My best advice is to monitor your plants every single day. Familiarize yourself with what disease or issue you think it might be. Send a photo to your local extension office or take it in personally for a correct diagnosis. Please don't rush out and buy the first product you see that covers lots of diseases and or insects. You'll only add to your plant's problems by using the wrong product.
Here's the direct link to find your closest extension office. Colorado State University Extension Offices
If you see infection starting at the bottom of the plant, then cut off leaves or stems with diseased leaves right away. Do this when the plant is dry and don't water right after pruning them. Sterilize your shears with a 1 to 10 bleach solution or spray the shears in a bucket with Lysol in between cuts. Be sure to wipe your shears clean after using products on them and dry them with a cloth.
Also note some of the lower hanging leaves, in fact they touch the ground, which only helps soil borne diseases get into the plant easier. I'll trim these up too, I should have done this weeks ago.
|The day after 2 1/2 inches of rain|
|A bit of yellowing and brown on a leaf, could be the start of early blight|