Understanding Plant Diseases, Pt. 2: Leaf Diseases

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 Some of the most common diseases observed in ornamental and vegetable gardens are those that occur on leaves. While there are many different leaf diseases affecting a wide range of plants, some of the more commonly observed diseases include powdery and downy mildews, and fungal and bacterial leaf spots. Though very common in the landscape and generally not fatal to plants, if left unchecked foliar disease can inhibit photosynthesis and weaken plants over time, exposing them to subsequent infestations by other insect and disease pests.


 Leaf diseases show up in a wide array of shapes, colors, and sizes. Identification begins with continued monitoring in the garden. Leaf diseases frequently start at the lower branches of plants where humidity is higher and there is less sunlight exposure. Spores of diseases can be carried by wind, or splashed by rain or irrigation, or may even persist in the soil from season to season if proper sanitation and management are not observed.

Leaf diseases may appear ‘powdery’, rounded or angular, or raised or sunken on leaf surfaces. Colors can be extremely variable, ranging from yellow-reddish, to tan or brown. The color of the spots on leaves can be a good indicator of what stage the disease is in. For example, a newly formed leaf disease can be yellow-green in coloration and as it progresses, will grow larger and change to a brown-tan color. When the disease is mature, you may even notice small black specks around old parts of the leaf spot. This is the reproductive fruiting body of the fungus that releases spores and infects other portions of leaf surfaces.

Downy mildew

Development of disease is favored in cool, from 60 to 75 degrees and in areas of high humidity, so even when daytime temperatures are not favorable, cooler night temperatures will still encourage the spread of downy mildew. Signs of downy mildew include mottling and yellowing of leaves, which may also resemble downymildewnutritional deficiencies. However, downy mildew also causes angular lesions on leaf surfaces that appear yellowish or brown. Inspect the underside of the leaves, as it may reveal gray-brown to purplish-black fuzz, which is the many spores produced by the disease. If the disease progresses further, leaves will become brown (necrotic) and begin to curl upwards.

Powdery Mildew

This disease starts as superficial growth on leaf surfaces and is a very common disease. Infected leaves will look as if they have been dusted with baby powder. This disease mostly affects new growth of plants and may cause leaves to turn yellow and drop prematurely. Powdery mildew requires cooler nights in high humidity environments, and is more severe in shady areas or on plants that do not have sufficient air movement between the leaves.

Cercospora Leaf Spot

 This particular disease can be extremely unsightly for ornamental landscape plants. Beginning in June and persisting throughout the summer, cercospora will cause leaves to turn bright yellow to orange-red. If infestation is severe, leaves may drop prematurely from plants. If you grow hydrangeas then you are likely familiar with the large, dark spots that infest leaves late in the growing season. This leaf disease is known as Cercospora leaf spot, and though it makes your plants look pretty rough, it is largely considered to be nonfatal. Many other plants are also affected by this disease and proper sanitation must be observed when symptoms are first noticed. Rake away fallen leaves and replace mulch around trees and shrubs.

Septoria Leaf Spot

This is one of the more common leaf diseases of tomatoes, but can infect all plants in the Solanaceae family. Symptoms are first observed on the lowest branches and leaves, and slowly progress upwards throughout the growing season. Small, circular spots with dark brown edges will be observed on leaves.

Controlling leaf diseases

In all cases, providing good air circulation around plant leaves is your first defense against leaf diseases. Drip irrigation in favor of overhead watering will cut down on disease occurrence as well. Choose disease resistant cultivars where available ad for mulched areas, replace mulch every few years where possible. Fungicides are available, but are preventative and require frequent, repeat applications.

Learn More!

 For more advice on plant disease, visit http://ces.ncsu.edu, where you can post your questions via the ‘Ask an Expert’ link, or contact your local Extension office. If you live in Pender County, call 259-1235. In New Hanover County, call 798-7660. In Brunswick County call 253-2610.