Holly (Ilex)
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.
Insect Problems:
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.