| Leaf spot is a common descriptive term applied to a
number of diseases affecting the foliage of
ornamentals and shade trees. The majority of leaf
spots are caused by fungi, but some are caused by
bacteria. Some insects also cause damage that
appears like a leaf spot disease. Leaf spots on trees
are very common and generally do not require
spraying. Leaf spot may result in some defoliation of
a plant. An established plant can tolerate almost
complete defoliation if it happens late in the season
or not every year. Small or newly planted trees that
become defoliated are more at risk of suffering
damage until they become established.
Symptoms and Diagnosis
The chief symptom of a leaf spot disease is spots on
foliage. The spots will vary in size and color
depending on the plant affected, the specific
organism involved, and the stage of development.
Spots are most often brownish, but may be tan or
black. Concentric rings or dark margins are often
present. Fungal bodies may appear as black dots in
the spots, either in rings or in a central cluster. Over
time, the spots may combine or enlarge to form
blotches. Spots or blotches that are angular are
generally referred to as anthracnose (see entry on
“Anthracnose of Trees”) Leaves may
yellow and drop prematurely.
Life Cycle
The organisms that cause leaf spots survive in fallen
infected leaves and twigs. Some may remain in dead
twigs on the tree. Most damage occurs in spring.
During wet weather, spores may splash or be
windblown onto newly emerging tender leaves where
they germinate in the moisture and infect the leaf.
Overhead watering can also provide prolonged wet
periods that are ideal for spreading leaf spot diseases.
Integrated Pest Management Strategies
1. Live with the disease. Most trees tolerate leaf
spots with little or no apparent damage. A tree
affected early in the year will re-leaf and the new
leaves may not be affected. Only if defoliation
occurs three or more years in a row will most
established plants be adversely affected.
2. Remove infected leaves and dead twigs. Raking up
and disposing of infected leaves as they drop and
pruning out dead twigs can help control the disease
by removing spores that can reinfect the new
leaves. This is not a cure but may help limit
infection by reducing the total amount of inoculum.
3. Keep foliage dry. Avoid overhead watering. Use
soaker hoses or water early in the day so the
foliage can dry off before night. Watering can also
spread the disease by splashing. Pruning plants to
allow for good air circulation and reducing
crowding will also help keep the foliage dry.
4. Keep plants healthy. Since most plants can
tolerate some defoliation, keep them in good
health so they can rebound quickly. Avoid over
fertilizing by testing the soil first. Abundant,
young, tender growth is very susceptible to attack
by disease and insects. Overuse of nitrogen can
cause an abundance of succulent growth.
5. Use fungicides if needed. In rare cases of severe
infection and where the size and value of plants
make it practicable, applications of fungicides
may be helpful. Sprays will not cure infected
leaves. Therefore, once the damage is noticed,
spraying may have limited value. Spraying
generally needs to be started as buds break in the
spring and repeated at 10–14 day intervals.
Recommendations will vary by disease and
fungicide used. Have the disease identified before
purchasing a control product.
6. Replace the plant. Though a drastic measure,
many gardeners find it less bother and more
rewarding to replace a plant that is continually
plagued with leaf spot diseases. Either replace
with a different kind of plant or a variety that is
more resistant or tolerant of disease. A nursery
can help you in your selection. | 
