Plant Disease Management Handbook

Cause Phoradendron juniperinum and P. densum. A parasitic seed plant, mistletoe depends on its host for much of its water and mineral nutrients. Fruit can be toxic to humans and livestock, but birds eat and disperse them to new trees. P. juniperinum belongs to the same genus as the mistletoe that affects oak but will not grow on oak or other conifers. The species on juniper are principally leafless.

Cause 'Tam' (Juniper sabina 'Tamariscifola') and 'Pfitzer's' (Juniper chinensis 'Pfitzeriana') juniper often are deficient in magnesium, and other cultivars may be as well. Competition with calcium-rich growing medium or acid conditions can also produce magnesium deficiency when it is not limiting.

See:

Cedar, Western Red (Thuja plicata) - Leaf Blight (Keithia Blight)

Pine (Pinus spp.) - Lophodermium Needle Cast

See:

Fir, Douglas and True - Armillaria Root Rot

Note Armillaria root rot was found on a few samples sent to the OSU Plant Clinic in the mid to late 20th century.

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Boston Ivy (Parthenocissus tricuspidata)

Cause Two fungi have been reported on English ivy in Oregon and Washington: Colletotrichum trichellum, also known as anthracnose; and Phyllosticta concentrica. Rainy or wet conditions favor both fungi. Wounded leaves develop large spots when inoculated with anthracnose.

Cause Heat stress is the primary cause of leaf scorch. Drought stress and use of excess fertilizer can also cause a leaf scorch symptom.

Cause Xanthomonas hortorum pv. hederae (formerly Xanthomonas hederae pv. hederae), a bacterium spread by splashing water and thus favored by overhead irrigation. Dense foliage and closely spaced plants favors disease spread. Bacteria may enter the plant through either wounds or stomata and hydathodes. They survive in infected plant debris and in the soil.

Cause Iris mild mosaic virus (IMMV) and iris severe mosaic virus (ISMV) are problems for bulbous iris; iris fulva mosaic virus (IFMV) is a problem for rhizomatous irises. All are in the potyvirus group. Transmission by aphids is nonpersistent, however controlling aphid vectors has not been an effective control tactic. Bean yellow mosaic virus (BYMV), turnip mosaic virus (TuMV) and narcissus latent virus (NLV) have also been found. Less commonly found are cucumber mosaic virus, broad bean wilt virus, and tobacco ringspot virus.

Cause Botrytis convoluta, a fungus that overwinters as large, convoluted, black sclerotia in affected plant parts and in soil. Diseased rhizomes are the principal way by which the disease is disseminated. Frequently, rhizomes appear healthy but carry the fungus as latent infections.

Symptoms Plants fail to grow in spring. Rhizomes rot, and many black, convoluted sclerotia are on them.