Why Did My Leaves Turn Black?

Insect secretions of honeydew and the resulting sooty mold biofilm buildup are why your plant’s leaves turn black. The sooty mold has a mutualist relationship with sap-sucking insect pests that create favorable growth conditions for the fungus. Blocking photosynthesis is the only harm to the plant caused beside the unsightly appearance of this fungus. “On pecan (Carva illinoensis) these molds can reduce light penetration and photosynthesis by factors of from 25% to 98%,” according to Science Direct. “In addition, the darkening of the leaf surface can result in an increase in leaf temperature of 4°C.”

Plants whose leaves turn black include azaleas, camellias, citrus trees, crepe myrtles, gardenias, elms, hollies, maples, podocarpus, and mostly evergreen plants. Susceptible plants tend to have rigid leaves that hold up well to prolong infestations.

Aphids, scale, mealybugs, mites, scale, and whiteflies pierce and suck out nutrients from the host plant. They then secrete the sticky honeydew waste product that fungal spores attach to as the leaves turn black. While other plants, such as herbaceous perennials, can be affected by the same insects, they are more likely to die before the buildup of secondary black sooty mold.

sooty citrus mold
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There are two identified forms of sooty mold categorized as deciduous and permanent. “The deciduous growth form is characterized by a thin hyphal layer that peels away from its substratum in drier, cooler months,” according to Science Direct. “It is found predominantly in lowland tropical regions but is absent from rainforests. The deciduous form also occurs along the coast of the southeastern United States and commonly is found on citrus trees.”

While the type of sooty mold commonly encountered can dry up and flake off during cool and dry seasons, it’s most effectively removed by washing with soap and water. Soaking the leaves and hand washing will loosen the mold away. This can be also be done with insecticidal soap or horticultural oil that also treats the primary pests. However, horticultural oils should not be used during freezing or over 90F temperatures or drought-stressed plants. You might see mention of ‘dormant oils’, which are horticultural oils used to treat overwintering aphids, mites, scale, and other sap suckers’ eggs. Always read label directions making sure that the treatment suits your specific plant. And don’t use horticultural oil on azaleas, begonias, or succulents. 

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sooty citrus mold research

There is ongoing research into biocontrols of sooty citrus mold and the associated insects. Bacillus velezensis MT9 and Pseudomonas chlororaphis MT5. “These bacteria were shown to produce antibacterial and antifungal activities in the form of water-soluble molecules and were effective in vitro in inhibiting the growth of two fungal strains, Penicillium sp. YM1 and YM2, isolated from citrus soot,” according to researchers at the University of Salento, Via Monteroni in Italy. “Altogether these in vitro findings and pilot experiments with a limited set of plants indicate that Bacillus velezensis MT9 and Pseudomonas chlororaphis MT5 can effectively combat sooty mold possibly by acting on both the insects and the fungi that inhabit the black biofilm, paving the way for an innovative and sustainable tool to fight this disease.”

Beneficial insects such as Ladybugs, Lacewings, and some predator wasps are a long-term natural remedy for sap-sucking insects. However, maintaining a sufficient population can take time and effort. If you want to introduce them in your yard, consider available species from Arbico organics. Earth Works garden center has the organic treatments mentioned in this article available for purchase.

Earth Works Lawn Care provides Integrated Pest Management treatment programs based on what we see with pest populations throughout the community and your specific landscape.

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