Diseases and disorders of Magnolia species

Issue 93
Diseases and disorders of Magnolia species in the
southeastern United States
Gary W. Knox, University of Florida, Department of Environmental Horticulture,
North Florida Research and Education Center
Mathews Paret, University of Florida, Department of Plant Pathology, North
Florida Research and Education Center
Amy Fulcher, University of Tennessee Knoxville, Department of Plant Sciences
William E. Klingeman, III, University of Tennessee Knoxville, Department of Plant
Sciences
Magnolia species are widely used in southeastern U.S. landscapes because
of their beauty, usefulness, low maintenance and broad adaptability to
the region’s climate and soils. Six Magnolia species are native to the region
(Magnolia acuminata, M. fraseri, M. grandiflora, M. macrophylla, M. tripetala
and M. virginiana) and many non-native species and hybrids have been
introduced. A review of commercial wholesale nursery production in
twelve southeastern states found the native M. grandiflora (southern magnolia) is the most widely grown--in168 nurseries--because its cultivars are
heavily favored for landscape use (Knox 2012; Knox et al., 2012a). Other
widely produced species are the native M. virginiana (sweetbay), found in
65 nurseries, followed by the non-native Magnolia stellata (star magnolia),
in 47 nurseries, Magnolia ×soulangeana (saucer magnolia) and similar hybrids, in 16 nurseries. Six other species also are produced.
Magnolias are considered to be “trouble-free” with few pests and diseases
under most landscape conditions (Dirr, 1998). A number of compounds in
Magnolia species exhibit antimicrobial, insecticidal and nematicidal properties (Kamikado et al., 1975; Li et al., 2009; Nitao et al., 1991). Nonetheless,
a wide array of pests, diseases and disorders are reported on Magnolia
species in the southeastern U.S. (Knox et al., 2012a; 2012b). Pests, diseases
and disorders on magnolia can cause significant economic or aesthetic
losses.
This article focuses on major plant diseases and disorders affecting Magnolia species in landscape settings, along with management recommendations. Many of the management strategies for diseases and disorders
ultimately relate to plant health, landscape placement and environment.
Weather may be beyond control, but magnolias can be placed in locations
that favor their growth and they can be cultivated in a manner that minimizes disorders and diseases; for example, by drip irrigating rather than
overhead irrigating, which can wet leaves and increase incidence and opportunity for foliar diseases. When management calls for use of specific
pesticides, these tools should be utilized properly and legally, noting that
they may only be available for specific species, areas, uses or applicators
(such as professionals only).
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Magnolia
Common diseases of Magnolia
Verticillium wilt, Phytophthora root rot and several foliar pathogens are the
primary causes of disease on magnolia. No canker or virus diseases are
reported.
Verticillium wilt. This disease is caused by Verticillium albo-atrum Reinke &
Berth and Verticillium dahliae Kleb., which are vascular pathogens of many
ornamental shrubs and trees, including Magnolia (Pataky 1997). Verticillium wilt causes leaf and branch dieback often at one side of the tree. Vascular discoloration can be noticed on infected plants. Symptom progression may be slow, but the plants can completely wilt and die in 1 to 2 years.
Sometimes large areas of cambial tissue die from infections by the fungus
and opportunistic fungi such as Nectria spp. (Fr.) Fr. develop in elongated
cankers (Chatfield et al., 1996). The characteristic symptom for Verticillium
wilt is the discoloration of xylem and cambial tissues, visible as streaks in
affected wood (Pataky 1997). However, other diseases and stress factors
can also cause these symptoms, and hence a definitive confirmation is required from a diagnostic lab.
Development of Verticillium wilt is favored by factors that stress roots,
including wounding and prolonged drought. The fungus penetrates
through root-wounds or directly into host tissues (Pataky 1997). Plugging
of the vascular system by the fungal organism restricts water and nutrient
movement, leading to plant wilting. The fungus survives in soil as longlived microsclerotia that can easily spread by wind, soil movement and on
equipment. Verticillium spp. can survive in soil for many years. In addition,
many weedy plants are also susceptible hosts; therefore, the cycle of contaminated soil is hard to break.
