Answer: The two problem may not be related. Diseases are usually specific to one plant species and won't attack to another. The powdery mildew on a rose is not the same at the one on a cucumber, for example.
Your marigold and verbena problem could have been a bacterial infections. Wet conditions favor this type of problem. Another possibility is root damage during transplanting followed by a fungal root rot which led to their demise. Again, managing soil moisture is the key to control. Give plants a good soaking and then allow them to dry out a bit before watering again.
Both plants are susceptible to spider mites and certain other tiny insects with piercing sucking mouthparts. Their feeding causes a bleaching of the foliage and sometimes tan areas appear where many of them are feeding. Turn a leaf over and check for these pests. Or, hold a sheet of white paper under the leaf and tap the leaf sharply. Mites are tiny brown to red specks smaller than a typed period that will begin to move around on the paper. Severe spider mite infestations are also identified by the fine webbing they produce on infested plant areas. Products containing neem are an organic control option for mites.
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