5 Greenhouse Plant Fungal Diseases To Watch For in Winter

Though the greenhouse protects tender plants from frost, it often invites fungal pathogens that thrive with standing air and consistent moisture. Watch for these five common diseases to keep your plants safe. Seasoned grower Jerad Bryant shares prevention and treatment solutions that work against the fungi.

Heliotrope plant leaves covered in powdery mildew, with additional foliage blurred in the background

Contents

Greenhouses come in all shapes and sizes, and there are varying degrees of control they can have. Some have fancy contraptions like humidifiers, dehumidifiers, and heaters that allow for complete control of the growing environment. Others simply provide an insulating layer from the cold, keeping inside air warmer than the outside. 

No matter how much control you have in your greenhouse, there are steps you can take to prevent and remove fungal pathogens. Many thrive in wet, humid conditions that are common in these spaces. Adding proper ventilation, airflow, and correct lighting conditions will all help in the battle against greenhouse fungal diseases. 

A yearly or biannual cleaning ensures the space stays in tip-top shape. Unlike the outdoors, the greenhouse needs some maintenance to stay in good condition. Remove leafy debris from the ground, clean the windows on glass ones, and replace old plastic on hoop houses. 

If problems persist despite the clean, sterile growing conditions, try one of the following solutions. We’ll cover how to tackle the major fungal pathogens in greenhouses. 

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Powdery Mildew

The leaf shows a white, powdery coating on its surface, with yellowing edges and a distorted, wilted appearance.
It typically grows white or gray fuzzy spots.

Powdery mildew is common outdoors and inside greenhouses! It’s a pesky fungal disease that spreads from leaf to leaf. It grows a white-gray powdery growth full of spores on leaf undersides and surfaces. The spores spread to other plants when the wind carries them to suitable hosts nearby. 

How to Identify

Powdery mildew typically grows white or gray fuzzy spots. They sprout in clusters on leaves and shoots, spreading wider over time under warm conditions. Pea pods sometimes turn brown with fuzzy growth, though the disease doesn’t often infect fruits and vegetables.

This condition grows differently on tomatoes, onions, artichokes, and peppers—it causes yellow and brown spots on the leaves instead of powdery patches.

How to Prevent

Extreme heat and direct sunlight harm this pathogen, preventing it from spreading further. Ensure your crops are growing under full sun, as shady sites encourage powdery mildew growth. Though temperatures above 90°F (32°C) can kill the spores, they often harm plants too. Cleaning the greenhouse regularly and using sterile, sanitized tools when caring for your crops prevents the disease from spreading.

How to Treat

Treat powdery mildew infections early and you may prevent them from spreading out of control! Always water plants at their base to prevent excessive splashing on the leaves. If spores start growing, prune off the infected growth and spray a preventative organic fungicide—horticultural oil, potassium bicarbonate, and neem oil all work well. Follow the instructions on their packaging for the best mixing and application rates.

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Root Rot

A close-up of tangled roots with signs of rot and decay, as a hand gently holds a damaged section showing soft, discolored tissue.
The pathogens attack sensitive roots in wet soil, causing them to decay and die back.

Root rot comes from several pathogens, though it’s often spread by fungi and organisms similar to fungi. Phytophthora and Pythium are two water molds that resemble fungi. They eat at sensitive roots in wet soil, causing them to decay and die back. True fungi like those in the genus Rhizoctonia grow similarly, infecting roots in soggy soil. 

How to Identify

By the time you see root rot symptoms in a plant, it’s often too late! The infected plant most often dies before recovering. You can, however, work to prevent further infections in the greenhouse. Watch for yellow and browning leaves, mushy plant tissue, and rotten, smelly roots. 

How to Prevent

Overwatering is the number one way to cause root rot. Excessive water creates soggy soil that’s perfect for water molds and fungi. They sprout from spores in the soil that rapidly multiply under moist conditions. Plant crops in well-drained soil inside the greenhouse. If root rot is a common issue in your garden, consider growing crops in raised beds with fresh soil and compost.

How to Treat

Root rot is difficult to treat. Because a variety of pathogens cause it, different treatment solutions are necessary for specific root rot diseases. The best way to treat it is to reduce watering, trim off the dead parts, repot your plant, and hope for the best. At the time of planting, add beneficial mycorrhizae to the soil to bolster your plants’ defenses.

