Just wait. Depending  on where you live it could be rain, cold or a fungal infection especially if you planted too early. The plants may outgrow the stress once the weather warms up. When to plant is determined by your climate but also the location of the garden and type of soil you have. White stuff could be mildew from dampness. Full sun and good air circulation helps to prevent mildew. Waiting until mid May is the most common time to plant veggies in most parts of the country. Keep growing, Marianne Binetti