Hi Jennifer, thanks for using Ask Extension.

There are several Septoria species that infect a wide range of garden crops. While it is not uncommon to have multiple crops infected by the various Septoria species. Be aware that there are problems both disease and abiotic that can resemble Septoria. If you have any doubts, we can help, or you can send samples the Plant & Pest Diagnostics.

The most notable condition of concern is the flooding. Some crops do not do well in wet soils. Plant diseases can flourish in continually wet soils. Improving drainage can help.  Using raised beds, adding drains, and deep aeration of the area to reduce water logging.  Creation of rain gardens with use of native plants in lower areas can help capture the water and also support native pollinators.

Here are some common control measures that can be effective against various Septoria species. The specific response can vary depending on the species and the host plant.   

Rotating crops can reduce the buildup of Septoria in the soil.

Sanitation is critical. This involves removing infected plant debris and removing diseased leaves and plants, if necessary, from the area.  Avoid working on wet plants and disinfect tools after working on diseased plants.  

Rain splash from the ground can infect a plant. Mulch can help if it doesn't add to the excessive water issue. 

Spacing plants properly can help in improving air circulation and reduce the humidity that promotes disease.  When irrigating, avoid overhead watering.  Prolonged leaf wetness promotes disease.

Use of fungicides can help.  These are several fungicides and biological controls effective against Septoria species; however, their effectiveness may vary depending on the host plant and Septoria species.  Be sure to Read the Label. 

Using resistant plant varieties can reduce the disease pressure. 

Using alfalfa as a cover crop, can improve soil conditions where the beneficial microbes can out compete the pathogenic microbes. Tarping can reduce pathogens if done correctly.

It's important to note that integrated pest management (IPM) strategies, which combine multiple control measures, are often the most effective approach to managing Septoria leaf spot. 

Check out the following links:

Crop Rotation in the Vegetable Garden | Yard and Garden

Garden Sanitation | Home & Garden Information Center

Tarping, Solarization and Occultation

I hope this helps.