Holly bushes, including Japanese and American holly, are prone to developing several fungal diseases which contribute to the development of blackened areas on the holly bush. Many of these diseases can be controlled with fungicide applications or by planting holly bushes in their ideal growing locations. Gardeners should check holly frequently for signs of disease, as it is easier to treat conditions before they spread.
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There are three common root diseases which affect ornamental holly bushes -- black root rot, pythium root rot and phytophthora root rot. Black root rot is caused by the fungus Thielaviopsis basicola. The first visible symptom of a black root rot infection is yellowing leaves, dropping leaves early in the growing season and a failure to grow. Holly growers should examine the roots for black fungus and root tissue, which indicates the fungus is killing the roots of the holly bush. Japanese, blue and meserve holly are most susceptible to black root rot, according to the Alabama Cooperative Extension System (ACES). Pythium root rot and phytophthora root rot feature similar symptoms to the black root rot disease. Brown to black rings are likely to develop on the trunk and twigs of holly bushes infected with pythium and phytophthora root rot.
Holly bushes are also susceptible to developing fungal leaf spots, which may cause black, necrotic tissue on the upper surface of the leaves. Cylindrocladium leaf spot causes leaves to develop tiny yellow spots. As the disease progresses, spots expand, become brown and develop a black border. Web blight is another fungal leaf spot disease which causes the infected holly bush to develop brown patches. Patches then expand into irregular black blotches on the leaf surface. Hot, wet weather contributes to the development of the web blight fungi.
Holly plants are able to fend off many fungal diseases when planted in ideal locations. Holly prefers loose, loamy soils, according to Fort Valley State University. Wet or boggy areas are not recommended for planting holly, as excessive water helps fungal spores develop. Transplant holly from one location to another from October to February, during the dormant season. Plant bushes 1 to 2 feet apart to allow for proper air circulation between the bushes, which lessens the likelihood of fungal disease development.
In addition to planting holly in well-drained soil with adequate air circulation, several fungicides can help prevent fungal diseases or slow the spread of disease already present in holly bushes. To treat black root rot, thiophanate-methyl and etridiazole + thiophanate-methyl fungicides can be applied to soil on a monthly basis. Also, thoroughly wash any potting material, as fungi often develop in plastic nursery pots. To treat pythium root rot and phytophthora root rot, apply etridiazole, etridiazole + thiophanate-methyl or fosethyl-AL in a trench around the soil of planted holly bushes. Cylindrocladium leaf spot can be treated with an application of thiophanate-methyl sprayed onto the leaf surfaces every two weeks through the growing season. Web blight also responds well to a topical application of fungicide spray. Some recommended fungicides include chlorothalonil, iprodione and thiophanate-methyl.
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