Greasy Spot

Zasmidium citri-griseum

Greasy Spot is a major Florida citrus disease caused by Zasmidium citri-griseum, historically called Mycosphaerella citri. Upper leaf surfaces develop yellow mottling that corresponds to raised, dark brown, greasy-appearing lesions on the lower surface. Severe infection causes premature defoliation and can reduce yield and tree vigor; grapefruit and some sweet orange or tangelo types are especially affected. Fruit may develop greasy spot rind blotch, a superficial blemish distinct from foliar lesions. The fungus matures in decomposing fallen citrus leaves and releases airborne ascospores after wetting. Infection occurs mainly through lower-surface stomata, followed by a long latent period. Management therefore targets leaf-litter inoculum and timely protection before symptoms are obvious. Huanglongbing mottling, mites, melanose, nutritional chlorosis and spray deposits must be separated carefully.

Identification

Confirm citrus and compare upper and lower leaf surfaces. The paired sign is important: diffuse yellow or chlorotic spots above and raised brown-to-black greasy lesions below. Lesions may be more obvious when leaves are viewed obliquely. Fruit rind blotch appears as small dark specks that coalesce into irregular patches, often on grapefruit. Collect mature leaves showing both surfaces, recently fallen leaves from the same canopy and affected fruit if present. Laboratory diagnosis may use microscopy or molecular testing, especially when HLB, mite injury or another citrus disease overlaps. Greasy spot mottling is generally more lesion-centered and symmetrical through the leaf than HLB's irregular blotchy mottle, but mixed infections occur. Nutrient deficiencies tend to follow vein or canopy patterns without raised lower-surface lesions.

Symptoms / Signs

Early foliar infection is invisible. Months later, upper surfaces show yellow speckling while lower surfaces develop brown, slightly swollen, greasy-looking spots. Lesions enlarge and merge, leaves yellow and drop, and repeated defoliation thins the canopy. Rind blotch creates dark superficial fruit blemishes without the sandpaper-like raised specks typical of melanose. Mite feeding may bronze or stipple rind and foliage; HLB causes asymmetrical blotchy mottle and additional tree decline signs. Because latency is long, visible symptoms do not identify the exact recent infection date.

Life Cycle / Process

The fungus develops pseudothecia in fallen decomposing citrus leaves. Rain or irrigation wetting triggers ascospore release, and wind carries spores into the canopy. Spores germinate and enter primarily through stomata on the lower leaf surface. A long latent period separates infection from symptom expression, often spanning weeks to months. Warm humid Florida summers, frequent rainfall and accumulated leaf litter support inoculum. Secondary spread from visible leaf lesions is less important than ascospore production from litter. Rind blotch risk is associated with susceptible fruit and inoculum exposure during development.

Prevention

Reduce the reservoir of decomposing infected leaves beneath trees. In small plantings, collect litter where practical; in groves, follow UF/IFAS guidance for urea or approved litter-decomposition practices when appropriate. Maintain tree health and airflow, avoid unnecessary prolonged canopy wetness, and prevent transfer of leaf litter between nursery areas. Choose less susceptible cultivars when site and market permit. Do not rely on waiting for symptoms because the critical infection period precedes visible lesions.

Management / Control

Base control timing on current Florida citrus guidance, cultivar susceptibility, disease history and weather. Commercial programs often use oil, copper or systemic products at defined periods, but product choice, compatibility, phytotoxicity risk and resistance management must be checked against the current label and production guide. Treating after severe lesions appear mainly protects later foliage or fruit; it does not restore dropped leaves. Address HLB, root disease and nutrition separately because they influence canopy condition but are not cured by greasy-spot treatments. If rind lesions could represent citrus black spot, obtain qualified diagnosis before moving fruit or nursery material.

Procedure Steps

1. Identify citrus cultivar and whether the objective is leaf retention or rind quality. 2. Examine paired upper and lower surfaces on mature leaves. 3. Photograph canopy thinning, lower-surface raised lesions and any fruit blotch. 4. Collect attached symptomatic leaves, matching fallen leaves and fruit separately. 5. Confirm uncertain cases through a citrus diagnostic laboratory, especially when HLB or regulated disease is possible. 6. Quantify leaf-litter accumulation and drainage. 7. Apply approved litter-management practices where suitable. 8. Consult the current UF/IFAS production guide for treatment timing and compatibility. 9. Record rainfall, application dates and cultivar response. 10. Evaluate retention and symptoms on later flushes rather than expecting old lesions to disappear.

Safety Notes

Use gloves and eye protection when handling concentrated oils, copper, urea or other products, and prevent runoff to surface water. Horticultural oils can injure citrus under high temperatures or when combined too closely with incompatible products; follow label intervals. Observe worker-protection and harvest requirements. Do not diagnose HLB or a regulated citrus disease away because greasy spot is also present; mixed disorders are possible.