Citrus

• Armillaria root rot
• Dothiorella gummosis
• Dry root rot
• Frost

•  
• Hendersonula tree and branch wilt
• Phytophthora spp.
  (gummosis and root rot)
• Sunburn

• Bud union disorder
• Cachexia viroid
• Exocortis
• Hyphoderma gummosis

•  
• Psorosis
• Tristeza virus
• Vein enation

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.

Phytophthora spp.
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.

Phytophthora gummosis
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.

Psorosis
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.

Exocortis
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.

Sunburn canker
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.

Frost
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 genetic incompatibility.

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 gum.

Dothiorella gummosis
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 attached.

Cachexia viroid
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.

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.