I have seen Marigolds perish before in the hot and dry mid and late summer but never was able to figure out the "why." The first sign of coming death is when leaves take on a bleached tinge. The foliage is green but has white extremeties.This image shows a more advanced spider mite infestation with bleached foliage and a shroud of webs encasing the plant -especially lower foliage.
They say spider mite attacks are frequently misdiagnosed since they are so tiny they go unnoticed. Once the marigold foliage turns brown the war is lost. Early treatment is important. Satisafactory control can be secured by soaking thoroughly with a horticultural oil or insecticidal soap.
Here's the recipe for insecticidal soap:
Add 1 to 2 tablespoons liquid soap to 1 quart water
Note: Buy a liquid soap and not a detergent. Health foodstores have liquid soaps, such as Dr. Bronner’s Pure-CastileSoaps.
We didn't want to put treatment off any longer since the spider mites were causing a lot of damage. We searched the house and barn for ingredients to make insecticidal soap. The closest thing we could find was a big bottle of ammonia. About 2 teaspoons were poured into a quart sprayer - shook it up and spritzed the affected areas on the marigolds. Some improvement was noticed the first day so we continued to spray each evening in each web infested area. Within 4 days of initial treatment we noted healthier marigolds with only vestiges of spider mites present. Damaged areas remained damaged but overall plant condition was much improved.
