Powdery Mildew is a very common and easily recognizable fungal disease in plants. It can cause some stress to your plants, as it leaches important nutrients from your plant if left untreated, but is very rarely ever fatal.
Powdery Mildew can affect almost any type of plant. Unusually for a fungus, this type of mildew actually prefers dry conditions - specifically warm and dry. It can actually appear first on the underside of leaves, but you may not start to notice it until it spreads elsewhere. In fact, it can spread just about everywhere - the top of the leaf, the stem, on buds and flowers and even on fruits. Signs to look out for are:
Light-colored white or gray spots, often on leaves. This may be confused for dust or dirt at first, but it is recurrent (i.e. if you just lightly brush it away, it will come back). This is the most common and distinctive symptom, but your plant may also show:
Withering/drying of leaves
Yellowing of leaves
Disfiguring of leaves and leaf tips
WHAT TO DO NOW
Isolate the infected plant to prevent the mildew from spreading.
If the damage is only moderate, try spraying the plant with a baking soda mixture: 1 tbsp of baking soda, 1/2 tsp of liquid soap, 1 gallon of water (try spraying on a small area of the plant first).
If the damage is more severe, it may be difficult to treat the plant - try to remove all infected material and destroy it completely to help stop the spread.
Consider using a fungicide. There are several good ones on the market, and some are even organic.
Check all of your plants from time to time to make sure it hasn't spread.
HOW CAN I PREVENT THIS FROM HAPPENING AGAIN? Try placing your plants somewhere where they are more exposed to the sun - as powdery mildew tends to develop where it’s shadier. You can also make sure to choose plants that are specifically bred to be resistant to powdery mildew. For example, cucumbers are particularly susceptible to powdery mildew, but you can buy seeds that are resistant.
WHEN IS POWDERY MILDEW MOST PREVALENT? It’s most likely to be a problem in spring to early summer, when the days are beginning to get warmer. Therefore, it’s a good idea to keep an eye on your plants during this time - check them regularly for early signs to try and stop it from spreading.