Plant Disease Management Handbook

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Rhododendron - Witches' Broom

Whole leaves to small parts of leaves can be affected.

Jay W. Pscheidt, 2006.

The strange looking tissue in the center is galled leaf tissue.

Photo part of OSU Extension Plant Pathology Slide Set

Image related to Azalea (Rhododendron spp.)-Leaf and Flower Gall

Note the healthy leaves on top and the galled leaves in the middle.

Photo by Robin Rosetta, 2006.

Cause Cylindrocladium scoparium, (sexual Calonectria morganii), a fungus but other species can be involved. The disease has been a problem only in greenhouses. Microsclerotia that form in plant tissues can survive many years in soil or media. New infected cuttings from other facilities can also bring in inoculum. Cuttings may have small inconspicuous leaf spots. High humidity and high temperatures (85°F) favor fungus growth.

Image related to Azalea (Rhododendron spp.)-Cylindrocladium Blight and Root Rot

Plants on the right have rotted roots resulting in wilting.

Photo by Ralph S. Byther

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Azalea (Rhododendron spp.) - Ovulinia Petal Blight

Greenhouse Plants, Ornamental - Gray Mold

Small lesions on petals develop into the characteristic gray fungal growth within 48 hours in high humidity.

Jay W. Pscheidt, 2025.

Cause Candidatus Phytoplasma asteris formerly referred to as a mycoplasma-like organism. This organism has a wide host range. Some of the 170 plant species that may be affected are: anemone, calendula, chrysanthemum, coreopsis, cosmos, delphinium, daisy, marigold, gladiolus, golden-glow, petunia, phlox, and statice. Many vegetables also are affected including carrot, squash, and tomato. The main vector of this organism is the six-spotted leafhopper.

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Marigold (Tagetes spp.) - Fusarium Wilt

Fusarium sp. was found growing on these dead stem stubs.

OSU Extension Plant Pathology Slide Set, 1983.

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Greenhouse Plants, Ornamental - Impatiens Necrotic Spot

Symptoms Both Tomato spotted wilt and Impatiens necrotic spot can result in necrotic spots and rings on the foliage of aster. Chlorosis of tip growth has also been observed in plants with Impatiens necrotic spot.

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Greenhouse Plants, Ornamental - Shoot Proliferation and Leafy Gall

Cause Golovinomyces asterum var. asterum (formerly Erysiphe cichoracearum), a fungus that overwinters in plant debris such as old stems and leaves. This fungus is favored by conditions that produce high humidity but dry leaves. The Podosphaera xanthii complex has also been reported worldwide. These fungi are favored by conditions that produce high humidity but dry leaves.

Note the numerous white colonies on these leaves.

Photo by Neil Bell

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Pine (Pinus spp.) - Needle Rusts

This aster (Blue Henry I) had rust (Puccinia sp.).

OSU Plant Clinic Image, 2016.

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Greenhouse Plants, Ornamental - Gray Mold

Gray spores can be seen extending from an infected flower (Aster - Peter III). Discolored spots can be seen on the petal to the extreme right in this photo.

Jay W. Pscheidt, 2013.

Flowers turn brown and develop the brown sporulation of the fungus.

Jay W. Pscheidt, 2013.