This week the Plant Disease and Insect Clinic at NCSU received a container rose with both downy mildew Botrytis blight. Both are favored by cool, moist conditions, and both can cause extensive damage in a short period of time via the dispersion of airborne spores. That’s where the similarity ends.
Cankers caused by Botrytis on rose canes are often light in color. This same fungus can cause spotting on rose petals and in the present case grew all over clusters of dead new leaves. It enters plants most easily in senescent tissue or through wounds. Recent cold snaps may have given Botrytis a leg up. On the other hand, downy mildew needs to infect and reproduce on living plant tissue. On rose leaves, it typically produces dark angular spots, though here it was found on green leaves that showed only a very slight mottle. On canes the usual symptom of rose downy mildew is purple or black blotching or spotting.
The downy mildew pathogen was found sporulating on the underside of this fairly healthy-looking leaflet.
Botrytis is sometimes called gray mold. This color is the combination of the black of the tiny thread-like stalks (conidiophores) and the white of the spores (conidia). Under magnification these look like pompoms. The spores of rose downy mildew (Peronospora sparsa) may or may not be visible on an infected plant. They are especially scarce on canes. On leaves they will be produced on the underside only. These structures are slightly smaller and more delicate than those of Botrytis, and the stalks (sporangiophores) are white. They branch repeatedly giving a more open, tree-like growth. At the tips of the sporangiophore’s branches, spores (sporangia) appear white or tinged grayish blue in mass.
Sporulation of Botrytis on a rose cane. Note the thorn in the upper right for size comparison.
Close-up of Botrytis sporulation Sometimes a second “pompom” of spores will form in the middle of the conidiophore.
Sporulation of Peronospora sparsa It’s unusual to see so much on a cane.
Close-up of Peronospora sparsa sporulation This and the previous three photos courtesy of Matt Bertone.
Prune out any affected canes you find, going well into clean wood. Sanitize shears frequently. Do not let debris or spent flowers accumulate, as Botrytis can reproduce on just about any kind of dead plant material. Keep foliage and stems as dry as possible by proper timing of irrigation and adequate plant spacing to allow good air movement. One reason these fungi were doing so well on this sample may have been that the plants are still in a greenhouse-like environment.
This time of year rose growers should maintain a spray program that includes products against both pathogens. Since the pathogens are unrelated, some products work only on one or the other. See pages 465-467 of Table 10-13 of the NC Ag Chemicals Manual. See also Table 10-14 (pp. 474-476) on relative effectiveness of different products. Follow all label directions. Remember that labels can vary depending on whether the plants are being grown outdoors or in an enclosed structure such as a greenhouse. Be sure to rotate among fungicides in different FRAC groups so as to delay the development of resistant strains. Test any new product on a small number of plants to be sure that there are no adverse effects.
Note: Early indications are that the plant pictured above also has a bacterial disease called Pseudomonas blight. Symptoms of this disease include cankers on stems and the death of new shoots, often following freezing temperatures. It affects not only rose but a wide range of woody hosts. Plants like this one are best discarded. For details, see last year’s blog post.