Plant Health Problems
Diseases caused by Physiological/Environmental Factors:
Purple spot or Spine spot.
Mechanical injury to holly leaves causes a purplish discoloration immediately surrounding the injured area. Wind-whipping in winter may drive the leaf points into other leaves. The problem may be reduced by planting holly in a sheltered area, out of severe winter winds. A similar injury may be produced by the holly leafminer.
Holly leaf miner, Phytomyza ilicis.
The adult is a small, black fly about 1/16" long. It emerges in late May or during June. Eggs are deposited in the underside of the leaves. Larvae make mines that broaden as the insects develop. There is one generation a year. Remove and destroy affected leaves in April. Spinosad may be sprayed to control larvae in leaves. Imidacloprid, which is among the compounds registered for control of this pest in Connecticut, applied as a soil drench in mid-May will provide season-long, systemic control. Consult the labels for dosage rates and safety precautions.
Southern red mites, Oligonychus ilicis.
Southern red mite can be a problem on several species of holly. This mite overwinters as reddish eggs on the under-surface of leaves. Adults and nymphs feed on both the lower and upper leaf surfaces. The oval-shaped adults are normally red, but can be green with lighter colored legs. Multiple generations occur each season. If not controlled in the spring, populations will rise again in the fall. Some damaged leaves drop. Among the compounds registered for control of this pest in Connecticut are insecticidal soap and ultrafine horticultural oil. Spraying with insecticidal soap will give good control if applied at least twice at 7-10 day intervals. The predatory mite, Neoseiulus fallacis, is most commonly found feeding where there are mite infestations. A single application of ultrafine horticultural oil (1/2 - 1% dilution) can be effective if predatory mites are present. Consult the labels for dosage rates and safety precautions. Special care should be taken with soap or oil to obtain thorough spray coverage, because they only work on contact. Hexythiazox or abamectin are effective restricted use products. Avoid applying carbaryl or pyrethroids, which tend to be much more toxic to beneficial predatory mites than to the pest mites.