Diseases of Shade Trees
Jim Downer, University of California Cooperative Extension 669 County Square Drive Suite 100, Ventura
CA 93003. ajdowner@ucanr.edu
Landscapes are composed of trees, shrubs, hardscape, turfgrass, color plants and other perennial
plantings. Because trees attain size and develop heavy woody branches, they can give dramatic effect
when planted and maintained strategically. When trees fail, from disease, loss of architectural integrity,
or slowly decline from various maladies, landscapes lose value. A single specimen tree can be worth
tens of thousands of dollars, so it is imperative to understand how to optimize tree health.
Trees are based on a cellular structure that requires energy for survival, growth and reproduction.
Energy for trees is derived from sunlight, through the process of photosynthesis. Energy is stored in
carbon molecules as sugars and starch primarily in woody tissues. Healthy trees capture and store
energy while plant pathogens use tree energy for their own growth and reproduction. Disease is the
process that pathogens (disease agents) cause in their host. Disease is a disruption of the normal
physiological functions of a tree. These disruptions cause host reactions that result in symptoms or
visible tree responses to the pathogen.
There are two categories of disease—Biotic and abiotic. Biotic diseases are caused by living pathogens:
fungi, bacteria, virus particles, parasitic seed plants, or nematodes--to name a few. Abiotic diseases are
caused by continuous irritation by a non-living factor such as moisture content of the soil, temperature,
relative humidity, soil mineral effect or other condition. For a thorough discussion of abiotic disorders
see ANR publication 4320 Abiotic Disorders of landscape Plants by Costello and others (2003).
Learning to distinguish and recognize various kinds of symptoms and signs of tree diseases is the first
step in maintaining tree health. Recognition of these subtle changes requires a keen interest in the
horticulture of each tree species. The more we know about tree horticulture the better the
understanding of their abnormalities.
Every part of a shade tree can become diseased. Pathologists categorize diseases in various groups such
as cankers, root rots, wilts, anthracnose, blights, leaf spots, and wood decay. While various kinds of
pathogens can cause these types of diseases, fungi are the most prevalent disease agents. Bacteria,
viruses and viroids, nematodes and parasitic seed plants also cause important diseases but with less
frequency. While disease symptoms are soon recognized as disease develops, the signs of disease
(which are parts of the pathogen) are often cryptic or microscopic. We can see some fungal fruiting
bodies, spores and parasitic seed
plants and sometimes nematodes, but,
bacteria and viruses are not visible to
the human eye.
Canker diseases form in the bark of
many trees. They can occur on stems,
branches or the trunk. Canker diseases
Figure 1. A perennial canker
cause symptoms of disfigurement as the stem’s growth is affected by the disease. When cankers girdle
stems, they elicit symptoms on the leaves and stems distal to the canker. Foliar discoloration, wilting,
necrosis and death are the usual symptoms. Paul Manion in his text Tree Disease Concepts (1981),
proposes that there are various kinds of canker diseases with resulting severities in the host. Cankers
can be annual, perennial or diffuse, the diseases they cause are either host or pathogen dominant. Host
dominant cankers cause slight or insignificant damage as the host can contain their spread, perennial
cankers may be contained by the host for a time but regrow and continue expanding on a stem for many
years. These cankers cause “target” lesions on stems. Diffuse cankers are pathogen dominant and are
very destructive, rapidly killing host tissue in a short amount of time. Cankers can be controlled by
selecting resistant varieties of trees, pruning out the cankers, and by controlling insects that spread
diseases. Rarely are cankers controlled by fungicides.
Root rots are very common pathogens.
They occur in all climate zones on almost
every kind of tree. Many roots rots are
caused by Oomycetes—organisms more
closely related to algae than to fungi. This
group includes the genus Phytophthora, one
of the most common and destructive
pathogens. Phytophthora means plant
destroyer. Most Oomycetes prefer wet
soils as they have infective spores that need
to swim to susceptible roots. Fungi such as
Armillaria , Fusarium, and Rosellinia, can all
cause root rot in trees, Armillaria being the Figure 2. Root rot often affects all the leaves on a tree causing
yellowing, wilt, necrosis and death.
most common of these. Root rots can also
be caused by anoxic conditions in highly
compacted or flooded soils. Correcting soil conditions that
lead to root rot is often the best control of these diseases.
While there are effective fungicidal controls for
Oomycetes, there are no fungicides for controlling
Armillaria. Manipulating soil microbial communities also
shows some promise in controlling Oomycete root rots.
Wilt diseases are diseases of the vascular system (usually
the xylem) of trees. They can be caused by fungi or
bacteria. Fusarium causes disease in relatively few
landscape trees the most notable ones Canary island date
palm wilt and Albizzia wilt. Verticillium causes wilt in a
number of landscape trees including Brazilian pepper, olive
and several maple species. Many wilt diseases are soilborne and may spread from tree to tree by root grafting
Figure 3. Fusarium wilt in Palm. Wilt diseases often
stain the vascular tissues.
(dutch elm disease). Spores can survive in soil for many years, even without a susceptible tree growing
there. Wilt infected trees are not easily treated as fungicides cannot be delivered to clogged xylem
vessels and tracheids. Removing infected branches and encouraging the tree to grow new xylem are
sometimes helpful.
Anthracnose diseases are diseases of young stems leaves and new shoots. Sycamore anthracnose is
perhaps the most known disease and the most treated fungicidally. Anthracnose diseases are easily
diagnosed by the symptoms they cause. Vein following symptoms, canopy defoliation from the lower to
upper canopy, and defoliation associated with warm spring rains. Anthracnose diseases are highly
regulated by the weather (rain and temperature) and also controlled when weather changes.
Appropriately timed fungicide applications
are very effective. Anthracnose resistance
exists in many species, so selecting a resistant
variety is a great way to avoid this disease.
There are many fungi and some bacteria that
cause leaf spots or blights on trees, these can
be defoliators or just minor disfiguring fungi
that are little noticed. Like anthracnose,
when the environment is conducive to
disease, these fungi can defoliate trees
causing slowed growth and great loss of
aesthetics. Rarely, are trees killed or severely
harmed by these organisms. One exception is fire blight on pears. This bacterial pathogen can cause
severe harm or death of major portions of the tree.
Figure 4. vein following symptoms of Sycamore anthracnose
Wood decay organisms are not thought to be pathogens by some because they affect non-living
portions of the tree (heart wood). However, many wood decay fungi also affect the sapwood and
therefore can limit water and sugar movement within trees. These limitations lead to development of
symptoms and thus these organisms are considered pathogens by many pathologists. Wood decay
diseases are categorized as brown rots (fungi
use cellulose but do not degrade lignin) and
white rots (fungi that decay both cellulose and
lignin). Most if not all wood decay fungi require
wounds to enter the tree. Control can be
achieved by limiting the size of pruning wounds
to prevent fungal entry. This can be
accomplished by pruning more frequently, so
that wounds are small and rapidly closed over
by wound response tissues.
Figure 5. Laetiporus gilbersonii, a common brown rot
pathogen of Eucalyptus, oak, and other hardwoods.
Cotello, L., Perry, E J, Matheny, N P, Henry, MJ and P.M. Geisel. 2003. Abiotic Disorders of landscape
plants a diagnostic guide. ANR publication 3420 University of California, Communication Services,
Oakland CA.
Manion, P. 1981. Tree Disease Concepts. Prentice-Hall Inc., 399pp.