Citrus Melanose

Diaporthe citri

Citrus Melanose is caused by Diaporthe citri and is especially relevant to Florida citrus. It produces superficial, raised, dark brown to black corky lesions on leaves, twigs and fruit. Individual specks feel like sandpaper; splash patterns may form tear-stain streaks, and dense infection on young fruit can create mudcake melanose. The lesions usually remain cosmetic on fruit, but severe foliar or twig infection can reduce nursery quality. The fungus reproduces abundantly in recently dead citrus twigs, so inoculum is tied to canopy dieback, freeze injury, pruning debris and declining trees. Rain releases conidia that infect young susceptible tissue. Existing lesions cannot be removed by treatment; management focuses on deadwood removal and protecting young fruit or foliage during wet periods. Citrus black spot, greasy spot rind blotch, rust mite feeding, wind scar and copper injury require exclusion.

Identification

Confirm citrus as the host and preserve lesion texture. Raised, small, dark corky specks that do not rub off are typical. On fruit, note whether lesions are scattered, aligned in tear stains below dead twigs, or merged into rough mudcake patches. Melanose lesions generally lack the expanding necrotic margins and pycnidia pattern of citrus black spot. Collect symptomatic young leaves or fruit together with recently dead twigs from above or within the affected canopy. Pycnidia may be visible in dead wood, and laboratory examination can recover Diaporthe structures, but Diaporthe species are diverse and molecular confirmation may be needed in unusual hosts or research contexts. Do not diagnose melanose on noncitrus plants from appearance alone. Record fruit age at likely infection, pruning history, freeze or storm injury, and rainfall because lesions may become visible after the susceptible period has passed.

Symptoms / Signs

Young leaves develop small dark raised dots that may distort tissue if infection occurs during expansion. Fruit symptoms range from isolated black specks to tear-stain streaks and coarse mudcake patches. Lesions are superficial and usually do not extend deeply into the rind. Twigs may carry spots, but the epidemiologically important structures develop in dead wood. Symptoms can be confused with citrus black spot, which has several lesion types and quarantine importance; greasy spot rind blotch, which is flatter and associated with greasy spot epidemiology; citrus rust mite injury; wind scar; scale insects; and copper phytotoxicity. A rough sandpaper texture and relationship to dead twigs and splash are strong clues, not definitive proof.

Life Cycle / Process

D. citri colonizes dead citrus twigs and forms pycnidia. During rain or overhead irrigation, pycnidia exude masses of conidia that splash short distances to susceptible leaves, fruit and green twigs. Tissue is most susceptible while young; fruit susceptibility declines as the rind matures. Warm wet weather and a reservoir of dead twigs create repeated infection opportunities. The organism may persist saprophytically in dead wood even when visible disease is low. Because symptoms appear after infection, removing dead twigs after lesions develop protects future flushes but cannot change the current rind. Florida citrus affected by HLB, freeze or storm damage may carry more deadwood and therefore greater inoculum.

Prevention

Maintain canopy health and remove dead twigs before major periods of fruit susceptibility. Prune during dry weather, remove cut material from beneath trees, and avoid leaving piles of dead citrus wood near nursery or grove blocks. Reduce unnecessary overhead irrigation and time irrigation to minimize prolonged wetness. Use clean nursery stock and separate young citrus from declining trees that shed inoculum. Manage nutritional and root-health problems that contribute to twig dieback. In small plantings, sanitation may provide more value than routine spraying when fruit appearance is not economically critical.

Management / Control

Assess whether the objective is protecting fruit appearance, nursery foliage or tree health. Prune deadwood first and correct sources of canopy decline. Where commercial fruit protection is warranted, follow the current UF/IFAS citrus production guide and product labels for timing around young fruit and rainfall; copper products are commonly used, but rates and timing must account for fruit injury risk, weather and other citrus programs. Do not apply unregistered products or assume a home-landscape schedule fits commercial citrus. Existing corky lesions are permanent. If citrus black spot is a plausible alternative, stop movement of suspect material and obtain qualified diagnosis because regulatory consequences differ sharply.

Procedure Steps

1. Confirm citrus species, cultivar, tree age and whether fruit or nursery appearance is the concern. 2. Photograph lesion texture and distribution, including tear-stain patterns below dead twigs. 3. Inspect the canopy for recent dieback, pruning wounds and dead twig density. 4. Collect unwashed symptomatic fruit or leaves and separate dead twigs bearing possible pycnidia. 5. Submit suspect regulatory look-alikes to an appropriate citrus diagnostic service. 6. Prune and remove dead twigs in dry weather, disinfecting tools when other diseases are possible. 7. Adjust irrigation to reduce splash and extended wetness. 8. Review current UF/IFAS timing guidance before any protectant application. 9. Record rainfall and fruit developmental stage. 10. Judge control by lesions on later flushes or fruit, not by unchanged old scars.

Safety Notes

Use eye and hand protection when pruning citrus and handling copper or other labeled products. Observe restricted-entry and preharvest intervals. Copper can injure plants and accumulate in soil, so never exceed label rates or apply solely to erase existing blemishes. Do not move suspect fruit, leaves or nursery stock when citrus black spot or another regulated disease has not been ruled out. Dispose of dead citrus wood so it does not remain an inoculum reservoir.