Dense and easy to grow, Japanese hollies (Ilex crenata) are slow-growing, evergreen shrubs used for foundation plantings and hedges. They tolerate shearing well, making them good substitutes for boxwoods. Hardy in U.S. Department of Agriculture plant zones 5 through 8, Japanese hollies have few serious diseases, according the Alabama Cooperative Extension System's website, although they are occasionally bothered by leaf spots or root rot. These problems can be easily controlled by homeowners.
- Skill level:
- Moderately Easy
Things you need
- Magnifying glass
- Pruners or loppers
- Alcohol or bleach
Look for circular lesions on the leaves, a sign of fungal disease. One common type, Cylindrocladium leaf spot, starts as tiny yellow spots that enlarge into tan or brown spots with a purple-black border. You may need a magnifying glass to see the initial yellow spots. Clean away infected leaves and remove dead shoots. Spray previously damaged plants with a fungicide approved for the control of this disease starting in early spring and repeat according to package directions until new growth stops.
Inspect leaf margins and bases for brown spots, an indication of web blight. These spots spread rapidly into black or brown blotches that cover the entire leaf. Dead leaves usually cling to the shrub rather than fall to the ground, and the disease spreads upward from the bottom of the shrub into the crown. Treat with a fungicide.
Check shrubs with yellowed, dropping leaves for cracked or shrunken patches of bark, which may be discoloured. The stem tissue above and below the canker also turns brown. Prune infected branches back to healthy, green wood. Fungicide is unnecessary, as this disease, Botryosphaeria canker, is caused by plant stress or mechanical injury. Prevent infections by watering plants well during hot weather and mulching to prevent lawnmower injury.
Look for swelling on young twigs and knobbly galls on large branches, signs of Sphaeropsis knot disease, which occurs most often in Florida. Other symptoms include clusters of leafless shoots and dieback of defoliated branches. Prune out diseased branches below the swollen part of the stem. Fungicides are not effective against this disease, so inspect new plants carefully before buying them.
Inspect potted hollies carefully before purchase for signs of Theilaviopsis black root rot. Look for yellowed leaves, twig dieback and black bands or rings on the white feeder roots of the plant. In the landscape, remove and discard affected plants. Fungicide is not effective in the landscape.
Check the bottom of plants and their root systems for Pythium or Phytophthora root rot if you see yellowing leaves at the tips of stems or twig dieback or if stems wilt and die. Infected stems have brown or black streaks that extend up from the roots. Roots turn dark and die away. Fungicide is not effective, and infected plants must be removed and destroyed.
Test the soil if you see yellow or white leaves with dark veins, a sign of iron chlorosis. A cultural problem, not a disease, it is easily confused with fungal diseases. Iron chlorosis is caused when the pH of the soil is too high, making iron in the soil unavailable to the plant's roots. After getting the results of your soil test, consult with your local extension service or garden centre for appropriate additives for your area to correct the pH.
Tips and warnings
- Fungal diseases such as leaf spots occur most often during extended periods of wet or humid weather. Space hollies far enough apart to ensure good air circulation and avoid getting water on the leaves by irrigating at the base of the plant.
- Hollies need good drainage to protect them from root rot diseases. Always plant hollies "high," with the rootball slightly above the soil level, and avoid siting them in spots that stay wet, such as near drain spouts.
- Always disinfect your pruners or loppers between cuts with alcohol or a solution of one part bleach to nine parts water. This helps control the spread of disease.
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