Management. Avoid stressful conditions for the plants, including overwatering, drought and root-wounding that will facilitate Verticillium spp.
infection. Avoid planting in areas with previous outbreaks of Verticillium
wilt. Fungicides are not effective in management of Verticillium wilt.
Phytophthora root rot. Phytophthora cinnamomi Rands causes this root rot
disease. Symptoms include yellowing of leaves, sudden wilting, premature leaf drop, slowed shoot growth, limb dieback and plant death. Infected plants typically have discolored feeder roots.
Phytophthora cinnamomi survives as hyphae in infected roots and resting
structures known as chlamydospores in plant debris and soil (Hagan
2001). This fungus is easily and quickly spread by contaminated water.
Root infection can be common and severe if soil is saturated.
Management. Avoid placing magnolias in areas that allow water to pool.
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If plants are irrigated, water only when needed and allow soil to dry between irrigations.
Foliar diseases. Bacterial leaf spot, Xanthomonas sp., is a relatively new disease found almost exclusively in commercial nursery production (Dankers 2012). Based on the time of the year and conditions prevalent during
disease occurrence, high rain and strong wind conditions, as well as heavy
overhead irrigation, easily move the bacteria from plant to plant. The bacterial pathogen can enter through wounds, stomates and hydathodes.
Symptoms of bacterial infection vary, but usually start as small necrotic
lesions with a yellow halo region and can progress to severe blighting
under ideal environmental conditions (Figure 1).
Figure 1. Bacterial leaf spot, Xanthomonas sp., starts as small necrotic lesions with a yellow
halo region and can progress to severe blighting under ideal environmental conditions. (Paret)
Bacterial blight is caused by Pseudomonas syringae pv. syringae Van Hall
and Pseudomonas cichorii (Swing.) Stapp. (Miller 1976). These Pseudomonas
spp. cause leaf spots (Mullen and Cobb, 1984) similar to bacterial leaf spot
on magnolia (Hagan 2001). The disease is characterized by small necrotic
spots with a yellow halo (Figure 2). It is difficult to separate the symptoms of bacterial blight from bacterial spot. However, bacterial blight usually occurs during cooler and wet weather conditions. Splashing water
and overhead irrigation help spread the disease from infected to healthy
leaves. Both Pseudomonas species are aggressive pathogens on southern
magnolia.
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Magnolia
Figure 2. Bacterial blight is caused by Pseudomonas syringae pv. syringae Van Hall and
Pseudomonas cichorii (Swing.) Stapp. ) and cause leaf spots similar to bacterial leaf spot on
magnolia. (Paret)
Anthracnose (Colletotrichum gloeosporioides Penz) causes large circular
spots towards the margins of magnolia leaves (Figure 3) and can lead to
premature leaf drop (Hagan 2001). Disease symptoms include a burned
appearance or angular spots that are surrounded by a yellow halo (Hagan
2001). The upper surface of the spot develops black, blister-like fruiting
bodies (acervuli). As the disease progresses, a pink mass of spores will
ooze from the fruiting bodies. Splashing water spreads the spores to new
leaves. Warm, humid and wet conditions favor disease development. The
fungal organism can overwinter in dead leaves and branches. The disease
is most commonly reported on southern magnolia.
Figure 3. Anthracnose, Colletotrichum gloeosporioides Penz, causes large circular spots towards
the margins of magnolia leaves. (Paret)
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Algal leaf spot, Cephaleuros virescens Kunze, is caused by a parasitic algae
(Vann 2007). Commonly seen on southern magnolia, the key symptom of
algal leaf spot is the formation of raised blotches on the leaves (Figure 4).
The raised blotches will develop a velvety appearance and leaf tissues beneath the spots die. Premature leaf drop can result from severe infection.
Algal spores have the ability to swim in water on plant surfaces. Continuous rain during warm and windy conditions serves as a major factor in the
spread of the spores. The algae may survive winter on infected leaves and
twigs, resulting in repeated infection during following years.
Figure 4. Commonly seen on southern magnolia, Algal leaf spot, Cephaleuros virescens Kunze,
causes raised blotches on the leaves. (Paret)
Pestalotiopsis leaf spot of magnolia (Kauffman 2011), Pestalotiopsis spp.
