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How To Identify, Prevent, And Remove Powdery Mildew Plant Disease

What Are The White Spots On My Plants? – My Experience With Powdery Mildew

One of the most common ailments to a plant can be seen as white spots.  Its a type of fungal disease known commonly as Powdery Mildew. What it really is, is a group of closely related species of fungus which have a very specific range of target plants with which it may affect.  

My Experience With Powdery Mildew

I grew a whack of fruit and veggies last season.  I had blueberries, tomatoes, herbs like oregano and chives, zucchini, beans, and squash.  All these were in relatively close proximity, to even touching each other. This picture was taken in August, the beginning of the mildew.

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We had a lot of rain here where I live this last year.  Great for some plants, my tomatoes did fantastic in their raised beds.  But my zucchini and squash became targets of a bad case of powdery mildew.  Now I was still able to get a huge crop of zucchinis harvested from just a few plants, but I had constant pruning and cleaning due to the fungal attack.   

It Started With Heavy Rains In Springtime

In the spring, I don’t recall there being much of an issue with the fungus.  In fact, I don’t recall seeing it at all until July. Nothing really until then.  If I recall correctly, we had a lot of rain in May, June, and July. I found some data actually so here it is:

Courtesy of Hamilton Weather Stats @ https://hamilton.weatherstats.ca/charts/rain-monthly.html

In this picture, you can see the spread of the mildew across the leaf. The spots multiply until it turns a whole patch white.  Then the leaf starts to die. 

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At this stage, you might be able to salvage the leaf with treatment, but otherwise, you will need to amputate the leaf.  Remember to disinfect your tools after as to not spread the fungus further.

This disease only attacked my zucchini and squash plants.  The blueberries and tomatoes were left untouched despite the fact they were even touching the infected plants at times.  The same occurred with the Arugula which was also not affected by the disease.

This disease, if left unchecked, will devastate your plants.  It’s strangely selective though. But no plant is safe from fungus if it’s the right fungus for that particular plant that is.  As mentioned, my experience with this affliction was direct with my zucchini and squash plants only. You can see in this picture how the tomato plants were actually touching but they were seemingly unaffected by this fungus.

The end result, when left unchecked, is a nasty spread of this white stuff.

Identification of Powdery Mildew

Identification of powdery mildew is actually quite simple.  If you look closely, it begins with small white specs. These do not move, crawl around or anything like that so if they are moving, you’ve got something else entirely.

The spots spread and multiply along with the plant with a focus on the leaves.  I found that the disease targeted only my zucchini and squash and left my tomatoes and blueberries alone.

This is not to say that it cannot attack families of plants.   There is actually a large variety of fungus’ which make up the group that is responsible for this apparent disease.  Some attack other groups of plants other than squash and zucchini.

The spots are usually seen on the top of the leaves, but I’ve seen them on the stalks and underside of the leaves as well.  The tops are the most visible.

The disease will often make the leaves twist, and they will yellow and rot/die starting at the edges and it works inward towards the stalk until the stalk itself dies as well.

The white fungus is kind of slimy and kind of powdery.  I recommend using gloves when handling any of the infected foliage.  Make sure you wash the gloves after to reduce the risk of spreading the disease to healthy plants.

Prevention and Treatment of Powdery Mildew

The best thing to do is to keep an eye on your plants.  Check them every day. It only takes a moment and will give you a sense of pride as you will get to spend time watching your accomplishments grow.  And that ties in my philosophy of growing your joy. But back to the fungus. Keep an eye out for the first signs and watch overwatering. Only water at the base, don’t water all over the leaves.  This will minimize the local humidity.

Here is a simple list to follow to help prevent powdery mildew and other molds and fungus.

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4 Tips To Prevent Molds And Fungus

1. Plant your crops with space between plants and rows.  Do not overcrowd.  When you overcrowd your plants, not only do you allow disease a much easier way of spreading from plant to plant, but you also eliminate good airflow.  Airflow is important in reducing mold and fungus due to reducing local humidity. Just like stagnant water is bad because it breeds nasty things like mosquitos, this is also trying for stagnant humid air.  It is a safe haven for molds and fungus so let those plants breath.
2. Don’t over-water your plants.  When you over-water, you increase the chances for mold and fungus to grow.  Most plants don’t want to overwater either as it can drown their roots. Some plants like a lot of water, and for these make sure you water the right way.
3. Water your plants at the base.  Try not to water your plants on the leaves.  Getting the leaves wet helps mold and fungus to have the moisture they need to grow.  It also increases the localized humidity. Watering at the base of the plant helps eliminate the excess evaporation and helps get the water where it’s needed: in the soil.
4. Use well-drained pots and beds.  A raised bed with a good gravel base under the soil allows the bed to drain properly.  This will help reduce the chances of mold and fungus to grow. It also helps by elevating the plant which also increases airflow around the plant.

If you do see white spots starting like this picture shows, then use this following recipe to make a spray treatment.

Recipe For Powdery Mildew Spray Treatment

1. 1 tbsp baking soda
2. 3 tbsps kitchen oil (canola oil works well)
3. ½ tsp dish soap
4. 1-gallon water

Mix together and pour into a spray bottle.  Thoroughly and liberally apply to the infected plant.  Apply to both the tops and bottoms of the leaves and apply enough that it is dripping off.  Reapply every 2-3 days as needed.

This fungus seems to spread the most in warm, humid conditions.  Keeping plants outdoors when it’s warm and humid can be an issue for trying to control the atmospheric conditions.  

Pro Tip:  Only water at the base of the plant and keep the leaves dry.  Do not over-water.  

Bibliography

• https://en.wikipedia.org/wiki/Powdery_mildew
• https://www.almanac.com/pest/powdery-mildew
• https://www.planetnatural.com/pest-problem-solver/plant-disease/powdery-mildew/
• https://science.sciencemag.org/content/330/6010/1543
• https://www.cabdirect.org/cabdirect/abstract/20023170351
• Author(s): Braun, U.;  Cook, R. T. A.;  Inman, A. J.;  Shin, H. D. Author Affiliation: Martin-Luther-Universität, FB. Biologie, Institut für Geobotanik und Botanischer Garten, Herbarium, Neuwerk 21, D-06099 Halle (Saale), Germany. Book chapter: The powdery mildews: a comprehensive treatise 2002 pp.13-55 ref.many

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