are.  However, taking the steps outlined in this article will help keep them at as low a level as possible.  If you also take steps to address the host and the environment, you will have knocked the legs off the three legged stool and made sure your pathogens do not infest your plants or spread throughout your greenhouse.  They will be kept at a low level and have minimal impact on your sales.

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