Steyaert, appears as unique circular spots with necrotic centers and black
borders (Figure 5). Fungal spores from infected leaves and plant debris
are spread by wind and water movement. This leaf spot usually occurs
during cool weather.
Phyllosticta leaf spot, Phyllosticta magnoliae Sacc., develops as small black
spots on the upper leaf surface (Hagan 2001; Pataky 1997). As spots develop, their centers turn off-white and borders become purple to black.
Black fruiting bodies (pycnidia) appear in the center of these spots. The
pathogen is spread to new leaves by splashing water. Warm, humid and
wet conditions favor disease progression. Although it is not common, this
disease may appear on southern magnolia.
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Magnolia
Figure 5. Pestalotiopsis leaf spot of magnolia, Pestalotiopsis spp. Steyaert, appears as unique
circular spots with necrotic centers and black borders. (Paret)
Powdery mildew on magnolia is caused by at least two species, Microsphaera alni DC. ex Wint. [M. penicillata (Wallr.) Lév.] and Phyllactinia corylea
Pers. ex Karst. [P. guttata (Wallr.) Lév.]. This disease commonly occurs on
saucer and star magnolias. Disease symptoms are white powdery patches
on the top section of the leaves (Figure 6; Hagan 2001). The entire leaf surface may develop these white patches during severe infection. In addition,
both leaf curling and plant stunting can occur. Powdery mildew fungi
overwinter as hyphae (fungal strands) in dormant buds or as spores in
Figure 6. Powdery mildew commonly occurs on saucer and star magnolias and appears as
white powdery patches on the top section of the leaves. (Paret)
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fruiting bodies (cleistothecia) on fallen disease leaves. During the spring
to early summer, spores are spread by wind. While primarily an issue in
nursery production, this disease is favored by warm to hot days and cool
nights with dew formation on leaves. Overcrowding of plants and lack of
aeration also contribute to disease occurrence.
Management. Most foliar diseases develop only when leaves are frequently wet, as during a prolonged rainy period or with frequent use of overhead irrigation, as is common in container nurseries. If plants are watered
with sprinklers, irrigation should be managed to avoid leaf wetness during overcast conditions, at night and other times when the pathogen may
be present. The spread of anthracnose, Phyllosticta leaf spot and algal leaf
spots can be prevented by removing infected leaves early in disease development. Selective pruning to thin the canopy can improve air movement
and light penetration, making conditions less favorable for Pestalotiopsis
leaf spot, powdery mildew and algal leaf spots. If environmental conditions for infection persist, copper-based fungicides can be used to manage
bacterial leaf spot and bacterial blight. Other fungicides may be used to
manage anthracnose, Phyllosticta leaf spot and powdery mildew.
Common abiotic and wildlife disorders of Magnolia
Some disorders resemble pest or disease damage or may render magnolias more susceptible to pests and diseases.
Winter burn. Evergreen and semi-evergreen magnolias grown in the upper South (USDA cold hardiness zones 6b through 8a) commonly exhibit
desiccated leaf margins (Figure 7). This winter burn, or scorch, usually is
more severe on the side of the plant facing the wind or receiving greater
sun exposure (Relf and Appleton, 2009). Evergreen or semi-evergreen
magnolias continue losing moisture through their leaves year-round.
These magnolias are especially vulnerable to winter burn on warm sunny
days when the ground is frozen and plants cannot take up water to replace that which is lost through transpiration.
Management. A few simple practices help reduce the incidence and/or
severity of winter burn. Gardeners should plan for irrigation in the fall
to increase the availability of water, especially for newly planted magnolias that do not yet have extensive root systems. Broad-leaved evergreens
should be planted earlier in the fall than their deciduous counterparts
(Adkins et al., 2010). This allows roots to become more established before
harsh weather and also helps ensure that the plant is optimally hydrated
until the soils freeze. Anti-desiccants have not been found to consistently
benefit magnolias during transplanting (Relf and Appleton, 2009). Planting evergreen magnolias in an area protected from wind can also reduce
the incidence of winter burn.
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Magnolia
Figure 7. Winter burn commonly occurs on evergreen and semi-evergreen magnolias grown
in the upper South, appearing as desiccated leaf margins. (Fulcher)
Cold injury. Magnolia stellata, M. ×soulangeana and some other magnolias
often flower in late winter or early spring. Freezes that occur at bud maturity or during flowering will kill or damage buds and flowers, turning
flower tepals brown. This freeze damage is unattractive, but the overall
health of the plant is not affected and subsequent management is not required.
