White Powder On Rosemary: Getting Rid Of Powdery Mildew On Rosemary

What Is White Powder?

The word “powder” comes from the Latin word “puellum”, which means white. So it’s basically a substance that looks like snow or dust. There are many different types of white powders out there, but they all have one thing in common – they’re not really white at all! They look white because they contain some sort of colorant. These colors range from light browns to yellows, blues, greens and even reds. Some of these colors may cause irritation when inhaled or ingested.

When you see white powder on your plants, it’s actually just a type of mold called Phytophthora infestans (or simply PIM). This fungus is found throughout the world in moist places such as soil and under rocks where it grows. It produces spores that germinate into tiny white fruiting bodies. These white mushrooms grow in clusters and can reach sizes of up to 2 inches long.

How Does White Powder Affect Roses?

PIM causes white powdery mold on roses, causing them to turn yellowish-white. This usually happens within a few weeks after the first signs appear. At first, you’ll notice the leaves starting to wilt and droop. As they dry out, they will start curling up and shriveling. These infected leaves will also become more susceptible to diseases.

As the disease spreads, the stems of the plants will become weak and start bending unnaturally. Eventually, you’ll see the entire plant collapse and die. Unfortunately, there’s no cure for this type of rose powdery mildew. It’s highly contagious and will spread to your other plants if you don’t take action right away.

The good news is that it can be easily prevented. The easiest way is to create some physical space between your infected plants and healthy ones. Just be sure you don’t destroy the roots when you do this!

What Do You Do When You See White Powder?

In addition to removing your infected plants, you’ll also need to take several preventative steps to ensure the rest of your crop doesn’t get infected. The first thing you should do is cut down any roses that show signs of the disease. After cutting them down, you’ll need to dispose of them properly. This means taking them out of the area and not just dumping them on the ground.

After doing this, you’ll need to disinfect everything. Grab a 5-gallon bucket and fill it with a mixture of water and household bleach (at a ratio of 1:32). Use this to disinfect your pruning equipment after each cut. This should prevent the spread of the disease.

Another thing you can do is create a barrier between your healthy plants and those that are diseased. This barrier should be at least 18-inches high and should be made from an opaque material such as plastic or cardboard. Make sure there are no holes in it!

After you create this barrier, spray everything on the healthy side with water. As the leaves begin to dry, apply a thin coat of vegetable oil. This should keep the spores from getting into your healthy plants.

Unfortunately, if you have a large number of plants that are infected you may need to destroy them. This is so important that it’s actually state law in some places! Be sure to check with your local government to see what the regulations are in your area. In some places, you may be able to legally burn them. If not, you’ll have to take them to a landfill.

You’ll also want to clean everything you used while disposing of the infected plants. Follow the same steps you did above for cleaning your tools after pruning healthy plants.

Finally, you may also need to apply some foliar fertilizer. This is because the plants are going into shock due to losing their roots and are no longer receiving nutrients from them. You can learn how to make your own foliar fertilizer by following this guide.

How To Prevent It From Happening Again

Once you’ve taken care of the infected plants, it’s time to figure out how this happened in the first place. The best way to do this is by figuring out where the disease came from. Below we’ll discuss some of the common causes that lead to powdery mildew in rose bushes.

Poor Air Circulation

One of the leading causes of powdery mildew is poor air circulation. This is because the fungus that causes it thrives in warm and humid conditions. The more stagnant the air around your plants, the greater the chances of them becoming infected.

White Powder On Rosemary: Getting Rid Of Powdery Mildew On Rosemary | igrowplants.net

Many rose gardeners combat this by pruning their bushes to promote growth and then thinning out the plants once they become adults. This promotes better air circulation around the plants and reduces the risk of powdery mildew.

Another thing you can do is train your rose bushes to grow up on a trellis. This will give them more room to grow, promote better air circulation, and give you easier access to them when it’s time to prune them!


Another reason you’re seeing so much powdery mildew in your rose bushes could be due to dehydration. This happens when the soil your plants are growing in doesn’t have ample moisture in it.

A sure sign that your bushes are dehydrated are if the leaves begin to curl up or look burnt. It can also manifest in the stem of your bush becoming dry and brittle.

Training your plants to grow up on a trellis is a good way to promote better air circulation, which helps prevent dehydration. However, there may be times where even this isn’t enough. If you live in a particularly dry climate, this may be the case.

The best way to combat dehydration is to water your plants more frequently. Use a moisture meter to test how wet the soil is before giving it a thorough watering.

If you don’t trust your green thumb, or you just don’t have the time to water your plants, you might want to look into installing an automated irrigation system. You can either create your own, or buy a pre-made one.

These systems are relatively inexpensive and can be set to water your plants at specific times every day. Just set it and forget it! If you’re really busy and don’t want to worry about it, this may be a good solution for you.

The last possible cause is the use of contaminated seeds or plants. It’s always important to buy new seeds every year. This helps prevent your plants from becoming too inbred.

It may also be beneficial to buy new plants every few years as well. If you start seeing a lot of powdery mildew and other diseases cropping up, it might be time to buy some new ones!

Another cause for concern is where you got your plants. It’s possible you got a diseased one without realizing it.

Sources & references used in this article:

Rapid and dynamic subcellular reorganization following mechanical stimulation of Arabidopsis epidermal cells mimics responses to fungal and oomycete … by …, D Takemoto, RG White – BMC plant …, 2008 – bmcplantbiol.biomedcentral.com

Immune suppression by neonicotinoid insecticides at the root of global wildlife declines by R Mason, H Tennekes, F Sánchez-Bayo… – J Environ Immunol …, 2013 – boerenlandvogels.nl

Rosemary by B Sasikumar – Handbook of herbs and spices, 2012 – Elsevier

Rosemary Production Guideline by M Mekonnen, B Mengesha, M Philophos, G Tigist – 2016 –



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