well they have some severe mildew ... but that didnt kill them.. but makes me wonder about you watering them EVERY night .. and then i look closer.. and wonder if they are in a planter of some kind.. and i wonder about drainage.. and whether you killed the root system with too much water..

a complete plant kill.. usually means.. the roots died .. aka root rot ...

can you give us some more precise facts other than a picture of a dead plant .. though that was a great start ..
facts as to ... the planter if any.. potting media or soil.. drainage.. watering.. etc .. and what large city you are near ... and any fertilizer applications ...

ken

H54,

Your second picture has a "misty" look in the upper lefthand part of the picture. That made me wonder if spider mites are present. Are there any very fine webs on your affected zinnias, with near-microscopic little creatures on the webs?

"I don't see any obvious pests... What is this stuff?"

I agree with Ken. A quick plant death usually means that something destroyed the root system. Powdery Mildew can do cosmetic damage, but it can take a long time to kill the plant. Your plants are probably suffering from a soilborne fungus disease. We don't have enough information to determine which one.

I don't know if you are far enough south to have Sclerotium Root Rot (also known as Southern Wilt, Southern Blight, or Crown Rot). It is caused by a soilborne fungus (Sclerotium rolfsii) that occurs primarily in the southern states.

Zinnias are also susceptible to a disease called Watery Soft Rot, which is also caused by a fungus (Sclerotinia sclerotiorum). The infection starts on the stem and spreads downward, rotting the roots. Watery Soft Rot is most active in wet weather in the temperature range of 55 to 75 F. It occurs in most of the US, with the exception of the upper mid-west.

Once a zinnia is infected, it isn't practical to cure it. As with many things, prevention is better than a cure.

ZM

Do you have a dog? When our dog pees on stuff, it can make spots like that. If so, could be that and any or all of the other stuff mentioned too. Quite a shame, looks like they were awesome plants!

Thanks everyone for your input! I water every other day, usually in the morning. I fertilize twice per month (Jack's water soluable, all purpose, triple 20)usig a hose end applicator. Brick planter has plenty of drainage...... No recent soil analysis, but it's river bottom soil amended with chicken manure/pine shavings compost.

I'm with ZM on the fungus issue. Prolly too late to try a fungicide.... Oh, well.......

OBTW..... It would take one very accurate dog to urinate on these, & I don't think my three 13 YO Dachshunds are capable! LOL

{{gwi:22040}}

{{gwi:22041}}