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Powdery Mildew starts out as whitish spots that spread quickly until the entire leaf is covered. The white powdery growth is a fungus that with time becomes gray to tan/brown felt like patches. Leaves may become stunted, curled, chlorotic and eventually wither and dry up. Mildew occurs from hot days , high humidity ,cool nights and at the change of season.

Fungicide recipes

Depending on the severity, spraying it with a baking soda formula is effective as a preventative when applied regularily.  For active infections spray daily for a week. 

1. Mix 1 TBsp each of  baking soda  and horticultural oil (dormant oil/vegetable oil) or a few drops of liquid soap to 1 gallon of water.  Spray weekly making a new mix each time.  It will not elliminate the disease but help control it.

2. Mix 1 tsp baking soda with a few drops of vegetable oil in 1 quart of water. Spray or paint on the leaves.  Works on houseplants, cucurbits & roses (balck spot).

Another suggestion is  a solution of 1/3 milk and 2/3 water and spray on plants. Use every other day.

I also heard of mixing 1 tbsp of pine sol to 1 gal of water as a mildew spray.  Neem Oil is also affectective in controling infections. Use 1 oz.(2 Tbsp) of Neem oil and 1/ 1/2 tsp of dishwashing detergent to one gallon of water. Spray once a week for two weeks.
Use chemical sprays such as Benomyl (systemic fungicide), sulfur/fungicidal soap early in the growing season as a preventative or as soon as symptoms appear. A synthetic fungicide Baylaton sold as Strike also works well on mildew. Always follow label directions, to make sure the product is approved for specific plants. The combination of Neem and baking soda is the safest control method. Once the disease takes hold, it is difficult to control.

Some cultural preventatives  that can be done are to remove the infected leaves, do not crowd the plants, provide good air circulation and keep plants well watered and stress free. Grow resistant plants when available.

Mandevilla tips and care

Mandevilla belongs to a large family that also includes plants that were formerly known as Dipladenia. It was named after a British diplomat/gardener Henry Mandeville. This exotic plant bears trumpet shaped flowers shades of pink, yellow and white (some fragrant) on twining vines with puckered, oval leaves.

This tropical plant is hardy (depending on cultivars) in zones 9-11 and therefore is for the most part of the U.S. treated as a houseplant. It is grown outdoors for the summer and in cold weather taken indoors to endure the winter.

Care
Outdoors: Mandevilla is cold sensitive and can be taken outdoors when the danger of frost has passed and overnight temperatures reach over 50ºF. It likes full sun to part shade in the summer and a deep rich, well-drained soil. It will also need a trellis to support its long trailing vines. Provide plenty of water during the hot days of summer. To keep a healthy blooming plant, feed it every other week with a high phosphorus fertilizer (10-20-10) in the spring and summer. In order to maintain the tangled growth during the season, pinch off new shoot tips which will produce a bushier plant. Trimming will not reduce flowering as it blooms on new growth. Use the cuttings to make new plants.

Propagation
It can be started from seed at temperatures of 70-80ºF. A faster way is to take cuttings in the spring and summer which will easily root and develop new plants.

Watch out for mealybugs, scales, whitefllies and red spider mites.

Winterover mandevilla