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How to Prevent Root Rot, Mildew, and Leaf Septoria on Cannabis Plants

July 5, 2016

Walking into a garden filled with healthy, vibrant, and strong cannabis plants is hard to beat. Unfortunately, anyone who has cultivated cannabis before also knows the feeling of discovering the onset of disease in their garden. Although diseased plants can be intimidating, they become manageable once you understand what is happening and learn how to protect your garden.

Here are three common cannabis plant diseases that you should know about and learn how to prevent.

What Is Root Rot (Pythium)?

Cannabis plant root rot

Root rot is a serious issue that takes ahold of unhealthy roots. When plants are overwatered and lack sufficient drainage, the roots cannot take in oxygen and start to die off. Unfortunately, under-watering plants can also cause roots to die off. Healthy roots require a balance of water and time to dry out to take in oxygen. Once there are dead roots in the soil, the next watering cycle will create an ideal environment for root rot to take over. Root rot weakens and destroy the roots, making it impossible for plants to take up nutrients and water.

Identifying root rot can be done in a number of ways. Hydroponic systems grant visible access to the plants’ roots, so check for roots that are brown in color, slimy, and lack the vigor you see in healthy roots. If you do not have access to the roots, you will notice the infected plants drinking less water, growing at slower pace, and a myriad of nutrient deficiencies. Root rot is nearly impossible to fix and results in either nutrient-deficient, stunted plants or having to discard the plants entirely.

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How to Prevent Cannabis Root Rot

You can lessen your chances of root rot by taking a few preventative measures:

  • Have healthy soil with beneficial microbe and bacteria populations. These populations help keep the fungus responsible for root rot under control.
  • Water your plants correctly. This means measuring the amount of water given to each plant and observing day to day how they respond to the amount of water given. It is better to see a plant begin to wilt than to overwater during this process.
  • Have breathable soil. Growing in smart pots and adding perlite to the soil are two ways to help facilitate oxygen flow and allow the soil to drain properly.

What Is Powdery Mildew?

Powdery mildew on a plant

Powdery mildew is a common disease that shows up on the leaves and buds of cannabis plants. Initially you will find it on the lower branches of a plant where there is less sun exposure, airflow, and higher levels of humidity. This type of mildew appears as a white powder that sits on the surface of the leaves. Once it appears, it spreads rapidly and can quickly make its way onto bud sites. Fortunately, because powdery mildew is so visible, it’s rare for a plant to die from it. The main concern is it renders the product unfit for sale.

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How to Prevent Powdery Mildew on Cannabis

It’s common for gardeners to take a preventative route with powdery mildew. Here are some best practices you might consider:

  • Spray your plants with organic products and fungicides
  • Prune your plants to increase airflow
  • Spray compost tea or solutions with varying PH levels to disrupt the spread of the disease
  • Pay attention to which genetics are susceptible to powdery mildew and consider focusing on other strains

If your garden does become infected with powdery mildew, there is a way to remove it by giving the harvested cannabis a bath in a H2O2/H2O solution. By mixing a small amount of 3% hydrogen peroxide (H2O2) with water, you create a solution that sterilizes the harvested plants and removes the mildew. This is a painstaking process, but it can save you from an infected crop.

What Is Leaf Septoria?

cannabis leaf septoria

Leaf septoria is a harsh-looking disease that shows up first on the lower branches and causes leaves to scab and yellow. It reveals itself during the summer when high temperatures combined with summer rains or moisture from watering leave the foliage damp. Nitrogen deficiencies can also serve as a catalyst to the disease.

Although leaf septoria will not kill your plants, it will reduce yields. Once you notice the infection, it’s important to remove and dispose of the leaves. Avoid putting the infected material in your compost pile to prevent future outbreaks. Spraying plants with Bacillus subtilis fungicides can also help slow the spread of the disease.

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How to Prevent Leaf Septoria on Cannabis

To prevent or reduce the chance of outbreak, there are a few actions you can take:

  • You must have a clean garden space with healthy soils. This is the most important preventative measure you can take. If you have an outbreak, you may need to replace your growing medium before planting again.
  • Clean your entire grow room, especially if you’re operating an indoor garden.
  • Use drip lines to water your plants so the leaves don’t get wet
  • Space the plants further apart to help keep direct moisture off the plants and the humidity levels down
How to tell if a cannabis plant needs watering

Root rot, powdery mildew, and leaf septoria are just a few of the more common diseases that can show up on cannabis plants. Other diseases include the TMV (tobacco mosaic virus), fusarium, and verticillium wilt. All of these diseases have a common theme when it comes to control: prevention. Gardeners must provide plants with the right nutrients, bacteria, and microbes to be ready to handle infections for the same reasons we as humans eat healthy and exercise to prevent illness.

Lastly, in addition to prevention, genetics play a significant role in dictating how debilitating a disease can be to a plant. Observe your garden, notice trends with specific genetics, and stick to strong-aged genetics that are stable.

Leaf septoria photo credit via Scot Nelson on Flickr

Trevor Hennings's Bio Image
Trevor Hennings

Trevor is a freelance writer and photographer. He has spent years in California working in the cannabis industry.

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