Fusarium Wilt

The pepper plant shows signs of fusarium wilt with drooping, yellowed leaves that exhibit a scorched appearance with a reduced number of stunted, malformed fruits.
You’ll notice infected plants exhibit wilting leaves and stems.

Fusarium wilt is similar to root rot—a greenhouse fungal disease that infects roots and stems, causing dieback, wilting, and mushiness. It thrives in warm soil, which is why it often occurs in greenhouses during winter. The insulated space offers the perfect growing environment for Fusarium fungal strains. 

How to Identify

Fusarium wilt occurs in crops in the nightshade family Solanaceae like tomatoes, eggplants, and peppers. It can affect other commonly grown plants, like cucumber, watermelon, and muskmelon. It stays dormant in the soil until optimal conditions arrive. You’ll notice infected plants exhibit wilting leaves and stems. The infection starts at the roots and moves upwards, killing the plant in stages.

How to Prevent

Prevent fusarium wilt by using sterile soil, and by avoiding bringing any foreign dirt from other gardens into your own. The disease persists on soil and plant debris inside chlamydospores—they’re like normal fungal spores, except they have a protective coating that allows them to live for many years. Practice crop rotation to further bolster your garden’s defenses against fusarium wilt, and avoid using too much nitrogen.

How to Treat

Like root rot, fusarium wilt is difficult to treat on infected plants. Start by quarantining diseased specimens to prevent the condition from spreading to nearby plants. Avoid planting nightshade family crops in the same space for a few years to let the fungus die out. Some seed retailers offer Fusarium-resistant varieties that perform well despite the presence of spores.

Botrytis Blight

Close-up of a green leaf with a brown, curled, dry tip due to disease.
Leaves, flowers, and buds will turn yellow, then brown, with lesions throughout.

Botrytis blight, or gray mold, is a common fungal condition that causes fuzzy, gray growth on susceptible specimens. It’s caused by fungi in the genus Botrytis that love moist, humid environments typical of greenhouses. It starts on dead tissue like spent flowers but can spread onto living tissue once it begins growing. 

How to Identify

Watch for blight-like symptoms first. They’ll appear before the fuzzy gray mold. Leaves, flowers, and buds will turn yellow, then brown, with lesions throughout. After the blight symptoms, fuzzy gray mold manifests on the infection sites. The fuzz spreads spores from the infection site to nearby plants, causing widespread outbreaks within the greenhouse.

How to Prevent

Prevent botrytis blight by practicing safe cultural techniques. Plant your crops with space between them; this allows for proper air circulation. Aim to maintain a dry environment, opening windows or doors once a day so that excess humidity leaks out. Remove dead or dying tissue to reduce possible infection sites.

How to Treat

Once gray mold symptoms appear, it is best to remove the infected tissue and quarantine the rest of your crops. This helps reduce future infections. Organic copper fungicides work well to prevent future infections, though they aren’t substitutes for proper cultivation practices. Use them as a part of a well-balanced treatment plan. Always rotate the types of fungicides you use, as botrytis develops resistances to them quickly.

Rust

Close-up of glossy green pear leaves displaying rust as orange spots on the surface.
Rusts manifest as orange, patchy spots on leaves and stems.

Rusts are a group of fungi that present orange, rusty patches on susceptible plants. They cause weak growth for long periods, causing the plants to wilt before dying completely. The fungi can overwinter on infected plant debris, causing issues throughout the cool months. Keep them away by practicing sterile cultivation methods. Use disease-free soil and plant stock inside the greenhouse.

How to Identify

Rusts manifest as orange, patchy spots on leaves and stems. They spread to fruits and vegetables in extreme infestations, causing bud, flower, and fruit drop. Many ornamental plants are susceptible like hollyhocks, chrysanthemums, and fuchsias. Watch for orange or brown patches—they start on the lower leaves and spread to the upper ones.

How to Prevent

Grow rust-resistant plants if available, and transplant them so they have sufficient space for air circulation. Bring a fan into the greenhouse to reduce condensation, humidity, and stagnant air. If rust symptoms manifest, remove the infected plant tissue and dispose of it far from the growing site. General pruning for airflow will prevent the spread if the disease does present itself.

How to Treat

Organic copper and sulfur fungicides work well to reduce future infections. Spray them around the greenhouse on susceptible plants. It’s best to wear a mask and eyewear to avoid accidentally inhaling the fumes. Many fungicides stay on the leaves, so refrain from spraying crops you intend to eat within the coming weeks.

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dry, yellow, rotting cucumber leaves.

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