Issue 3, May 8, 2009
Holly Leaf Discoloration
Holly species in Illinois are not typically bothered by many diseases. The leaves pictured were submitted recently to the University of Illinois Plant Clinic as a disease suspect. We did not find a pathogen associated with these leaves. The black blotches are most likely environmental in origin. We see blotching on holly leaves in winter, especially in heavily shaded locations. Scorch would appear as leaf edge burn. No fungicides were warranted. Hollies grow best in light, well-drained, slightly acidic soils, so action recommended was to try to maintain these conditions.
The foliar diseases found on holly include tar spot (Phacidium curtisii), Coniothyrium leaf spot (Coniothryium ilicinum), and Alternaria leaf spot. Sometimes hollies also have algal leaf spot. These pathogens cause distinct spots as compared to the general black blotching of the image. Although all occur in Illinois, we rarely see these diseases.
The light color of the foliage suggests a possible root problem or possibly low nitrogen. It is suggested that growers inspect roots in these cases. Root decline from wet conditions, root rot pathogens, mechanical injury, etc could cause smaller than normal leaves that are light in color and exhibit weak growth. There are two root diseases that occur on holly. These are black root rot (Thielaviopsis basicola) and Phytophthora root rot (Phytophthora cinnamomi). Both will cause roots to turn black, internally and externally. If roots are rotted, consult the help of a diagnostic lab.
Commercial growers might also be concerned with root feeding nematodes on hollies. These nematodes are microscopic. They feed on roots causing aboveground symptoms that include chlorosis, stunting, leaf drop, and plant decline. Most diagnostic labs can test the soil or potting medium for the presence of high populations of nematodes. Of concern are ring, root-knot, and stunt nematodes. Nematodes are more of a problem in field grown hollies where the soil is light textured and sandy.--Nancy Pataky
Author:
Nancy Pataky