I had a Nandina that I decided to remove today after this winter’s freeze. While digging it up I noticed that the root tissue was bright yellow. The roots seemed fine otherwise–no odd smell, not mushy. Is this color normal for Nandina, or is this a sign of a virus? I lost another Nandina to a mysterious disease that looked like a mosaic virus, but that plant was nowhere near the one I just removed. I planted a Grevillea victoriae in that spot, assuming that diseases affecting Nandina would not affect it. Is this correct?
The yellow roots of your Nandina are normal. Nandina is in the family Berberidaceae, along with Mahonia and Berberis, which also have yellow, fleshy roots. Here is more information about Nandinas:
Clemson University page on Nandina
Nandina sometimes suffers from powdery mildew, which is usually not serious, and there is information about this plant occasionally suffering from a mosaic virus and from anthracnose. Here is information about the diseases occasionally affecting Nandina.
Image of Nandina leaf spotting from University of California, Davis Integrated Pest Management
Grevillea is in the Proteaceae family, and I have not found any information that suggests it would be either vulnerable or immune to the diseases which affect Nandina. Some plant diseases affect multiple plant families and others are more narrowly focused. In their native Australia, Grevilleas are not known to suffer from diseases. In our climate, I imagine that cultural and weather conditions would be of greater concern than diseases.
Gardening Australia factsheet
Oregon State University Landscape Plants database information about Grevillea victoriae, excerpted here:
“Sun to partial shade (best). Well-drained, sites, may need occasional summer water to prevent bud drop. Fertilize lightly, avoid fertilizers with a high content of phosphorus.”