Hopefully, we’ll only have one or two plant diseases that pop up in the home garden, even though they may recur. That’s not bad out of some possible 50,000 different problems.

While some do only minor damage, severe cases of others can be detrimental. Plant diseases adapt to and target specific hosts to survive, or can have a broad range.

Diseases not to overlook include those that not only hinder growth but can kill our garden plants, from edible crops to ornamentals to trees. These are the famine-inducing, most notorious diseases that spread to surrounding plants and have no available treatment but swift removal. We’ll explore common fungal, bacterial, and viral offenders capable of irreversible damage and how to prevent them.

Setting plants up for success with their optimal growing conditions is the best prevention. Sun exposure, soil type, drainage, moisture level, and spacing are key preferences to follow for the best health. To help stave off plant diseases:

• Grow disease-resistant species and varieties
• Interplant with companions and a diverse selection
• Increase air circulation
• Adjust watering sessions and methods
• Manage insects that are disease vectors
• Disinfect tools
• Keep any diseased debris out of the compost pile

Root and Crown Rot

Rot occurs in waterlogged conditions.

Certain conditions are ripe for fungal plant diseases like root and crown rot, which hinder root and stem moisture and nutrient uptake. Over-saturated soils are one of the primary situations that promote the problems.

With overwatering, an inundation of rainfall, or poorly draining soils, the roots sit in soggy conditions. While some plants prefer “wet feet,” many do best in well-draining areas. And, plants can’t survive in standing water devoid of oxygen. They’re likely to succumb to root and crown rot. 

Root rot and crown rot occur most in warm, humid environments, in prolonged wet conditions, and in sites with poor air circulation. With root rot, the pathogens Erwinia and Phytophthora (among others) impact feeder roots, which become unable to absorb water and nutrients. Signs of root rot include drought-stressed leaves and stems that turn yellow and wilt as they’re not receiving water from the roots.

Check for rot by inspecting stems and roots slightly below the soil line. Rotting roots will be yellow or brown rather than a healthy white. With crown rot, brown patches and rotting appear where the leafy stems emerge just above the soil line. Brown or black lesions may also crop up.

Botrytis

Look for brown cankers in the stems and leaves.

Botrytis cinerea, or gray mold, is a blight that presents as light grayish-brown watery spots on petals and leaves. They discolor, and a fluffy mold appears. Stems may develop tan-brown cankers that cause them to collapse.

Botrytis thrives in high humidity, especially in the cool and damp conditions of spring and fall. It proliferates with limited air circulation.

Leaf Spot

There are several types of leaf spots to watch out for.

Bacterial or fungal leaf spot plant diseases are common infections that grow on leaf surfaces. Brown or black spots with tan centers become more significant and spread across leaves and stems over time. As the pathogens spread, affected leaves and stems blacken.

The likelihood of leaf spot increases in wet conditions, with spots appearing when leaves stay wet from prolonged moisture from rain or overhead watering. 

Some of these plant diseases, like Alternaria, can spread to the roots. The roots rot on the tips and are unable to function to support the upper growth. Others, like anthracnose, affect leaves, stems, and fruits, causing lesions and impeding photosynthesis.

Aster Yellows

This disease is common on certain flowering plants.

Aster yellows is a plant disease that spreads via leafhoppers. The sap-sucking pests spread the phytoplasma (similar to a bacterium) as they feed on infected hosts and travel between specimens. 

The phytoplasma causes weak, distorted leaves and shoots, overall yellowing, and malformed, greenish flowers where tissues form instead of blooms.

Wilt

Sudden wilting is a cause for concern.

Fusarium and Verticillium wilt are fungal plant diseases that cause sudden wilting, blackened leaves and stems, and decline. They spread from spores in the soil and also arrive on infected seeds or transplants. Wind, water, and tools also spread the spores. 

The fungus impacts the roots and affects nutrient and water uptake. With fungal wilts, you may see one half of the plant wilting more than the other, followed by leaves yellowing from the bottom of the stem on up. When cut, the stems may show brown tissues.

Rust

Rust is easy to identify by its coloring.

The fungal disease rust is most prevalent in hot and humid climates. It can cause leaves to yellow, wilt, and drop early. 

With these plant diseases, you’ll find powdery spores on the undersides of yellowing leaves. The spores erupt with yellow mold caused by the highly specialized pathogens like Aecidium and Puccinia, and some 7,000 more.

Mosaic Virus

This virus leaves mosaic patterns on leaves.

Mosaic viruses reflect a number of plant diseases that disrupt normal cellular function. The virus invades a cell, which replicates the virus instead of carrying out plant processes. Leaves get a mottled mosaic pattern in light and dark greens and may curl and distort.

Common mosaic viruses include tobacco mosaic virus, cucumber mosaic virus, tomato mosaic virus, and rose mosaic virus.

Mosaic viruses spread easily. They enter tissue through insect or pruning damage, on tools, and via splashing water.

Rose Rosette

This virus is specific to plants in the Rosa genus.

Rose rosette is a Rosa-specific virus that’s highly contagious and results in a rose dying. Also called witches’ broom, it causes growth malformations and thick, thorny canes. Foliage can stunt and deform, and buds distort.

The plant disease is spread by a mite that feeds on infected host plants (often multiflora roses) and travels via wind and infected tools.

Fire Blight

This severe plant disease is common in fruit trees.

Fire blight is severe among fruit trees like pears and apples, as well as ornamentals like quince. The bacteria Erwinia amylovora overwinter in small cankers in the branches and trunks and enter healthy tissues from the point of infection.

During blooming and in early summer, you may see withered flowers, buds, and blackening leaves and stems that appear scorched and wilted. Branches produce less fruit, and eventually, the entire tree may succumb.

If you suspect blight but aren’t sure, cut into the impacted stem to inspect the tissue. With blight, it will be dark and moist. If other issues are the cause of the stress, the tissue will be greenish or dry brown.