Plant Disease Management Handbook

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Boxwood (Buxus spp.) - Boxwood Blight

Image related to Boxwood (Buxus spp.)-Volutella Leaf and Stem Blight (Canker)

Twigs and leaves become tan, and leaves turn upward along the stem.

Photo by Ralph S. Byther

The sporodochia of this fungus can be seen on the underside of leaves on this Korean boxwood.

Melodie Putnam, 2008.

Branches die back and leaves turn from green to light green-yellow and finally to shades of tan.

OSU Plant Clinic Image, 2013.

Cause Phytophthora cinnamomi and P. parasitica, have been reported from the Eastern United States while P. citrophthora, P. cinnamomi and P. occultans have been found in Oregon nurseries.

Image related to Boxwood (Buxus spp.)-Phytophthora Root and Crown Rot

The cambium of this boxwood has been exposed using a pocket knife. Note the discolored areas on the lower portions of the main stems.

Photo by Carrie R. Foss

Not every plant looks the same in this hedge. The problem was diagnosed as Phytophthora root and crown rot.

Neil Bell, 2007.

When roots rot then the portion of the plant above dies.

Neil Bell, 2007.

Necrotic, discolored roots can been seen on these plants.

Melodie Putnam, 2007.

Standing or splashing water encourages disease development.

Jennifer Parke.

Cause Phyllosticta sp. and Dothiorella candollei (formerly Macrophoma candollei), fungi, have been found infrequently by OSU's Plant Clinic. The fungi survive on dead leaves under the shrub.

Necrotic marginal leaf necrosis due to Phoma sp.

OSU Plant Clinic Image, 2017.

The fungus Macrophoma candollei was found in these leaf spots along with small black fruiting bodies.

Kathy Merifield.

Cause Boxwood blight caused by the fungus Calonectria pseudonaviculata (synonyms Cylindrocladium pseudonaviculatum or C. buxicola) was found for the first time late in 2011 in three Oregon and four British Columbia nurseries.

Spots on both leaves and stems can be seen in this image.

Luisa Santamaria, 2012.

A yellow or orange halo around leaf spots may also be observed as you can see on this 'Dee Runk' plant.

Jerry Weiland, 2019.

Whole leaves and stems can become blighted as on this 'Green Velvet' boxwood.

OSU Plant Clinic Image, 2013.

Dark or light brown spots on leaves, often in a circular or zonate pattern.

OSU Plant Clinic Image, 2014.

Bowood plant with green leaves upper canopy, dead leaves lower canopy.

Lesions of various shapes on the leaves.

Karl Puls, Oregon Department of Agriculture, 2011.

Stem on the right has easy to see stem lesions and defoliation.

Karl Puls, Oregon Department of Agriculture, 2012.

Defoliation of lower branches.

Karl Puls, Oregon Department of Agriculture, 2011.

Boxwood blight infected plant is on the right while Phytophthora infected plant is on the left. Phytophthora infected plant still has many leaves attached to stem while the plant with boxwood blight is almost completely defoliated.

Jay W. Pscheidt (thanks to Jerry Weiland), 2019.

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Greenhouse Plants, Ornamental - Black Root Rot

Note Out of over a hundred samples there have been 5 that were diagnosed with black root rot by the OSU Plant Clinic.

The fungus can be found in the black root sections of this Buxus sempervirens 'Suffruticosa' bush.

OSU Plant Clinic image, 2012.

Above ground symptoms of discolored yellow leaves. (Marginal yellowing is normal for this cultivar.)

OSU Plant Clinic, 2020.

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Grape (Vitis spp.) - Powdery Mildew

Image related to Boston Ivy (Parthenocissus tricuspidata)-Powdery Mildew

Powdery mildew on the closely related Virginia creeper.

Photo by Jay W. Pscheidt, 2002

Cause The fungus Phyllosticta ampelicida (formerly Guignardia bidwellii) has been found in Oregon. This is a slightly different form of the fungus that causes black rot of grape, which is not common in the PNW. Note that Phyllosticta parthenocissi would be the name of the pathogen on Virginia Creeper (P. quinquefolia).

Small black fruiting bodies (pycnidia) of the fungus can be seen in the center of the necrotic lesions.

Jay W, Pscheidt, 2008.

Numerous spots can occur on leaves.

Jay W, Pscheidt, 2008.

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Ivy (Hedera helix) - Leaf Scorch

Image related to Boston Ivy (Parthenocissus tricuspidata)-Leaf Scorch

Vines on the left side are showing heat stress due to high temperatures and lack of water. Notice the leaves with necrotic edges.

Photo by Jay W. Pscheidt, 2002

It is common to find leaves with dark to black areas on the upper exposed side of the leaf. The lower middle leaf has the same discoloration on upper leaf surface but none on the lower leaf surface. This can also be a symptom of ozone damage.

Photo by Jay W. Pscheidt, 2002

See:

Grape (Vitis spp.) - Downy Mildew