Bark cracking. Many magnolias are thin-barked and therefore are considered susceptible to bark cracking. In particular, yellow-flowered magnolias can be prone to bark cracking at the base of the trunk. Excess nitrogen
fertilizer or warm temperatures late into the fall season can extend active
plant growth. Consequently, the base of the trunk may not sufficiently
harden before cold weather (Hartman et al., 2000) and freezing temperatures may result in bark cracking (Figure 8). Cracked bark usually becomes evident in the spring, but may occur due to freezing conditions
anytime from late fall to early spring.
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Figure 8. Many magnolias are thin-barked and therefore are considered susceptible to bark
cracking. (Fulcher)
Management. Bark cracking can be avoided by ending liquid or soluble
dry fertilization programs early enough to prevent growth. Use of controlled-release fertilizers is generally not a problem because most of these
products release substantially less nitrogen during cold temperatures.
Other management tools include reducing irrigation frequency and volume towards the end of the growing season to slow plant growth and
allow them to adapt to cold temperatures. However, allowing the root
zones of evergreen magnolias to become too dry during late fall and winter can lead to winter burn.
Frost cracking and sunscald. Bark cracking on magnolia also occurs due to
frost cracking and sunscald (Clatterbuck and Franklin, 2004). These similar but separate disorders both cause vertical cracks through the bark to
the wood. Both often occur on the south or southwest side of a tree since
this is where winter temperature fluctuations are greatest. Frost cracking
and sunscald are linked to root injury, aboveground wounds and pruning
cuts. Inadequate plant hydration is also linked to sunscald, but not frost
cracks (Harris et al., 2004; Hartman et al., 2000).
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Magnolia
Frost cracking occurs when water in the wood expands and contracts as
a result of dramatic temperature fluctuations such as those occurring on
warm, sunny winter days with periods of dense clouds or at nightfall
(Clatterbuck and Franklin, 2004). Frost cracks are a physical separation of
the wood and often they will close and heal.
Sunscald occurs when the same dramatic temperature changes damage or
kill the cambium and bark. In winter months, the temperature of the cambium receiving direct sun exposure can be 20°C (68°F) or greater when
the ambient temperature is at or near freezing; snow exacerbates the temperature increase (Sinclair and Lyon, 2005). Sunscald often is not detectable until spring growth resumes (Harris et al., 2004) when the damage
appears as sunken or discolored bark that may later split. If healthy, the
tree will develop a callus roll around the wound as the season progresses.
Management. Tree wraps made of insulating paper may mitigate temperature fluctuations and prevent sunscald if applied in early winter. Also,
avoid large pruning cuts at the bases of trees and large shrubs since some
basal bark cracks have been observed in conjunction with large pruning
wounds. Remove branches before they become more than 1/3 the diameter of the branch or trunk from which they are being removed, in order to
minimize the size of the pruning wound and loss of carbohydrates stored
in the wood.
Sapsucker damage. Sapsuckers (Sphyrapicus spp.) are a type of wood-
Figure 9. Sapsucker damage is characterized by many uniform rows of holes forming rings
around the trunk and large branches as on this saucer magnolia. (Knox).
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pecker native to North America. Sapsuckers peck at trees and feed on sap,
bark and small insects that are attracted to the sap (Vann and Robbins,
2008). Sapsucker damage is more common on large landscape trees. It
can be distinguished from trunk-boring insect damage by the many uniform rows of sapsucker holes forming rings around the trunk and large
branches (Figure 9), whereas borer damage occurs more randomly and
usually with much less frequency. Extensive sapsucker damage creates
entry points for disease-causing organisms and boring insects and generally weakens the plants, making plants more susceptible to other types of
stresses.
Management. Sapsuckers are protected by federal law and may not be
killed or harmed. Recurrent damage may be prevented by wrapping damaged areas with netting, burlap or similar materials. Sapsuckers may be
repelled by sound-making devices, visual deterrents (e.g. movement of
reflective strips or a fake owl) or coating the damaged area with a sticky
substance to discourage bird visitation.
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