Forest Health > Insects and Diseases publication
Hardwood Foliage Diseases
Causal Agents: |
Various Organisms and Abiotic Factors |
Common Hosts: |
Hardwoods |
Significance:
Leaves are among the most conspicuous features of hardwood trees and are often the reason certain trees are planted as ornamentals. As a result, foliage abnormalities (symptoms) are often more readily observed than other types of tree disease problems, especially by the layman. Many FOLIAGE DISEASES are just that - diseases resulting from foliage infections or injuries of one type or another. In other cases, foliage symptoms (color irregularities, wilting, etc.) are not actually the result of foliage diseases, but are INDICATORS of other problems such as nutritional imbalances, root diseases (refer to index), chemical injuries, or environmental stresses that are affecting the tree as a whole.
As a rule, foliage diseases do not constitute a serious threat to Florida's hardwoods, although in situations where blemish-free foliage is necessary or desirable (ornamental nurseries, etc.) they can be troublesome. When foliage symptoms are indicators of other problems, the significance of the indicated problem is often of greater potential consequence. By the time indicator symptoms are expressed by the foliage the tree has already sustained serious injury or is in an advanced stage of disease development.
Recognition:
Foliage diseases are exhibited in a variety of ways on hardwoods in Florida. Table VI outlines the more typical symptoms types, together with their related causes and most common hosts.
Infection Biology:
The fungi that cause foliage infections (true foliage diseases) produce spores on symptomatic leaf tissues in or on various types of small fruiting bodies when temperature and moisture conditions are favorable. Spores are disseminated largely by wind and rain-splash to the surfaces of newly emerging and/or disease-free leaves. In most cases, spore survival and germination require several hours of high relative humidity, water droplets, or a water film on the host leaf tissues. Following spore germination, the fungi penetrate the leaves directly or through minute pores (stomates) on the leaf surface.
With the exception of powdery mildews and sooty molds, the fungi causing foliage disease typically develop parasitically within host leaf tissues, eventually giving rise to their associated symptoms (leaf spots, blotches, etc.). The powdery mildews develop on the surface of host leaves and establish their parasitism by producing microscopic penetration pegs called haustoria (sing., haustorium) which penetrate into individual leaf cells. These haustoria serve as absorption structures to provide the pathogens' needed nutrition. In contrast, sooty molds do not actually parasitize leaves at all. These fungi are superficial nuisances that typically feed on the honeydew of aphids and excretions of certain scales or other insects, although naturally secreted exudates are also fed upon in certain host plants as well.
The Fire Blight Bacterium (Table VI) survives throughout the winter in a dormant condition within blighted twigs and branches of infected hosts. With the onset of warm spring rains the pathogen becomes active and produces a bacterial ooze which is picked up by various types of insects, especially bees. Through their normal activity these insects carry bacteria to susceptible uninfected host tissues (blossoms, leaves, twigs) where new infections are initiated. Other infections result from simple rain splash of the bacterial ooze. The bacterial pathogen enters host tissues through natural openings (stomates, lenticels, etc.), insect-created injuries or man-made openings (pruning scars, etc.).
The infection biologies for the indicator type of foliage diseases are related directly to their specific causes and can be best understood by referring to other sections of this book (root diseases, etc.).
Control:
In most cases foliage diseases have very little impact on infected trees. Accordingly, control measures are often unnecessary. In other cases some foliage diseases may be quite damaging. If control is warranted, most foliage diseases can be effectively controlled with carefully applied applications of suitable fungicides or bactericides. In addition, raking up and destroying fallen, infected leaves is recommended. This practice reduces chances of new, disease-free foliage sustaining infections via spores produced on the previously infected leaves. In the same vein, selective pruning of diseased foliage and/or branches may be helpful in some situations. Providing adequate aeration and/or minimizing excessive moisture accumulation (via lawn sprinklers, etc.) on the leaves of shade trees is advisable also. Moisture is a necessary ingredient for most foliage infections.
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Bulletin No. 196-A | Printed October, 1983 | Contact the Forest Health Section |
