Plant Disease Management Handbook

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Radish (Raphanus sativus) - Black Root

Cause Aphanomyces raphani, a fungus-like microorganism that overwinters in soil and on diseased plant residue in soil. It invades the fine rootlets of young seedlings and develops resting spores in large numbers within the roots.

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Seed Crop, Crucifers (Brassica, Eruca, and Raphanus spp.) - Black Leg

By C. M. Ocamb

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Lawn and Turf

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Maple (Acer spp.) - Verticillium Wilt

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Garlic (Allium sativum) - Blue Mold Rot

Iris, Bulbous (Iris spp.) - Blue Mold

Cause Rhizoctonia tuliparum, a fungus that survives in the soil for at least 3 to 5 years as sclerotia. After 10 years there may still be 10% viable sclerotia. Sclerotia germinate and infect bulbs after planting or in the very early spring. Can be transported on diseased bulbs. Volunteer plants from previous bulb crops may help fungus survive from year to year. Primarily a problem on tulip and iris but daffodil, crocus, gladiolus, and lily are also possible hosts.

Cause Fusarium oxysporum f. sp. tulipae, a fungus that lives in soil and on plant debris. The disease became a problem with shorter rotations that favored buildup of inoculum. The disease is widespread but not as common or severe as Fire (Botrytis blight); it is worse on some tulip cultivars and occurs primarily in storage. Symptoms may be more severe at higher (61°F) than lower (54°F) temperatures.

Cause Botrytis tulipae, a fungus that overwinters as sclerotia in soil, affected bulbs, and diseased plant residue. Germination of sclerotia and infection of emerging shoots is greatest when soil is 41°F. Flowers are very susceptible to this fungus. Flower petals that drop onto leaves can promote further development on the leaves, especially in the leaf axils and on stems. High humidity, rainfall, and moderate temperatures favor the blight phase of this disease.

Cause Sclerotium delphinii, a fungus that can survive many years in soil as sclerotia. Infection usually occurs on the stems near the soil surface. A low percentage of sclerotia survived after only a month in the soil when dried first. The disease is widespread but not as common or severe as Fire (Botrytis blight), yet it can cause serious damage, especially in moist, warm soils. In the Pacific Northwest, crown rot is more common on bulbous iris, tulips and lilies than daffodils.

Cause Break is a disease name for any of several viruses including tulip breaking virus, tulip band-breaking virus, tulip top-breaking virus, Rembrandt tulip-breaking virus and lily mottle virus. Each is in the potyvirus group and is long flexuous rods. These viruses are spread by several species of aphids in a nonpersistent manner. They can also be mechanically transmitted in plant sap. Viruses survive in infected bulbs.