Thank you for reaching out to ask2.extension.

It looks like your loosestrife has Septoria brown spot which is a fungal disease.  It probably is a result of the wet conditions we’ve had this spring.  Also over crowded beds that don’t allow a lot of aspirin flow can contribute.  It  can be treated with fungicides or by removing infected leaves. The effectiveness of treatment depends on how far advanced the disease is and the type of plant.

Organic fungicides can be used if the disease isn't too advanced, organic fungicides containing potassium or copper bicarbonate can be effective. 

 Chemical treatments can be necessary for weakened or stressed plants, or in rainy and humid seasons.

Remove infected leaves If detected early enough, remove and destroy lower infected leaves from the plant.  At the end of the season, collect all foliage from infected plants and dispose of or bury it. Don't add infected leaf or debris to the compost pile.

Avoid overhead watering.  Water can help spread Septoria leaf spot, so try to keep it off the leaves by watering at the base of the plant only.

You may have to trim the plant way back and destroy the leaves by putting in the garbage.  Do no compost the infected leaves.  This can be spread to other plants.  Keep an eye on surrounding plants.  It may be that the loosestrife has to be removed if a fungicide doesn’t work.

Please follow instructions on the fungicide.