Answer: Based on your description I am not certain exactly what killed your Gerber daisies, but in most instances it is unwise to replant the same plant in that location the following year as that would be an invitation to reinfection if it is a soilborn problem. For this reason you may want to plan your planting carefully for this year.
The best defense against infections in general are to maintain healthy soil conditions, purchase healthy plants and maintain them in vigorous condition. This helps the plants resist problems. Rotating your plants can also help avoid a long term buildup of a problem.
Each fall, and during the season if problems occur, carefully clean up and remove from the garden area any damaged or diseased plant debris. (Put it in the trash, not in the compost.) This can go a long way toward preventing reinfection from year to year. Adding a clean layer of mulch on top of last year's remaining mulch can also be helpful in some cases.
Steps to preparing healthy soil in a flower bed include regularly adding ample organic matter such as compost, old rotted leaves, or well aged stable manure and bedding and working it into the top ten inches or so of soil. Soil that is well cared for in this way over time will hold ample air and water, both of which are needed by plant roots and the activity of "good" microorganisms in healthy soil can help combat diseases through competition as well.
Using an organic mulch and maintaining it at a depth of two to three inches will also help feed the soil and improve its structure as it breaks down over time. The mulch layer will also help moderate the soil temperature, keep the soil moist, and keep down weeds. This makes a big contribution toward overall plant health, too.
Correct watering can also contribute. When you water, water deeply but less often to encourage deep rooted plants. A daily light sprinkling is less effective and uses more water in the long run. Use your fingers to check the soil moisture below the surface and water as needed to keep the soil evenly moist but not sopping wet.
In some cases an overly wet soil can encourage root diseases. Also, avoid wetting the foliage when you water as this can encourage foliar problems such as fungal infection. In the same vein, make sure your plants are located in their preferred type of light conditions and in a spot with good air circulation.
Finally, you may want to run some basic soil tests and see if the fertility levels and pH are adequate for healthy growth. Over and undersupply of nutrients can both affect plants' ability to fight off problems, so it pays to feed based on the test results. Your county extension should be able to help you with the tests and interpreting the results.
I hope you have better luck with your flowers this year. If however you see signs of the wilting you might want to consult right away with the county extension as to what exactly is causing the problem and what additional control measures you could take to combat it -- since it is impossible to recommend a specific control without a definite identification of the problem.
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