Rainy, cool weather is Downy Mildew weather. So, with all of the rain and flooding in the Southeast over the last few days, it seems to be an appropriate topic.
HOW TO IDENTIFY
Downy mildew got its name because when it is sporulating (the fungal equivalent of fruit), the spores make a fluffy white mat on the undersides of the leaves. This Black-Eyed Susan leaf has a terrible infection:
When you turn the leaf over, you can see that it is purple in the infected areas. That's because the fungus really lives on the inside of the leaf, taking nutrients from the cells. When areas on the leaves turn purple, it is a signal that the chlorophyll and sugars have been used by the fungus.
One of the interesting things about the fungus that causes Downy Mildew is that it is bound by leaf veins. That means that it creates angular leaf spots, like on this Butterfly Bush:
Downy Mildew is a common disease of Roses during the fall and spring. If the infection is bad, the entire plant will quickly defoliate. Cool, foggy and wet weather is the trigger for this disease.
MANAGING DOWNY MILDEW
The best way to control this disease is to keep the leaves dry. Use drip irrigation, if possible. When using sprinklers to irrigate, water early in the day so that the leaves can dry before nightfall.
Plant mildew-susceptible plants in areas where there is good air circulation (this lowers the humidity). And rake up fallen leaves in the winter.
Some varieties are more susceptible than others- so choose strong plants. For example, yellow and white flowering roses are typically more prone to Downy Mildew than other colors.
I haven't really shown my dorky side in a while and I'm feeling a little repressed. So, I want to show you what the spores look like up close (this is how I identify a fungus). I love this one, because the part that makes the spores (sporangiophores) look like deer antlers! At the tips, round spores are formed:
The spores splash in water droplets from plant-to-plant, causing new infections. Wow, right?