Basil Fungus Making the Rounds
Nice basil leaf--better watch out! (Leighton Pritchard/Flickr)
I think it was on a cooking show on the radio the other day (or maybe just a news report) that I heard that a fungal disease was playing havoc with basil plants in some parts of the U.S. I figured it might be worth looking into. Sure enough, a Google news search for “basil” and “fungus” does turn up a few results, and it seems that the initial report I heard was an interview with plant pathologist Margaret McGrath on the NPR program All Things Considered (see Basil Downy Mildew Threatens Pesto Lovers):
There’s bad news for gardeners who grow basil to sprinkle over Italian food or Thai dishes — or anyone who thinks pesto is the best flavor on Earth.
A potentially fatal fungal disease called downy mildew has been attacking basil plants in New York, New Jersey, Ohio and Florida. Gardeners are worried that it could spread farther throughout the summer, turning delicate green basil leaves an ugly shade of brown, yellow or gray.
Although the news outlets just seem to be catching up with the issue, the onset and spread of the disease has been known about for over a year now. According to a blog post (from Margaret McGrath) Basil Disease Hits Northeast:
Basil downy mildew, which was first reported in the U.S. in October 2007 in Florida and the Northeast in 2008, is unfortunately back again. And it’s being found several weeks earlier!
Like the late blight that has been killing tomatoes and potatoes in the region, the pathogen causing downy mildew in basil produces a spore that can be dispersed long distances by wind, and then deposited onto plants by rain.
What is the Fungal Culprit?
Although Fusarium oxysporum is known to cause a wilt disease of basil, the fungal culprit on this recent outbreak seems to be something new. I noticed that most of the articles discussing the basil disease that have appeared in the press have failed to identify the fungus causing the disease. A search for “basil downy mildew” was quite productive turning up over 15,000 hits, one of which was Downy Mildew on Basil (PDF) from the Florida Extension Service, which provides this additional information:
Symptoms initially appear as a faint yellowing of leaves concentrated around the middle vein. On the underside of leaves, a gray, fuzzy growth or russetted appearance may be apparent. Although symptoms may appear relatively mild in the field, the disease causes major post harvest problems causing the leaves to blacken and become un-saleable once the crop is harvested and packed.
The greenhouse industry has also reported that a new downy mildew that affects coleus has recently been discovered in multiple locations around the United States and is causing alarm for both coleus growers and researchers. This new coleus downy mildew disease is caused by a microorganism that may be new to North America.
Tests of coleus downy mildew DNA, however, have indicated that it is a different species — one that has previously been seen in Europe on greenhouse-grown basil in Italy and Switzerland. For now, the coleus pathogen can be referred to as Peronospora sp. It is known that it can infect both coleus and basil, but the possibility it can go to additional plant hosts certainly exists.
So is the Coleus-infecting species the same one that is attacking basil? According to Identity of the downy mildew pathogens of basil, coleus, and sage with implications for quarantine measures from researchers in Germany, the answer appears to be yes:
The downy mildew pathogen of basil (Ocimum spp.) has caused considerable damage throughout the past five years, and an end to the epidemics is not in sight. The downy mildew of coleus (Solenostemon spp.) is just emerging and here we report that it was very recently introduced into Germany. Although it has been recognised that these pathogens are a major threat, the identity of the pathogens is still unresolved, and so it is difficult to devise quarantine measures against them. Using morphological comparison and molecular phylogenetic reconstructions we confirmed in this study that the downy mildews of basil and coleus are unrelated to Peronospora lamii, which is a common pathogen of the weed Lamium purpureum. In addition, we conclude by the investigation of the type specimen of P. swingleii and downy mildew specimens on Salvia officinalis that the newly occurring pathogens are not identical to P. swingleii on Salvia reflexa. The taxonomy of the downy mildew pathogens of hosts from the Lamiaceae and, in particular, from the tribes Mentheae and Elsholtzieae, is discussed, and a new species is described to accommodate the downy mildew pathogen of basil and coleus, which is the first downy mildew pathogen known to be parasitic to hosts of the tribe Ocimeae.
Some follow up comments on the NPR website give a few tips on how to salvage your basil crop if you notice signs of the disease (scroll down to the bottom of the page for the basil comments).
More on Downy Mildew of Basil and Related Topics
Fungus threatens basil plants for this year and beyond
Worst year ever for plant diseases
Basil: Growing, Harvesting, and Avoiding Downy Mildew
First Report of Downy Mildew Caused by Peronospora sp. on Basil (Ocimum basilicum) in France
Tags: basil, downy mildew, Margaret McGrath, Peronospora
Oh cool! I just saw a bunch of lesions on the basil in my garden plot this evening. If I had known DM was going around I would have checked under the leaves!
Peronospora belbahrii Thines, the basil downy mildew as is now called. In the 2008 Mycological Research paper you cite, the authors observe that the downy mildews of basil and of coleus might in fact be two separate species–more data are wanted, but there is some evidence for genetic separation. If they are two species, the basil downy mildew will keep the name P. belbahrii, since the type specimen is on basil. It’s worth noting too, that the downy mildews are not true fungi, but rather relatives of Phytophthora that mycologists have long treated as “oomycetes.” My plan? I’m going to make a ton of pesto this year and freeze it.
Awesome update Kathie. Thank you very much!