Download Holly Leaf and Twig Blight

Holly Leaf and Twig Blight
Identification, Biology and Management
Drew Zwart—Plant Protection Specialist
Twig blight on Holly (Ilex spp.) is a fairly common disease in the
Pacific coastal region of Canada and the northwestern United States.
This disease is caused by a fungus-like organism, Phytophthora ilicis.
There are several well known pathogens in the genus Phytophthora,
many of which cause root rots or trunk cankers, including the ‘Sudden
Oak Death’ pathogen, P. ramorum. Phytophthora species in general
require free water to complete their infection cycle, and this is an
important factor in any Phytophthora disease situation.
SymptomsSymptoms of holly twig blight normally begin as small purple to black
spots on the leaves, or necrotic bunches of berries. As the disease
progresses, the spots develop into purple or black blotches and the
infection will also move into the twigs, causing dieback, defoliation,
and cankers. The disease is favored by cool temperatures and moist
weather, and often the symptoms will subside in the summer months.
Inoculum persists in soil, and also is produced on the cankered
areas of twigs. During wet and cool spring or fall weather,
spores are produced which splash onto leaves, penetrate the
tissue, and cause the initial leaf spot symptoms. Often the
symptoms will be worst on the lowest leaves and branches
where inoculum has splashed onto foliage.
TreatmentsThere are several simple cultural methods which can dramatically reduce the severity of this
disease. Pruning plants to increase air circulation and light penetration will help to avoid wet
leaves, which are essential for infection. Also, overhead irrigation that wets foliage should be
avoided. If the affected holly is tree-form, the lowest branches can be pruned in order to remove
the possibility of inoculum splashing up from the soil below. This pathogen does not readily
penetrate bark but will infect leaves.
In addition to cultural control, there is an effective chemical control option. When applied as a
soil drench, this option has proven to be effective against the fungus-like organism and will stop
spread of the disease; however, it will not eradicate the pathogen from the soil. Conditions that
are conducive to disease development must be corrected before discontinuation of treatments in
order to prevent this disease long term.