The first thing to note is that this fungus does not just affect plants with spotted leaves. It affects any plant affected by it. In fact, it’s so common that most people don’t even think about it when they see their plants are covered in spots or have some other symptom. However, there are several things you need to know before you go out and try to treat your own plants.

What Is Leaf Spot?

Leaf spot is a fungal disease caused by the species Phytophthora infestans. It’s a very common problem, affecting many types of plants including vegetables, fruits, flowers and trees. Some examples include spinach, lettuce, cabbage and kale.

It usually appears on the underside of leaves (the lower surface) where it forms patches that may appear dark brown or blackish in color. They may appear anywhere from light tan to dark brown. These patches are called septa and they’re actually tiny pockets of infection. When these spores come into contact with moisture, they germinate and grow rapidly until they burst open releasing spores which fall off and infect other parts of the plant.

How Does It Affect My Plants?

If left untreated, leaf spot will eventually kill your plants if not treated immediately. It is very important to treat plants with this disease as soon as you see signs of it.

If it’s not treated immediately, leaf spot will spread rapidly across the surface of the leaves. Eventually, the leaf will turn completely brown and wither up, falling off the plant entirely. This process can take as quickly as a matter of days or over a period of weeks.

What Are The Symptoms Of A Leaf Spot Attack?

As mentioned above, leaf spot is a fungal disease. The symptoms of which are quite distinctive if you know what you’re looking for. They usually appear on leaves as:

Dark brown or blackish spots that sometimes have yellow borders. These spots are called septa and are actually tiny pockets of infection that appear on the underside of leaves.

Spots may occasionally have a lighter tan or gray color.

Leaves may wither and shrivel up, fall off the stem entirely.

They may also turn yellow and curl up as they age.

What Do I Do If My Plants Are Affected?

If you see any of these signs on your plants, you should immediately separate them from any other plants you have as these spores can be easily transferred by wind and will quickly infect any other nearby plants.

Treat your plants using any of the following methods:

If your plant is still fairly small, you can remove any infected leaves by hand. Be sure to dispose of them properly so that no other plants come into contact with them as they can still infect them. You can disinfect your pruners by wiping them with rubbing alcohol after use.

You can also use a garden hose to wash off the leaves of your plants. Be careful not to directly water the stems or buds as this can cause mold or other types of fungal diseases to form.

If you have a larger number of plants and/or they are too high up for you to reach, you can try using an anti-fungal spray on them such as neem oil or potassium bicarbonate (both of which you can buy online). Be sure to follow the application instructions on the packaging of whichever product you choose to use.

How Do I Prevent My Plants From Getting Leaf Spot?

Leaf spot is usually passed from plant to plant by spores that are carried by the wind. If you grow your plants outdoors, it is important to take steps to protect them from airborne spores as they can quickly send your plants into a slow and painful death.

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Solutions for outdoors growers:

There are several steps you can take if you grow your plants outdoors such as:

Raise the heights of your plants.

This may not always be possible, especially if you have a limited growing space but if you can, raising the heights of your plants (whether by using a larger growing space or supports) will create a barrier between the spores in the air and your plants. This helps to stop any spores in the air from being able to reach and settle on your plants.

Create a barrier.

You can also create a barrier using greenhouse plastic to cover the plants. This is also effective in helping to keep the airborne spores off of your plants. You can either lay the plastic flat on the ground and let the plants stand up above it or you can raise it up a few inches off of the ground by standing it on eggs (filled with water) and placing the plants in between this raised plastic and the ground. This second method will create a small space for you to walk beneath to tend to your plants.

You can also cover your plants with floating row covers (you can make these by stretching mosquito netting over frames that you create using bamboo canes and twine). This is more lightweight than the greenhouse plastic and can be easily let down if you want to tend to your plants or pick any flowers.

Use a fan to blow air away from the plants.

This isn’t going to have as much of an effect as the other two methods but using a fan will at least create a bit of a wind barrier which will stop spores from settling on your plants.

Solutions for indoor growers:

Since you have a greater ability to control the environment inside your home, there are more options available to you. These include:

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Venting your grow room.

Vent the area outside your grow room into another area of the house which has high circulation. This way, air from outside is pulled into the room and discharges out into another area of the house. You can easily create a hole in the wall (using drywall or wood sheets) for a vent to go through and attach it to a fan to direct the air flow.

Install an exhaust fan.

If venting the grow-room into the rest of the house isn’t an option, you can instead just exhaust the air directly out using a fan. You will want to either purchase a fan made for this purpose or create your own by rigging up a normal household fan to a hole in the wall leading outside.

Use air filters.

Using air filters can help remove spores from the air which would otherwise settle on your plants. You can either purchase an air filter made for this purpose or create your own using a normal household fan and some furnace filters (these are better than allergy filters since they catch a wider range of particles but either will work).

Ventilate often.

You can further reduce the amount of disease in your room by ventilating it more often. Open the room up for a few hours each day. This will allow fresh air to come in and foul air to be released.

Tips on Controlling Pests

While you have many natural predators in the wild, indoor growing tends to eliminate most of them. Most pests that are likely to affect your plants can’t survive without a food source provided for them and as such you are in control of when they eat, what they eat, and how much they eat. This tends to make them grow larger, faster and meaner than they would in the wild.

A good rule of thumb is that if a plant is big enough for a bug to crawl under, it’s big enough for a bug to crawl on. If you prevent access for bugs to your plants, they will be unable to eat. While this won’t kill them (the bug), it will prevent them from spreading disease or laying fertile eggs which will hatch into more hungry bugs.

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For each unique type of pest, there is a different solution which you can implement. These range from simple, to complex, to simple but requiring a bit of patience. I will go over a few common pests and how to deal with them.

