Leaf spot on Plants

Symptoms

Start with the leaves, where leaf spot appears as discrete, diseased sections of leaf tissue. These sections are called leaf spots or lesions, and each one is a distinct, separate patch of diseased tissue on the leaf.

Next, check whether the spots have started to run together. Numerous leaf spots can merge into blights that affect large portions of the foliage. At that point the infection covers large portions of the foliage instead of a few small marks on single leaves.

Finally, look at the twigs and branches. Infections there can cause cankers, which are sunken, discolored, and cracked areas. Cankers appear on twigs and branches, while leaf spots appear on leaf tissue.

The three signs to look for are separate spots, merged blights, and cankers. Keep these three signs in mind as you read on, because the diagnosis section uses them.

Causes

In many cases, the fungi behind leaf spot are host specific and infect only one plant species. Examples of the fungi that cause leaf spot include species of Alternaria, Diplocarpon, Cercospora, Cladosporium, Stemphylium, and Ovulinia.

Most of these organisms produce spores that spread by wind currents and by splashing water from rain or sprinklers. The splashing water can come from rain or from overhead sprinklers. Rain and sprinklers are both named as sources of splashing water.

Moist, humid conditions favor infection and the development of the disease. The fungus can also arrive on infected seed, cuttings, and other propagative material.

In short, the facts name four causes: host-specific fungi, spores carried by wind and water, moist and humid conditions, and infected propagative material. Leaf spots, merged blights, and cankers on twigs are all signs of infection by fungi of this kind. Spores travel in two ways: on wind currents and in splashing water.

Diagnosis

Begin by looking for discrete, diseased sections of leaf tissue. If you find them, check next for merged spots that have become blights across large portions of the foliage. Then look at twigs and branches for sunken, discolored, and cracked cankers.

These signs together point to a fungal infection rather than a general care fault. The causes give you more to check. Think about whether the weather has been moist and humid, since those conditions favor infection.

Think about whether rain or overhead sprinklers have been splashing water onto the plant, because spores spread that way. Ask whether the seed, cuttings, or other propagative material came from a source you can trust, since infected material can carry the fungus.

Because the fungi are host specific in many cases, check whether only one kind of plant shows the problem. This page cannot name the exact fungus on your plant, because many different fungi cause leaf spots.

Treatments

Start with the cultural methods, which come first in the list of treatments. Use pathogen- and disease-free seed, cuttings, transplants, and other propagative materials. Infected seed, cuttings, and other propagative material can carry the fungus.

Put sanitation measures in place when you are dealing with containers, flats, benches, and pruning tools. Avoid using overhead sprinkler irrigation. Spores spread in splashing water from rain or sprinklers.

Rogue out, which means remove, diseased cuttings, transplants, and plants. Sanitize your hands or gloves after you handle diseased plants. In short, five cultural steps are listed: clean planting material, sanitation, no overhead sprinklers, removal of diseased plants, and clean hands or gloves.

Cultural methods and the chemical method are the two types of treatment listed. Fungicides provide some control of leaf spot. Rotate fungicide mode-of-action groups to avoid resistance.

The active ingredients listed for this method are azoxystrobin, pyraclostrobin, trifloxystrobin, and fluoxastrobin, which are all part of the chemical type of treatment. Fungicides are listed as one chemical option alongside the cultural steps.

The facts for this page list no organic treatment, so none is described here. This page gives no dose, rate, or spray interval; the product label is the legal instruction, and you must follow it for the crop and site being treated.

Prevention

Prevention is about the conditions around your plants. Moist, humid conditions favor infection and disease development. Spores spread by wind currents and by splashing water from rain or overhead sprinklers.

Avoiding overhead sprinkler irrigation is listed as a cultural practice. Start with seed, cuttings, transplants, and other propagative materials that are free of the pathogen and of disease. Infected propagative material is one way the fungus arrives.

Apply sanitation measures to containers, flats, benches, and pruning tools. Sanitize your hands or gloves after you handle diseased plants. Remove diseased cuttings, transplants, and plants when you find them. These steps focus on conditions you can control, such as irrigation method, planting material, and cleanliness.

Plants most often affected

• Adam's Needle
• American Basswood
• American Sycamore
• Aztec Marigold
• Baby Rubberplant
• Barbados Aloe
• Black Baneberry
• Black Cherry
• Black Walnut
• Blackgum
• Blue Giant Hyssop
• Boxwood
• Camellia
• Canada Goldenrod
• Cast Iron Plant
• Cherry Plum
• Chinaberrytree
• Chinese Banyan
• Christplant
• Common Asparagus Fern
• Common Buckthorn
• Common Evening Primrose
• Common Filbert
• Common Hackberry
• Common Milkweed
• Creeping Jenny
• Crepe Myrtle
• Eastern Redbud
• Eastern Teaberry
• English Ivy
• English Lavender
• English Walnut
• European White Birch
• Flamingo-lily
• Flowering Dogwood
• French Hydrangea
• Glossy Buckthorn
• Inchplant
• Joshua Tree
• Kentucky Bluegrass
• Lilac Chastetree
• Littleleaf Linden
• Madagascar Periwinkle
• Monkeypuzzle Tree
• Muscadine
• Norfolk Island Pine
• Northern Catalpa
• Oxeye Daisy
• Panicled Hydrangea
• Pothos
• Pride of India
• Purple Coneflower
• Purpletop Vervain
• Red Maple
• Sassafras
• Silktree
• Silver Maple
• Southern Catalpa
• Sugar Maple
• Sweet Cherry
• Tuliptree
• Vilevine
• Virginia Creeper
• Weeping Willow
• White Poplar
• Yaupon