Plant Disease Management Handbook

Cause Phytophthora ilicis, a fungus-like microorganism. The disease develops in cool, rainy weather of fall and winter and subsides in warm, dry summer months. It may be very severe in dense orchards where air circulation is poor. In some orchards on the Oregon coast, the disease has caused almost complete defoliation and severe twig blighting, particularly on lower limbs.

Image related to Holly (Ilex spp.)-Phytophthora Leaf and Twig Blight

Fruit, which turn a dark color, can also be infected.

OSU Extension Plant Pathology Slide Set, 1962.

Notice the defoliation, shoot dieback and necrotic berries.

Photo by Jay W. Pscheidt, 1994.

Japanese holly plants that tested positive with ELISA for Phytophthora (the twigs).

Lyle J. Feilmeier, Collier Arbor Care, 2006.

Splashing water from a roadway has encouraged severe defoliation from leaf and twig blight.

Robin Rosetta, 2007.

Defoliation and twig dieback are common.

Jay W. Pscheidt, 1990s.

See:

Holly (Ilex spp.) - Scab

Holly leaf spotting due to Trochila ilicina.

OSU Extension Plant Pathology slide collection.

Cause Green algae. This elementary plant grows on leaves, twigs, and trunks in wet or humid weather. Algal growth on conifer needles is a response to high nitrogen deposition and may be the same for holly.

Symptoms Yellow-green powdery layer on leaf surfaces, twigs, and trunk, usually appearing first on the north side of the lower trunk and gradually spreading up and out to twigs and leaves.

Dull, green, algal colonies develop on leaves.

Photo by Jay W. Pscheidt, 1993.

See:

Holly (Ilex spp.) - Red Leaf Spotting

Cause Many commercial holly plantings have incipient boron deficiency. PNW soils west of the cascades are naturally low in boron. Many crops in these areas need supplemental boron.

See:

Hemp (Cannabis sativa) - Fusarium Wilt

Cause The fungus, Rhizoctonia solani, which can survive in soil for at least several years. The fungus has a broad host range and can be spread by movement of infected plant debris. Disease is favored by warmer soil temperatures.

See:

Hop (Humulus lupulus) - Powdery Mildew

By C. M. Ocamb

A portion of a hemp plant infected with Golovinomyces ambrosiae, the powdery mildew fungus that typically occurs on hemp.

Photo by C.M. Ocamb, Oregon State University, 2021.

Hemp leaf showing the small, mostly diffuse, powdery, white colonies that are characteristic of Golovinomyces sp.

Photo by Cynthia M. Ocamb, Oregon State University, 2020.

A hemp branch infected with Podosphaeria macularis, the fungus that causes powdery mildew on hop plants. These powdery mildew colonies typically exhibit a more dense hyphal growth and sporulation than colonies incited by Golovinomyces spp.

Photo by Taylor A. Bates, Oregon State University, 2020.

Cause Phytophthora cinnamomi, P. cactorum, P. cryptogea, and P. drechsleri, fungus-like microorganisms found under wet conditions such as prolonged periods of soil saturation. Generally called water molds because they produce a swimming spore stage (zoospore) that is attracted to roots.