Can you compost diseased plants, namely, powdery mildew leaves? There are a lot of conflicting answers to this question, but maybe this article can help to clarify some of them using a bit of research and, of course, personal experience.

First, let’s look at the compost bin and how it works. Personally, I don’t think it’s possible to manage common garden diseases in an environmentally responsible way without a compost system.

The three-bin compost system

My compost system employs the standard three-bin system. I have a compost bin which I am currently filling until the end of this year (2021) which will eventually get thrown on the garden beds in spring 2023. I also have an old compost bin that I filled last year (2020) which I will use next year (2022) and another compost bin from 2 years ago (2019) which I am using this year for planting and topping up the garden beds.

This is the reason why you need at least three compost bins for effective composting – each bin needs on average a year to decompose, during which time you shouldn’t add more organic matter to the bin.

Put simply, you need three compost bins for a three-year plan…

  • FIRST YEAR: A bin you are currently filling, 
  • SECOND YEAR: A bin that you filled last year and is currently decomposing, and 
  • THIRD YEAR: An old bin that has finished the decomposition process and that you can spread out throughout the year.

Coincidentally, this three-bin system also helps to manage powdery mildew and other diseases in your compost.

  • FIRST YEAR: The bin you are currently filling has powdery mildew infected plant matter and their spores. The whole bin gets turned over at the end of the year and I throw a layer of mulch on top to prevent anything “escaping” from the compost on the wind.
  • SECOND YEAR: The bin is half-decomposed and may still contain small amounts of spores that over wintered in the infected plant material, but without a fresh source of plant material, they will eventually die off over the summer as the compost pile heats up. Scrape back the mulch and turn the whole pile over again.
  • THIRD YEAR: The bin has finished decomposing and is safe to use on the garden. The number of spores in it is no greater than what is naturally in the environment anyway.

You can safely compost powdery mildew leaves because the process of composting makes it impossible for powdery mildew spores to survive. When the compost is finished, it no longer contains discernable plant matter for the powdery mildew to feed on, so the fungus eventually dies off.

A few facts about powdery mildew

Now let’s look at the powdery mildew in a bit more detail. The fungal spores and hyphae require orgainc material from the host plant to be able to survive the winter. Host plant? Yes. There are a wide variety of powdery mildew fungi which are adapted to specific plant hosts. For example, the powdery mildew that infects your melons is not able to infect your legumes or onions because it is not even in the same genus.

Many powdery mildew species utilize both sexual and asexual reproduction, but it is the sexual stage that gets them through the winter. Luckily, even the sexual stage is host-specific and an obligate parasite, so requires host material (preferably fresh, green growth) to survive the following spring and summer – something it is unlikely to receive while being buried in the compost bin during its second year of decomposition.

With regards to powdery mildew on annuals, if you have successfully removed all of the host plant matter (unlikely, but we can all dream…), then the powdery mildew spores in your melon garden bed won’t have a host to overwinter in. The remaining spores will eventually die off as long as you don’t replant the bed with melons or gourds for the next two years.

With respect to powdery mildew on perennials, I remove as much of the diseased plant matter as possible, especially closer to the ground which increases light and ventilation. Then I rake around the base of the plant to remove any fallen leaves and everything goes straight on the compost pile along with an armful or two of weeds and/or autumn leaves on top.

Finally, give the plant a top-up with some thoroughly decomposed compost. This only decreases the risk of the plant developing powdery mildew next spring since there is a good chance that the fungus still remains in the dormant buds over winter.

Powdery mildew doesn’t require water for the hyphae to colonize a leaf, but it will proliferate in humid conditions and quickly produce spores which subsequently become wind-borne and rapidly infect other plants.

This ability to become easily windborne may be one reason why some people are against composting powdery mildew, but an extra armful of organic matter thrown on top of the diseased plant matter makes a decent physical barrier against any wind dispersal out of the compost bin, at least, no more than what is naturally floating around us every day. Let’s face it, that’s probably how it got into your garden in the first place.

Compost vs Powdery mildew

Now it’s time to look at how a compost pile can kill powdery mildew.

Temperatures above 90° F (32° C) for at least 12 hours will halt a powdery mildew infestation and weaken the spores and hyphae on the plant, in the soil, and in your compost pile. Most weed seeds and pathogens, including powdery mildew, are killed at temperatures above 130° F (54° C) for 10 consecutive days.

In my experience in warm climates, a single hot summer is enough to manage powdery mildew in the compost bin. Cold climate compost bins are less likely to reach these killing temperatures, but the temperature can be raised each summer by placing a clear plastic sheet over the whole bin to mimic a greenhouse

Composting powdery mildew doesn’t have to be complicated. As long as there is enough brown and green plant matter evenly distributed through the compost pile, there will be enough energy and heat produced to at least break down the plant matter and starve the host-specific fungi. This is why it’s recommended to turn the pile over at least once a year to mix those autumn leaves in with the weeds you pulled out in spring.

Often in small gardens, the compost system is a single bin tucked away in a dark corner where little grows anyway. In these situations, it will be very difficult in cold climates for the compost to reach the desired temperature to kill most of the fungi, even with a plastic cover. This is where I would suggest employing a pair of long-term compost bins

Questionable advice

Be wary of people advising not to put any plant matter infected with powdery mildew in the compost; for fear of “contaminating the whole compost bin”. This implies a lack of understanding of how the nutrient cycle works, the life-cycle of powdery mildew, and the prevalence of fungi spores and hyphae around us, everywhere. When done correctly, (i.e. with enough brown to green, time, and heat), by the time the compost is ready to use, the spores in the compost bin have essentially died from starvation. 

Perhaps some people have had problems with powdery mildew after spreading compost after only one year of decomposition. In this case, the sexual spores that have overwintered in the half-decomposed plant matter can become wind-dispersed when the compost is spread around the garden, instead of being contained and “starved” within the compost bin during the second year.

There are plenty of people advising to bag up anything powdery mildew related – plant matter, roots, soil and all, and throw it in the rubbish bin to be sent to landfill! Such a shame! Perhaps they are incredibly risk-averse? Is it possible that their compost bin is in a poor location and doesn’t get hot enough? Maybe they are impatient and never give the pile enough time? Or do they assume a wind-borne infestation from the neighbor’s garden originated from their own compost bin?

In any case, I recommend letting mother nature do her job and allow the natural processes in the compost bin deal with powdery mildew.


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