More than 600 species of holly are in the plant genus Ilex, including Japanese holly (Ilex crenata), a popular ornamental. Hollies are especially susceptible to black root rot caused by the fungus Thielaviopsis basicola that appeared in the late 1970s. To a lesser extent, hollies are subject to Phythophthora root rot caused by the water mold fungus Phythophthora cinnamomi.
Black Root Rot Symptoms
The fungus Thielaviopsis basicola mainly infects the feeder roots. Starting at the tips, infected roots turn dark brown or black. Above ground, the distance between the stem nodes that produce buds may be shortened. The leaves may turn yellow between the veins. Plants suffering from extensive root rot lose vigor and become stunted. Hot dry periods in the summer may kill them.
If a Japanese holly shows symptoms on its leaves, its roots have already suffered extensive damage; the plant should be removed.
Black Root Rot Causes
Black root rot in hollies is usually confined to poorly drained soil in nurseries, not in landscape plantings. Its spores can linger in the soil for years. Plant pathologists at Virginia Tech University recommend that nurseries grow susceptible hollies in new containers containing soil-less pine bark. Containers that are recycled should be rinsed in a 10 percent solution of bleach to kill surviving spores.
Black Root Rot Treatment
There is no known cure for plants infected by black root rot. Virginia Tech plant pathologists recommend preventive fungicide treatments only for newly planted hollies and those in nurseries. Under those conditions, the Alabama Cooperative Extension Service recommends preventive applications of fungicides containing the active ingredients thiophanate-methyl, etridiazole plus thiophanate-methyl, or metalaxyl plus thiophanate-methyl.
Phytophthora Root Rot Symptoms
The symptoms of Phytophthora root rot are similar to that of black root rot. The leaves turn yellow, especially at the tips of shoots; they drop early. Infected plants grow slowly and twigs die. Limbs may later wilt and die back to the trunk; a brown to black streak of dead tissue may appear from the rotted roots to the infected limb. The disintegration of roots may cause the plant to die.
Phytophthora root rot is caused by hollies that are planted in poorly drained, warm soil. They should be planted in well-drained soil or in raised beds. Stress contributes to infection by phythophthora fungus; only hollies adapted to local soil and climate should be planted.
Fungicides will not cure infected plants. Repeating applications of fungicides containing the active ingredients of etridiazole or mefenoxam may prevent infection.