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Armillaria root rot
Identification tip: Pale and wilted foliage,
few leaves, and limb dieback are symptoms of Armillaria
root rot, dry root rot, Dothiorella gummosis, and Phytophthora
root rot. Look for cankers and oozing gum, cut under bark
to inspect cambium and wood, and examine roots to help
diagnose the cause.
Phytophthora root rot
Identification tip: Causes of leafless, dead
branches include citrus red scale, inappropriate irrigation,
fungi, and Tristeza virus. When Phytophthora citrophthora or
certain other fungi are the cause, bark may exude resin.
Dry root rot
Identification tip: Before tipping over, this tree exhibited pale
foliage and an unusually heavy crop of lemons. A girdling canker on the lower
trunk (not shown here) and the absence of any oozing gum are other indications
that the cause is infection by Fusarium solani.
oozing, cracking, peeling or distorted growth
Identification tip: Cracked, dry bark on the
lower trunk may be due to Dothiorella gummosis, Exocortis,
Hendersonula tree and branch wilt, Psorosis, or infection
by Phytophthora spp. Look for other symptoms, such
as discoloration beneath cankered bark and the presence
of oozing gum, and have samples tested by a laboratory
to help you diagnose the cause.
Identification tip: Phytophthora gummosis
is the most common cause of profuse dark exudate from bark.
Dothiorella gummosis, Hendersonula tree and branch wilt,
and Psorosis also produce gum. But sometimes there is no
obvious oozing when these diseases are present.
Identification tip: Psorosis is due to a viral
infection that causes a scaling and flaking of bark on
the scion. It cracks and peels bark high up the tree, but
Psorosis does not cause symptoms below the graft.
Photograph not available.
Hendersonula tree and branch wilt
Identification tip: Infection by Nattrassia
mangiferae (=Hendersonula toruloidea) causes
bark cracking and peeling or dead bark that remains
tightly attached to dead limbs. Black, sooty growth
may develop beneath infected bark; injured limbs may
bleed profusely; leaves on infected limbs suddenly
wither, turn brown, and dry up. Dead leaves typically
remain attached to the twigs.
Identification tip: Cracked, bark that peels off (bark shelling)
in small pieces, but only on old trees, is characteristic of Exocortis. Damage
is limited to around the root crown. In comparison, Phytophthora gummosis affects
trees of any age and damage often extends from the soil to several feet up the
trunk. If Hendersonula tree and branch wilt is the cause, bark cracking can occur
even higher on the trunk and also on limbs.
Vein enation (woody gall)
Identification tip: The cause of this gnarled bark on trunks is
unknown. Small bumps also develop on leaves (vein enation). An a aphid-vectored
virus is one suspect. This malady is rare as it is eliminated during propagation.
wood or cankers beneath bark—Top
Identification tip: Sunburn cankers are limited to outer branches
exposed to direct sunlight, usually in the south or west portion of trees. Mechanical
injury cankers can occur at any location where bark is impacted by equipment
or tools. The location of pathogen cankers does not depend on sun exposure.
Identification tip: Bark cankers and limb
dieback may not appear until weeks after cold weather.
More immediate symptoms include fruit drop and brown
dead leaves that remain attached, causing cold-damaged
trees to appear scorched.
Bud union disorder
Identification tip: Cut-away bark (the two pale squares) reveals
a dark horizontal line paralleling the uneven growth where the rootstock and
scion meet. This "crease" at the bud union is a delayed symptom of
Dry root rot
Identification tip: Cut under bark where
the lower trunk is sunken and discolored. Wood infected
by Fusarium solani will be dark and discolored,
in contrast to the healthy greenish white wood, as
shown adjacent here. Dry root rot's discoloration
extends deeply into wood. It does not produce oozing
Identification tip: Cutting underneath to expose the inner bark
and cambium reveals discolored, yellowish brown wood. Unlike dry root rot, Dothiorella
discoloring is lighter and infected bark may ooze dark liquid. On the surface,
Dothiorella cankers can have a grayish cast with dead bark that remains tightly
Identification tip: Pits in wood and bark, and brown discoloration
and gumming in the phloem underneath pits, are symptoms of Cachexia. This disease
is rare as the viroid is eliminated during propagation.
or fungal mycelia
Armillaria root rot mushrooms
Identification tip: During the rainy fall
and winter, short-lived mushrooms often grow around the
base of Armillaria-infected trees, such as this almond.
Armillaria root rot mycelia
Identification tip: The most reliable sign of Armillaria root rot
is large white fan-shaped mycelia plaques growing beneath bark. When the entire
tree aboveground declines, exposing the root crown and cutting under bark may
reveal Armillaria mycelium.
Photograph not available.
Hyphoderma gummosis fruiting body
Identification tip: Pink to white fungal
growth of Hyphoderma sambuci appears around
wounds after wet weather. Reported in the field only
on lemon, this wood decay fungus causes branch wilting
and dieback that ultimately results in tree death.