Downy mildew (caused by a species of the oomycete, Peronospora) on the herb/groundcover Sweet Woodruff (Galium odoratum) was found on a recent sample. At first glance the symptoms suggested iron deficiency (based on yellow foliage and other indications by the grower). However, a closer look shows leaf dieback and gray lesions which indicate a pathogen may be involved. Flipping the leaves over (see photos below) reveals the characteristic fuzzy growth of downy mildew.
Downy mildew loves cool, wet conditions! Management should involve reducing leaf wetness (which is required for the spores to germinate). There has been no research done on fungicide control on Galium spp., so proceed with caution if choosing to use a fungicide. Test in small areas to ensure that phytotoxicity isn't going to be an issue. The Chase Research Gardens, Inc. out of California have done quite a bit of work on downy mildew fungicide control in ornamentals and some results can be found here.
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