Barb, Its probably powdery mildew, take a look at the link for a picture to see if thats it. Powdery mildew is a disease that occurs worldwide. For decades, organic gardeners had to rely on making a spray from baking soda to control the disease. Now, instead of measuring out the baking soda and combining it with a surfactant (a "sticking" substance) of either oil or soap, gardeners need only head for their refrigerators. A weekly spray of milk at a concentration of at least 10% (1 part milk to 9 parts water) significantly reduced the severity of powdery mildew infection on the plants by 90%. How does milk control powdery mildew? Scientist aren't 100% sure how milk works to control this disease. It seems that milk is a natural germicide. In addition, it contains several naturally occurring salts and amino acids that are taken up by the plant. From previous experiments using sodium bicarbonate, potassium phosphate, and other salts, researchers have found that the disease is sensitive to these salts. It is possible then, that milk boosts the plant's immune system to prevent the disease. Worth a try :) Here is a link that might be useful:

the issue with mildew.. is going into darkness ... with wet leaves ... on hot humid nights when i used to do phlox in suburbia.. i would get home and in the evening.. wet down the whole garden.. it was a stress reliever.. unbelievable mildew on monarda.. phlox.. roses .... once i stopped wetting things down.. i reduced the PM 50% ... the bane of such is also air movement at night.. so the next step was cutting away some of the jungle around them.. to get the best air movement.. at night that i could ... finally.. i found out that it is prevented with early application of baking soda and water.. in a spray tank ... no chemicals necessary ... [might want to verify that .. coffee buzz warning.. lol] it took me a long time to learn all that without the worldwideweb ... then i moved out to the country... former horse pasture.. much wind .. pure sand .. and full sun ... and have never had a PM issue ... frankly it is just too dry ... and i dont water them at all once established ... whether a given plant will resist or not.. is irrelevant .... if the plants are placed in a situation where it is impossible to resist ... sooo.. i would suggest you plant them in full sun... and once they get established.. insure they never have wet leaves in darkness ... and if its a lawn sprinkler issue.. figure out how to avoid such ... they are not a 'stuff them in a crowed corner' type plant ... and if you cant do that.. NOTHING.. repeat NOTHING .. will stop PM completely ... what all that means.. is that the plants 'CULTURAL REQUIREMENTS' will outweigh any claim of resistant.. IMHO ... 'culture' meaning how and where its planted ... all that said.. the fragrance is unique .. and one of my childhood memories.. i think grandma had some back in the polish ghetto of detroit in the 60's ... i will never be without them ... good luck ken BTW ... mine get 4 to 5 feet tall with total abject neglect.. and seed themselves somewhat... i started with the pink eyed one.. and the seed has given a pink.. and a white ... i never knew they seeded until i neglected them. lol .. so now i have 3 varieties ....

You can if you want to, but it's not necessary. I've left mine and cleaned them up in the spring, but this year I'm trying to get ahead of the game, so I've cut them back. What do you mean by struggling? Plant not growing as it should in terms of size? Mildew on leaves? No flowers? Is this a new plant or an established Phlox? I divide my Phlox every 5 or so years. Seems to rejuvenate them. At this time of year, I mulch with compost for the winter. (My compost is made out of seaweed, coffee grounds and shredded leaves.) In the spring, I amend with a bit of cornmeal and sometimes alfalfa pellets. I'll do this again in late June. If powdery mildew (PM)is the problem, this will help. I no longer have any PM in the gardens. Also, make sure the plant is in an area which has good ventilation. Good air circulation around these plants discourages PM. And if PM was the problem this year, yes, I would cut it back this fall and throw all their debris in the trash - ie. not the compost.

I cut my phlox back to just a couple of inches above the base. Phlox foliage often gets a bit of powdery mildew late in the season so I want to get that out of the garden before winter. To collect phlox seed after the flowers fade they will form seeds. They are ripe when they are dark and I pluck them off as they ripen. Some put a nylon or small bag over the seed head to catch ripe seeds as the ripe pods open easily and seed falls to the ground. I no longer add winter mulch to my phlox as they are hardy and survive well without it. We've had a couple of nights in the twenties. The few clematis and mums that are blooming seem to be unaffected by the low temps. Same for several other blooming perennials. Think the temps need to be low for a longer period of time to end their bloom. In my zone pansies are more readily available and displayed in the spring. Some mornings they look terrible from being frozen but as they warm the foliage and blooms perk up and they are lovely again.