Organic control of plant diseases in the garden.

Plant Diseases in the Garden: How to Prevent and Control Them

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Even the healthiest gardens sometimes fall victim to plant diseases. While giving your plants plenty of room to grow and caring for them properly can help cut down on disease prevalence, there are times when a gardener will need to step in with a product control. To help prevent and manage plant diseases in the garden, we’ve compiled a list of the best products for the job. 

Plant disease prevention

As with all ailments — whether human or plant — prevention is key. Maintain a healthy garden environment through proper maintenance. Keep pruning equipment clean and in good repair. Don’t over-fertilize, and because fungal diseases love wet environments, always water in the morning so foliage has time to dry before nightfall.

But, even when you do everything “right,” diseases can still strike. It’s important to remember that nearly all fungicides are protectants, meaning they’re best used before, or very soon after, the pathogen first strikes. Full-blown disease outbreaks are very difficult to manage once they’re established. During very wet springs, be on constant lookout for signs of disease, early and often, and do your best to nip problems in the bud early in their development. This is key to controlling plant diseases in the garden, especially if you’re going to use one of the products described below.

Foliar diseases of tomatoes can be managed with the right organic products.

They key to managing plant diseases, like this tomato blight,  is keeping a keen eye out for early signs of disease and tackling the problem quickly.

When to apply a plant disease control product

If you feel a pathogen is affecting the production, yield, or aesthetics of your garden in a negative way, it’s okay to step in with a product control. But, it’s important to choose the products you use wisely as not all are effective against every disease. For example, using a fungicide on a bacterial disease will get you nowhere, and using a pesticide on a foliar disease is nothing but a waste of time and money. It’s absolutely essential that you properly identify the disease affecting your plant before stepping in with a product control. There are many online and printed guides to identifying plant diseases, including two of our favorite books, What’s Wrong With My Plant?  and The Organic Gardeners Handbook of Natural Pest and Disease Control.

Though all of the products we recommend below are far more environmentally friendly than most synthetic chemical-based products on today’s market, they still should be used with care. Follow all label instructions and protect yourself appropriately. Do not spray when pollinators are active, and be smart about using them only when necessary.

Maple tar spot is a fungal disease of trees.

Fungal diseases, such as this maple tar spot, can create unsightly aesthetic issues, but issues like this one don’t necessarily cause significant harm to a plant’s health. It’s important to properly identify pathogens before determining if control measures are necessary.

Effective natural fungicides for the garden

Bicarbonates:
Sodium bicarbonate (baking soda), potassium bicarbonate, and ammonium bicarbonate have been used as fungicides for preventing plant diseases in the garden for years. However, potassium and ammonium bicarbonate-based products are considered by many to be more useful than baking soda (sodium bicarbonate) because in order to effectively combat fungal pathogens, baking soda must be mixed with horticultural oils, whereas the other two bicarbonates do not.

Bicarbonate products are used as contact fungicides, and they’re labeled for use on many different plants against many different fungal pathogens, including the plant disease powdery mildew, anthracnose, rusts, botrytis, and various blights and leaf spots, to name just a few. They work by inhibiting the growth of the thread-like mycelium of various fungi and/or damaging the cell walls of the fungus. Like most fungicides, they’re best used as a preventative, before the pathogen takes hold.

Powdery mildew is one of many plant diseases in the garden.

Bicarbonate-based fungicides are great for managing a broad range of fungal diseases, including the powdery mildew affecting this zucchini crop.

The toxicity of these products to humans and beneficial insects is almost non-existent. Pay careful attention to the label of these products as each different kind of bicarbonate is effective against different plant diseases in the garden. There are many different brand names for bicarbonate-based products, two of the most common include GreenCure® and Monterey Bi-Carb®.

Bacillus subtilis:
This biological fungicide uses a naturally occurring bacterium often found in soils and even in the human gut to combat fungal organisms. In other words, it uses one living organism to manage another living organism; in this case the bacterium inhibits fungal spore germination and interferes with the fungi’s ability to penetrate into plant foliage. It also has some action against certain bacterial pathogens as well.

Fungicides based on B. subtilis control many different types of plant diseases in the garden, and they’re very good at doing it with no negative impacts to birds, insects, or humans. Useful against black spot, powdery mildew, early blight, anthracnose, botrytis, and many other common fungal pathogens, these products prove very useful and effective. There are several different brand names; among the most common are Serenade®, Companion®, and Cease®.

Black spot on roses can be managed with biofungicides.

Black spot on roses is one of many fungal diseases that’s readily managed with biofungicides based on B. subtitles.

