Thank you for your question to Ask a Master Gardener Volunteer with eXtension.

Let me preface with this statement that is very difficult to diagnose problems (even with the pictures you sent) on-line.
There could be a host of issues.  Let's list them and take it from there.

The first could be watering (too much/too little) they can present symptoms very much alike.

Insects - are those white fly on some of the leaves or water droplets?

Proper air flow around plants.  Some plants are more finicky about this than others.

Mulch issues - Using mulch in gardens to excess may also encourage the plant to root in the mulch and not in the soil, which will cause root decay, especially when the mulch dries out. Another garden mulch problem caused by thick application is possible fungi establishment, resulting in the creation of water repellent conditions.


Fusarium is spread in contaminated soil and infected cuttings and is favored by warm temperatures, high relative humidity, overwatering, and poor drainage. Fusarium diseases on ornamentals are difficult to control. Prevention is the best approach, even when fungicides are used.

Unfortunately, there is no cure for fusarium wilt. The only option is preventing fusarium in your plants. Once your plants are infected, they must be removed and destroyed.  See the following for further diagnosis and information:

https://ipm.illinois.edu/diseases/series600/rpd650/rpd650.htm

Good luck to you as your work thru the issues which might be affecting your snapdragons.