Mold and fungi

Mold and fungi tend to be the biggest threat to your plants because most molds produce toxins to kill the plants they are growing on. This means that if they grow on your plant, it will either die or you will have to throw it away.

White powdery mold:

If you see this, you’ve got a problem. Throw the plant out, it’s history. This is a toxic mold which can make you really sick (and it makes the weed taste horrible too). If you have other plants in the area, check them as well because this mold spreads fast and will soon infect everything it can.

Aerosol insecticide (direct contact kills the mold without harming the plant)

Peroxide (diluted! this stuff is strong and will kill your plant if you use too much!)

Isopropyl alcohol (diluted!)

Grow room cleaning schedule:

Grown rooms collect a lot of junk which builds up on the floor. This junk includes food crumbs, small scraps of paper, and other things which are great food sources for pests (as well as giving them places to hide and breed).

You should sweep, vacuum, or at least toss out the junk on the floor of your room every couple of days. This will prevent pests from overrunning your room as well as keeping your plants healthy. Mold tends to grow faster in dirty rooms than clean ones so this is doubly important!

Now that you have some idea how to control mold and pests, let’s move on to controlling fungus (mold).

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The key to prevention of fungus is a two-pronged approach: keep the humidity low and keep the room clean. Most rooms will have a humidifier installed to them, however, if you’re growing outdoors, this isn’t as much of an issue (however, rain sometimes carries mold spores so there are still precautions to take).

When it comes to keeping the plant itself free from disease, there are many tools at your disposal.


If you’re serious about growing, you’ll want to get yourself some high quality seeds (ask around, there are specialty shops online which I’m sure you’re aware of). While the seeds aren’t absolute top quality, they’re most definitely good and will produce very potent weed. This will ensure that your plants will be less prone to disease. Of course, if you’re really serious about growing, you’ll want to get yourself some clones (they’ll be a lot cheaper).

Clones are cuttings from a plant and will grow pretty much the same as the original. They’re great for ensuring that all your plants will be very similar and therefore easy to breed and care for.


Always make sure to feed your plants and nourish them. If you’re growing indoors, you can simply buy some ready-made plant food and add a bit to their water every couple of days. If you’re outdoors, plants tend to feed themselves with what they find in the soil, however, if your plants don’t seem to be doing well you can always add some guano (seabird droppings) to the soil. You can get this at most organic gardening stores.


As I said before, humidity can cause mold and mildew to form on your plants. This is very bad for their health. If you’re growing indoors, a good dehumidifier should do the trick. If you’re growing outdoors, try to grow your plants in regions which have low levels of humidity.


Good airflow will prevent mold from forming on your plants. If you’re growing indoors, try to make sure that your plants aren’t in a sealed off room. If you can open a window or something else which allows air to pass through, all the better.

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UV Radiation

If you’re only growing one plant (or a few) and you aren’t very concerned about their health, placing them somewhere with strong levels of UV radiation will prevent insects from infesting your plants (and mold and mildew from forming). In other words, you can stick your plants in the direct sunlight and they’ll be safe (though obviously this isn’t as cost effective as other measures).


Plants love fresh herbs. If you’re growing indoors, placing some pots of rosemary, lavender, or mint near your plants will help to keep insects away (rosemary also keeps away mold and mildew). If you’re growing outdoors, planting these herbs somewhere nearby will do wonders.

Now, bugs aren’t the only things you have to worry about. There are certain things you can do which will attract pests, so you need to be careful. For example, if you’re keeping your plants indoors, keep the lights on 24/7 (unless you want to pay for electricity). This is because certain plants only bloom when they experience a period of darkness (like many types of tomatoes and onions).

While this won’t bother all plants, it will bother some and in rare cases can cause a mutation.

If you’re growing outdoors, keep an eye on the temperature of your soil. The last thing you want is to have a big clump of ice forming underground, as this can cause the destruction of plant roots and the death of your crop. Those styrofoam coffee cups you get from starbucks make for excellent insulators when stuck into the ground.

In Conclusion…

As you can see, there is a lot to know about in the world of gardening. It isn’t something to be taken lightly, but if you’re dedicated and prepared to put in a lot of hard work into your plants, you’ll be able to reap the benefits of your labor.

Good luck!


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P.S. If you’re interested in growing medical marijuana, you can use these same techniques to grow healthy plants. You may also want to buy an airstone and a thermometer, which you can place in the soil to learn about the temperature of your soil.

You also may want a book on the effects of light spectrum on plants, as this will help you to grow top quality bud (a little knowledge can be a dangerous thing, but only if you don’t have the common sense God gave a gnat. ALWAYS play it safe when growing things which can get you thrown in jail).

WRITER’S NOTE: This guide came about as a result of my own experiences with hydroponics and indoor growing. I do not pretend to be an expert on the subject, but I do know what it’s like to learn from the school of hard-knocks (or in this case, the school of research). I hope this has helped.

If you have any questions or comments, feel free to send me a private message.

Have fun!


Sources & references used in this article:

The efficacy of acibenzolar-S-methyl, an inducer of systemic acquired resistance, against bacterial and fungal diseases of tobacco by DL Cole – Crop protection, 1999 – Elsevier

Associations between fungal and abiotic leaf spotting and the presence of mlo alleles in barley by …, ND Havis, R Hackett, JI Burke, JKM Brown – Plant …, 2007 – Wiley Online Library

Yield loss in plantain from black sigatoka leaf spot and field performance of resistant hybrids by KN Mobambo, F Gauhl, D Vuylsteke, R Ortiz… – Field Crops …, 1993 – Elsevier



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