What Fungicide is and How it Works to Fix Plant Diseases

Fungicides are most often liquid treatments used to prevent or manage fungal diseases in plants. They work by either:

• Killing fungal spores before they can spread

• Blocking spore germination (growth) on plant surfaces

• Strengthening plant defenses to help resist future infection

There are two types of fungicides:

• Synthetic fungicides (typically chemical-based)

• Biofungicides (made from beneficial microbes or naturally derived compounds) include things like Bacillus subtilis and Trichoderma harzianum—both helpful bacteria and fungi

Biofungicides are especially popular with home growers because they’re generally safer for pets, pollinators, and edible plants. Plus, you don’t have to gear up like a lab technician to apply them.

Synthetic fungicides are formulated to control serious fungal infections quickly. They're commonly used in outdoor gardens or larger growing spaces where fast action is needed. However, it’s best to avoid using them on edible plants close to harvest and in indoor environments, since they can leave behind chemical residue and may disrupt beneficial microbes in the soil.

Fungal infections in plants often show up as discoloration, spots, mushy patches, or unusual growth on the leaves and stems. If you’re noticing symptoms like wilting or decay, especially alongside moisture issues, fungus could be the cause. Here are the most common red flags:

These spots often start small and spread outward, sometimes with yellow halos around them. Over time, the infected tissue may dry up and fall away, leaving holes behind.

If the leaves are turning yellow and feel mushy or water-soaked, it’s likely a fungal infection setting in. This usually starts at the base of the leaf and moves outward.

Powdery mildew looks like someone dusted your plant in flour. It spreads quickly, especially in dry but humid environments, and can distort new growth if left untreated.

If your plant is well-watered but still drooping, the problem may be below the surface. Fungal infections like root rot cut off the plant’s ability to take up water, even when the soil feels damp.

Stems or roots that turn dark, feel squishy, or smell foul are signs of rot—usually caused by fungi like Pythium or Phytophthora. Once the base of the plant is affected, it's tough to recover without removing damaged parts.

When a plant seems fine one day and struggling the next—but you don’t see any bugs or damage—it’s often a fungal issue working beneath the surface. These infections can spread fast, especially in warm, humid conditions.

It’s not always easy to tell the difference between fungal infections and other plant problems. Stressors like underwatering, overwatering, poor lighting, or sudden environmental changes can cause similar symptoms—like yellowing leaves or drooping. The key is to look for combinations of signs, like leaf spotting paired with mushy areas or powdery growths, which point to fungus.

The best time to use a fungicide is before there’s a problem, not after. Fungicide should be applied every 10–14 days as part of a regular plant care routine, especially during warm, humid months when fungal spores are most active. Think of it as a preventative, not just an emergency treatment.

One of the biggest mistakes plant parents make is waiting until visible damage shows up. But by that point, the infection is often already well-established and harder to control. Preventive use helps protect your plant from stress and disease before it can take hold!

In addition to routine use, you should reach for fungicide:

• After repotting or pruning

• When bringing home new plants

• If another plant nearby is showing symptoms

• If you’re growing fungus-prone varieties (like Begonias or Calatheas)

• Anytime conditions are especially damp, dark, or stagnant

Proactive applications, paired with airflow, clean tools, and proper watering, give your plants the best shot at staying healthy and fungus-free.

Follow the recommended dilution and application method. Overuse can stress plants or disrupt the beneficial microbes in the soil.

Avoid applying during peak sun hours to prevent leaf burn or quick evaporation.

Fungal spores love to hide out on the backs of leaves or along stem joints.

Many fungicides require multiple applications over time—especially if you’re treating an active infection like powdery mildew or root rot.

Fungicide is not a miracle cure and should be combined with proper watering, routine pruning, and good airflow for the best results.

Skipping fungicide when your plants are showing fungal symptoms can lead to:

• Total leaf loss

• Stem droop

• Root rot and irreversible damage

• Spread of disease to nearby plants

• Stunted growth or plant death

And with certain aggressive fungi like Pythium or Botrytis, damage can spread in just a few days, especially in high humidity.

If you’re looking for a natural fungicide option that’s safe for indoor use, Arber’s Organic Bio Fungicide is one of our top picks.

Made with Bacillus amyloliquefaciens, a beneficial bacteria, it helps create a protective barrier on leaves and roots—without synthetic chemicals or harsh ingredients.

Why we like it:

• Safe for pets, kids, and pollinators

• Works on over 30 types of plant diseases (including root rot, leaf spot, and mildew)

• Easy to use—just mix and spray or drench

• OMRI-listed and approved for organic gardening

• Doesn’t require gloves or goggles (unlike many chemical fungicides)

We’ve used it on everything from our rare houseplants to outdoor herbs and leafy greens!