Question
Why are my Marigolds turning black. They are in large pots(outdoor) with other annuals???

Thanks,

Joe

Answer
Michigan State University Extension posts a page on one of the more common Fungi known to attack ornamentals this time of year, Botrytis:

'Botrytis normally establishes itself on stressed, aging or inactive plant tissues. Once established, the fungus can attack healthy flowers and leaves. Whereas Botrytis can infect from just above freezing to above 77 degrees F, it is strongly favored by temperatures between 62 and 72 degrees F and very moist conditions.'  Spring weather.

The Michigan authorities have a test for this: 'If you suspect Botrytis, place some affected tissue in a plastic bag with a wet paper towel. If Botrytis is the problem, a gray mold should appear within one to two days.'

Here's the URL:

web1.msue.msu.edu/msue/imp/modzz/00001692.html

In the case of Botrytis on Marigolds, you usually find symptoms affecting flowers in addition to, or instead of, leaves; they turn Brown and begin to decay.  Cold, wet weather is the perfect setup for these spores.

There are a number of other Fungi that would cause symptoms similar to the ones you describe.  Also, there are nutritional deficiencies that also turn leaf sections black ('necrosis') as the problem progresses -- but this is highly unlikely.  Flowering annuals are susceptible to attack by Fungi because they are heavily treated and revved up for profuse growth and bloom to get them marketable; tissue is weak and vulnerable, a disease just waiting to happen.

Plan of Action: First, separate these from your other plants.  You don' t want this to spread.  Your other plants may not be as vulnerable, but if you hit them over the head enough with a disease, they're going to catch something.

Next, water carefully - keep water off the leaves, bring the pots in at night -- or at least when it rains or if fog or humid weather is forecast.  Better however to keep them indoors until morning, to keep dew off the leaves.  Bottom water only, and make sure roots are soaked but that water does not bounce off Soil and land on leaves.  That's where the spores wait until they find a host.

The cure: I would recommend a double-dose of two fairly recent products that might work alone, but would work better if you used both.

One is a natural Fungicide, 'Serenade', sold by Agraquest Inc., a California company.  Actually a bottle of Bacillus subtilis, a strain of bacteria that devours certain Fungi on its menu of preferred entrees, it has been effective in trials against some strains of Botrytis as well as other Fungus problems.  I don't have a ieaf sample from your plant, but I think this is definitely worth a shot.

The other product is a patented Harpin Protein spray, 'Messenger', which stimulates 'healing' activities by damaged cells.  Tt's very different from a fertilizer.  Spray this on leaves, watering only with the excess.  It comes in small package and is sold in garden centers but may be hard to find just because it's so hard to see.  Ask if you can't find it.

Don't fertilize the Marigolds -- if this has crossed your mind.  They should not be fertilized anyway.

Why not use a traditional, old fashioned Fungicide, then?  Well, Copper oxychloride, the most effective Fungicide around these days, gets an EPA warning lecture with a list of side effects (including a skull and bones):

www.pesticideinfo.org/Detail_ChemUse.jsp?Rec_Id=PC35070

Master these OTHER alternative bacterial controls, and

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