Copper-based products:
Sprays based on copper can be used to prevent various fungal and bacterial pathogens, including the plant disease powdery mildew, anthracnose, leaf blights, bacterial leaf spots, fire blight, and many other kinds of ornamental plant disease, though you cannot use it on some plants due to an adverse reaction (see later section on phytotoxicity). There are many different copper-based fungicides approved for use in organic farming and they may have different active copper-based ingredients, but they all work because copper ions on the surface of plant leaves destroy pathogens before they can enter the plant’s tissue. However, once the disease is symptomatic, copper is ineffective. These products are meant to be used as a preventative only.

Though many copper-based products are certified for use in organic agriculture, they are highly toxic to humans and other mammals if ingested or inhaled, and they’re toxic to fish and other aquatic invertebrates and should not be used near waterways. Care should also be taken when using copper formulations when bees are present. They may also negatively impact earthworms when copper build up occurs in soils.

Brand names include Monterey Liqui-Cop® and Bonide Copper Fungicide®.

Sulfur-based products:
Sulfur-based fungicides have been used for thousands of years, especially on agricultural crops. For homeowners, when it comes to managing plant diseases in the garden, they’re effective preventatives for powdery mildew, leaf spot, black spot, and many other fungal issues. The sulfur prevents spores from taking hold and are best used before disease establishment. Sulfur-based products should not be used when temperatures are above 80 degrees F. Brand names of sulfur-based products include Bonide Sulfur® and Safer Brand Garden Fungicide®.

Septoria leaf spot on Rudbeckias.

Septorial leaf spot is a common plant disease in the garden. This time it’s occurred on the leaves of a Rudbeckia plant.

Neem oil:
Neem oil is an extract from the seeds and fruits of the tropical neem tree. Though it’s typically though of as a pesticide, neem oil is also an effective fungicide against many plant diseases in the garden, including powdery mildew, black spot, rusts, leaf spots, and scabs. Like most other fungicides, it’s best used as a preventative. Look for Bonide Neem Oil Concentrate® and Garden Safe Neem Oil®. Use caution when applying neem-based products as they are slightly toxic to fish and other aquatic life.

Plant disease in the garden include hollyhock rust and many others.

The rust affecting this hollyhock is easily managed in the early stages by neem oil and other natural fungicides.

Streptomyces griseoviridis (Mycostop®) and Streptomyces lydicus (Actinovate®):
These soil bacteria-based products prevent some pathogenic fungi from infecting plant roots. They can be used as a soil drench to prevent various seed and root rots and wilts, including Fusarium, Alternaria, and Pythium. It can also be used as a soil or foliar spray to prevent botrytis, blights, and other plant diseases in the garden. It does not effect beneficial insects or earthworms.

Trichoderma harzianum (Root Shield®):
Made from a naturally occurring soil fungus, this product suppresses soil-borne diseases such as Pythium, Rhizoctonia, and Fusarium that can cause root rots. This beneficial organism parasitizes the pathogenic fungi and prevents damage to plants. The granules are sprinkled around susceptible plants and are best used as a preventative measure where these pathogens have been present in prior years.

Using garden fungicides safely

Before you spray any product on a plant, check the label carefully to ensure it’s safe to use on that particular plant. Some plants can have an adverse reaction to a particular product — a reaction called phytotoxicity. Phototoxicity could result in discolored leaves, stunted plant development, defoliation, and even plant death due to the application of the product. A list of contraindicated plants are on the label of each product. These are the plants you do NOT want to use the product on. There will also be instructions on how to avoid phytotoxicity that can occur from spraying when temperatures are too low or too high, or when the product is not mixed at the correct rates. Label instructions are there for a reason. Follow them carefully.

Basil downy mildew is a tough plant disease to manage.

Before using any natural fungicide, be sure to properly identify the issue and make sure the product is labeled for use on the specific plant you want to apply it to. Some plants, like this basil that’s been infected with basil downy mildew, will show phytotoxicity from certain fungicides.

Getting a grip on plant diseases in the garden

Growing a healthy, disease-free garden is within your grasp. Controlling plant pathogens in the garden starts with being smart about prevention. Select naturally disease-resistant plant varieties, then put careful thought into using the best maintenance practices for limiting diseases, space plants properly, and pay attention to how you care for your garden. The product controls described above should only be used as a last resort.

For more advice on handling issues in the garden check out the following posts: 

Organic weed control tips for gardeners
Using beneficial nematodes to tackle pests
Preventing pests in your garden: 5 strategies for success

Have you faced a plant disease before and managed it without turning to synthetic chemicals? Tell us how in the comment section below. 

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One Response to Plant Diseases in the Garden: How to Prevent and Control Them

  1. Stacy says:

    Generally, I don’t worry about a lot of diseases. Roses get black spot and rust, but other than being unsightly, the plants seem unaffected. Squash gets powdery mildew, but this happens every year, as the days get shorter. I’m not so worried about these things. Choosing for varieties that survive and still produce seems the best option for me. Of course, I’m not a display garden, so I can accept a little ugliness.

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