Spots on plant leaves can be either a sign or a symptom. To be a symptom, they have to be the results of something the plant does in response to an injury or infection. A sign would be the result of some outside influence such as spots left by sprays or frost damage or other physical damage.

Many plants are susceptible to an array of ailments collectively called leaf spot diseases. The vast majority of these are caused by fungal organisms. Spores land on the leaf surface and, under the proper conditions of moisture and temperature, a fungus begins to grow. It begins to kill cells in the leaf resulting in discolored tissue in the form of a circle or a blotch.

With many fungal leaf spot diseases, close inspection will reveal the presence of tiny, black specks within the spot. These are called fruiting bodies and are the organs that release sports into the air to go out and infect other plants. Such structure may be seen on deciduous leaves or on needles of pines and other evergreens.

A few fungal leaf spot diseases such as the rust diseases will develop finger-like projections from the bottom of the leaves. These are the fruiting bodies. As the name implies, another characteristic of these diseases is that they develop a rusty dust material that covers the spots.

Although not technically "spots", there are leaf discolorations that are caused by virus infections. These symptoms are more generally termed as "mottling" which displays as streaking of yellow mixed in with the normally green leaf tissue.

The following are some common leaf spot diseases of ornamental plants.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.


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