As much as any gardener, including myself, looks forward to warm weather, I admit that late summer is my least favorite time in the garden. The days are too hot and buggy for working outdoors except in the early morning, the long days and warm soil have helped the weeds grow to epic proportions, and pests and plant diseases are at their peak.
One of the visible signs of late summer in my garden is the start of powdery mildew, a common fungal disease that affects many types of plants. In my own garden, I see it first on my monarda (aka bee balm) after its flowers start to fade and then on my cucurbits. The large leaves on my heirloom Red Kuri Squash already look like someone’s come by and sprinkled them with talcum powder. Usually, you can see the start of powdery mildew first on the plant’s most mature leaves and the white powdery fungus will then spread to the rest of the plant.
Powdery mildew is sometimes what ultimately kills my cucurbits. In my perennial beds, it can be merely unsightly but it won’t do any permanent damage. Once my summer-flowering perennials have finished blooming, I typically cut them way back to prevent powdery mildew and encourage a flush of fresh growth in the plant. For vegetables though, there are a few things you can do to slow down powdery mildew and keep your plants producing for as long as possible.
First, powdery mildew is like most fungus in that it loves places where air isn’t circulating. One of the easiest things you can do to prevent spread of the disease is to do some strategic pruning of the plants to open up air space and encourage plant health. I tend to plant with little space between things, so mid-season pruning is an important part of my fight against powdery mildew.
When pruning, use good hygiene. Dip your pruners in a bleach solution (1 part bleach to 9 parts water) and wipe them clean after pruning any plants that have powdery mildew – or any fungal disease for that matter. You don’t want your pruners to spread the fungus to healthy plants. Remove any leaves that have heavy mildew and dispose of them. Do not compost diseased foliage since temperatures rarely get hot or cold enough to kill the fungus and you could then spread it when you use the finished compost.
Finally, use a milk solution to prevent and slow down powdery mildew. Don’t wait until you have a significant problem, as a milk solution works best as a preventative measure. The jury is still out about exactly why this seems to work for powdery mildew, but it’s thought that there is a protein in milk that, when applied to leaves on a sunny day, coats leaves and stops the fungus from gaining a foothold. I begin using half milk-half water solutions beginning in early July. I use a spray bottle and coat the leaves and stems thoroughly during the heat of the day. After the initial treatment, I spray once every 710-14 days. Milk solutions are completely safe and won’t harm anything except the fungus.
When fungus and bugs seem to be overtaking your garden, it’s easy to get discouraged. But, these simple tips can greatly improve the look of your plants and your harvest as long as you are diligent.