Fungi infect plants at every stage of growth, from the small seedling to the fruiting tree, and they spread especially when high humidity combines with poor ventilation. Many begin as a small spot no one pays attention to, then within days turn into an infection that covers the whole field or greenhouse.
In this guide we divide fungal diseases by the affected part of the plant: leaves, fruits and aerial parts, and roots and seedlings. This division helps you identify the disease from the look of its symptoms, and each disease has its own article explaining its details, with the link placed under each type.
Quick overview
Main diseases: downy mildew, powdery mildew, early and late blight, gray mold, rust, anthracnose, and root diseases such as Fusarium and Verticillium.
Affected crops: vegetables, fruits, fruit trees and field crops, in open fields and in greenhouses.
Symptoms: spots on leaves, a white powder or gray fuzz on the surface, rot in fruit, or sudden yellowing and wilting with brown roots.
Most important step: early diagnosis and lowering humidity around the plant, which are easier and cheaper than treating a widespread infection.
Which fungal diseases affect plant leaves?
The leaves are usually where an infection shows first, and the shape, color and location of the spot are important clues to the type of disease. Checking both the upper and lower surface of the leaf saves you a lot of guessing.
Downy mildew
Yellow spots on the upper leaf surface and gray or purple fuzz underneath; it spreads in cool, humid weather.
Powdery mildew
A white, flour-like coating on leaves and stems; it appears even in relatively dry weather when nights are humid.
Early blight (Alternaria)
Brown spots with concentric rings that begin on the older lower leaves and gradually move up the rest of the plant.
Fungal leaf spot (Septoria and Cercospora)
Small spots with defined edges that merge, dry out the leaf and make it fall, weakening the plant and reducing its fruit.
Rust diseases
Orange or brown pustules carrying spores that spread with the wind, usually appearing on the underside of the leaf.
Anthracnose
Dark sunken spots on leaves, stems and fruit, which increase with rain and splashing droplets.
Scab in apples, pears and potatoes
Rough dark spots on leaves, fruit and tubers that deform their shape and lower their market value.
Which fungal diseases cause rot in fruit and aerial parts?
These diseases attack fruit, flowers and tender stems, and thrive when moisture builds up inside dense foliage or after harvest.
Gray mold (Botrytis)
Gray fuzz covering flowers, fruit and wounds, spreading quickly in poorly ventilated greenhouses.
White mold (Sclerotinia)
White cottony growth with hard black bodies forming inside it, which stay in the soil for a long time.
Black sooty mold
A black layer growing on the honeydew secreted by sucking insects, which blocks light from the leaf.
Late blight
Large water-soaked spots that quickly turn brown on leaves and fruit, and become severe in cool, humid weather.
Which fungal diseases affect roots and seedlings?
A soil-borne disease is hard to detect because its above-ground symptoms resemble a lack of water or nutrients. If a plant wilts despite adequate watering, pull it up and examine its root and the base of its stem.
Damping-off
The seedling collapses and its stem rots at soil level; it happens in dense plantings and water-saturated soil.
Rhizoctonia
A fungus that causes lesions on roots and stems near the soil surface and weakens plant growth.
Root rot
Soft brown roots with yellowing and general wilting, usually linked to poor drainage and overwatering.
Pepper blight and crown rot (Phytophthora)
Sudden wilting and rot at the base of the stem, which becomes severe in heavy soil where water stagnates.
Fusarium wilt
Yellowing and wilting that starts on the lower leaves with browning of the stem vessels; the fungus stays in the soil for years.
Verticillium wilt
Wilting that often appears on one side of the plant with browning of the transport vessels inside the stem.
How do you reduce the risk of fungal infection?
Prevention works better than cure for fungal diseases, and these general steps apply to most types:
- Avoid irrigation that wets the leaves, and prefer watering at the base of the plant.
- Keep good ventilation between plants and do not plant them too close together.
- Remove infected leaves and fruit away from the field and do not leave them on the soil.
- Improve drainage and avoid overwatering, especially in heavy soil.
- Rotate crops and do not repeat the same crop in the same spot for consecutive years.
Chemical control requires knowing the type of fungus first, because each group of diseases has its own suitable fungicide, and using the wrong one wastes time and money. Whatever product you use, follow the directions on the label.
Have you noticed symptoms on your plants and do not know the disease? Contact Orkida's engineers on WhatsApp and send photos of the infection so they can help you diagnose it and choose the right solution.
Image source: Shutterstock
Frequently asked questions about fungal plant diseases
How do I tell a fungal disease from a nutrient deficiency or insects?
Fungal diseases leave defined spots, a white powder, fuzzy growth or rot, while nutrient deficiency shows as even yellowing, and insects can be seen or leave signs of chewing and sucking.
What conditions encourage fungi?
High humidity, poor ventilation, watering on the leaves, dense planting and mild, humid weather.
Is spraying enough after the disease appears?
Fungicides work best preventively or at the start of infection. Once the disease has spread, damaged tissue is hard to repair and treatment is limited to protecting the healthy parts.
How do I reduce infection next season?
By rotating crops, disposing of infected plant residues, improving ventilation and drainage, watering at the roots, and using a preventive program that alternates active ingredients.