Diseases in the Organic Garden

Post #15 Diseases in the Organic Garden

Post Description: How to deal with diseases in a sustainable way.

Welcome to a brand new day and a brand new post on curing what ails our benevolent plant friends sans chemicals! 

Diseases, like pest infestations, can seem to appear overnight. One evening, everything is right with the garden, and the next day you wake up to an entire colony of aphids munching down your tomato plants with vigor! 

The truth is, it does not happen overnight. Just as diseases in our human bodies generally do not appear out of nowhere, neither do plant diseases. Under most circumstances, there are many factors leading up to the presence of disease.

This is good news. 

Why? Because it means that once you understand what these factors are, you can do your best to eliminate them. We’re not playing God here, but we can certainly facilitate an environment that does not take well to common plant diseases, thus discouraging the diseases to take root, literally!

The same practices that keep pests down keep diseases under control as well. Bacteria, fungi and viruses strike plants that are stressed, and will spread to other plants if too many of the same type are grown together. 

There’s no need to reach for chemicals to eradicate disease. In fact, keep in mind that most chemicals used to prevent or kill plant diseases are neurotoxins. And I don’t know about you, but I’d rather not eat a neurotoxin-laden tomato sandwich.

For more information on how using chemicals can affect the ecosystem and your own body, refer back to our post on Pests. 

Preventative Measures

• Rotate your annuals each season so that species-specific diseases do not build up in the soil
• Train vining crops to grow vertically, such as cucumbers. This will keep their leaves off the ground and help prevent soil diseases from occurring, plus it saves space in your garden bed!
• Plant virus and disease resistant organic or heirloom varieties – you can look these up or simply look at the labels on the seed packets or the information in seed catalogs when purchasing.
• Companion plant,. This simply means putting plants next to each other that can help each other prevent disease. For example, intersperse calendula among your tomato plants to attract beneficial pollinators and repel tomato worms and nematodes. Insects will leave plants vulnerable to diseases.
• Water thoroughly and regularly, but not too much. A soaked bed that never dries to just moist is a breeding ground for mildew. A dry bed stresses plants and invites disease. Plant varieties with similar water needs in the same bed.
• Do not crowd your plants. If you like to freely sow seeds, be sure to thin your starts to allow ample space for the plants to grow. Crowded plants are a breeding ground for disease. 
• Some issues that look like a disease, such as yellow or spotted leaves, are actually caused by soil deficiencies. So feed your soil well, and pay attention to the nutrient needs of the plants you are growing.

Bottom line: Give your plants the right amount of water, food, sun, shade and airflow so they do not become over-stressed. 

Damage Control

But wait, an ounce of prevention is worth a pound of cure, right? 

Yet we all learn as we become more seasoned gardeners that some things are beyond our control. So here are some tips for when all else fails.

Before you reach for even a natural “solution”, stop and ask yourself: 

• Why am I having this issue? 
• What is out of balance? 
• Is this plant stressed? 
• Is it getting the right amount of light, airflow, water, and food? 

If these things are good, troubleshoot some more:

Pests?

• Are there larger pests like snails, slugs or caterpillars attacking your plants?
  • Treatment: Handpick and dispose of (if you have chickens or catfish, they will devour these.)
• Are you dealing with smaller larvae?
  • Treatment: Try BT (Bacillus thuringis), a natural, bacterial spray that affects only larvae.
• Insects in general?
  • Try a  homemade preparation of spices such as hot pepper sauce mixed with water.

For more information refer back to our post on pests.

Diseases and Treatments

• Blossom end rot – Do your peppers or tomatoes have  brown, leathery spots on the bottom or tip?
  • Treatment: This is actually caused by a calcium deficiency in the soil, so not a true disease. Correct the soil’s calcium deficiency by adding lime several times throughout the year. Be sure to test your soil first to confirm this is the problem.
• Rust fungi – Yellow spots on top of leaves with orange spores on the underside
  • Treatment: Use a prepared or homemade neem oil spray on the leaves. (Bonus: You can grow your own neem tree if you have space!)
• Botrytis Blight – Appears as gray mold on leaves
  • Treatment: Keep soil around plants clear of fresh debris such as fallen leaves, water only in the morning, and use a prepared or homemade neem oil spray on the leaves.
• Downy Mildew – White, gray, or black mildew usually found on the underside of leaves, often caused by cool, moist conditions. We tend to see this in our cool season.
  • Treatment: Correct airflow, remove affected plants and dispose of or, alternately, if you catch it early and want to save the plant try this:
    • Put 6 Tbsp 3% hydrogen peroxide in 1 gallon of water and spray the entire plant (make sure to spray every surface area, including fruit) and surrounding soil.
    • 24 hours later mix 2 Tbsp baking soda in 1 gallon water and repeat.
• Powdery mildew – White to gray powder on leaves, often caused by warm, dry conditions. We get this mainly in our cool season.
  • Treatment: If it’s minimal and affecting annuals only, you can get away with doing nothing. If it is rampantly spreading and going to affect your harvest, try this:
    • Mix 1.5 Tbsp baking soda, 2 Tbsp mineral oil, and 1 Tbsp dish soap in 1 gallon of water and spray the entire plant (make sure to spray every surface area, including fruit) and surrounding soil.
• Black Spot – Appears on roses as black spots on yellowing leaves. Caused by humid weather, so we see it more in our warm season.
  • Treatment: Make sure your roses are getting enough sun (minimum 6 hours per day.) Correct airflow. Remove all affected foliage and dispose of it away from the garden – do not compost! Add a layer of mulch under the plant to cover the old so spores do not splash back onto the leaves when it rains. Do not water past early morning to give the leaves time to dry off. If the problem keeps coming back, try one of the mildew treatments above, or this milk-based spray:
    • Mix 2 cups of milk into 3 cups of water and spray the entire plant and surrounding soil.

These are some of the most common diseases you may come across in your zone 10/11 garden, but certainly not an exhaustive list.

If you are uncertain or completely perplexed by what appears to be a disease on your plant(s), take a cutting, put it in a sealed bag, and bring it to your local county agricultural extension agent for a diagnosis and recommendation. 

Note: They are getting wiser about the ecosystem, but old habits die hard, and the major manufacturers do have pull. So the agent may recommend an unnatural solution first, such as synthetic chemical sprays. In that case, record the diagnosis and then do further research on natural solutions, and try these first. 

Have you encountered any of these diseases in your garden? If so, how did you deal with the situation? Did your solution work? 

Have you encountered any diseases we did not cover here in your South Florida or zone 10/11 garden? Do tell! 

You know I love to hear from you.

Until next time, Happy Gardening!

Mary