Diseases of the Magnolia Tree
The evergreen southern magnolia (Magnolia grandiflora) grows wild along streams and riverbeds, to a height of 80 feet. The Magnolia genus has more than 100 species in the Americas and Asia. Magnolia was first collected in Martinique in 1703, and named for Pierre Magnol (1638-1715). Magnol was a physician and pharmacology professor at the gardens of the University of Montpelier, France. Magnol, who lived a generation before Linnaeus, invented the concept of plant families. Does this Spark an idea?
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Health
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Established, healthy magnolia trees do not usually sustain long-term damage from pests. Magnolias should be planted in rich, sandy, acidic (pH 5.0 to 6.5), well-drained soils in full sun or partial shade. In alkaline or heavy, poorly drained soils, magnolias do well for a few years and then decline. Planting pit soil should be half leaf mold. Magnolia wood is soft and can be damaged by mowers and weed-eaters. Magnolia trees shed leaves in spring. Yellow leaves falling in March and April are not a sign of disease.
Algal Leaf Spot
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Algal leaf spot (Cephaleuros virescens) is parasite that first appears as a green, circular spec with wavy edges, and turns red as spores develop. When a nonparasitic fungus colonizes the alga, the specs turn gray. In magnolias, algal leaf spot infects both leaf and twig. Good hygiene is the best defense. Remove infected leaves; rake and burn fallen leaves. Set lawn sprinklers at a distance, and prune overhanging trees to encourage evaporation. Apply Bordeaux mixture (copper sulfate and hydrated lime) at 8 tsp. per gallon every other week.
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Weevils
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Yellow poplar weevils (aka sassafras weevils, magnolia weevils) are unsightly, but will not affect the long-term health of an established tree. Adults over-winter among leaf litter, emerge in mid-spring and feed on buds and the undersides of leaves. 1/8 inch holes in leaves are the first indicator. In June, larvae mine the leaf from the midrib toward the edge, giving the leaf a brown, scorched appearance. They pupate in a silk cocoon. For organic control, remove and burn leaf litter to eliminate their winter habitat. For chemical control, apply acephate, carbaryl or chlorpyrifos after 10 percent of leaves appear damaged.
Wax Scale
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Female magnolia wax scale (Neolecanium cornuparvum) grows up to 1/2 inch diameter in a convex elliptical shape. Mature scale ranges pinkish-orange to brown, armored with white wax. Scale over-winters on young twigs as a dark gray nymph with brown median ridge (tendon), reaching maturity in July. Males are short-lived flying insects. Damaged twigs are swollen and purple. The scale secretes sticky honeydew colonized by black mold. Introduce ladybugs for organic management. Apply dormant horticultural oil (unsulfonated oil such as Ferti-lome Dormant Spray) BEFORE leaf buds open to avoid clogging stomata and lenticels. Chemical pesticides in late summer control the newly hatched "crawlers."
False Oleander Scale
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False oleander scale (Pseudaulacaspis cockerelli, magnolia white scale) was misidentified as oleander scale (Phenacaspis cockerelli) when introduced to California on palms imported from China in 1897. The life cycle is 30 days. Tiny yellow spots appear on the upper surface of leaves where the scale feeds. Females are pear-shaped and shiny white. Apply dormant horticultural oil before leaf buds open. Manage with introduced ladybugs. Two wasps, Arrhenophagus albipes and citrinus, lay eggs in immobile mature scale. Chemical control requires applications every six weeks. Dead scale remains attached by their sucking mouthparts, but will decompose to dry flakes.
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