Why Powdery Mildew infected my pumpkins

Powdery Mildew infection is one of the most obvious: a plant’s leaves and stems – pumpkins and many others – are suddenly covered with what appears to be a white powder. It spreads within days and if not treated can pose a significant threat to susceptible plants.

cantaloupes growing in sweet corn

First week of August-healthy, green foliage and a melon growing at the edge

This summer  has been quite an adventure for gardeners in Pennsylvania and elsewhere, starting with our near-drought conditions. We weren’t officially in drought as much of the country was, but tell that to my thirsty vegetable garden. Plants barely held on if at all, and my garden hose got more of a workout than it’s seen in six years. But all in all I was doing okay, and there were no serious drought or disease issues, with the exception of some blossom end rot  on a few tomatoes. Low yields however were the norm, due to lack of water and lots of heat.

Over the past few seasons, I’ve been planting melons at the ends of the raised garden beds where I grow sweet corn. The long vines and lush foliage of melons, members of the Cucurbitaceae family of plants , provide a natural mulch for the corn, which has an extremely shallow root system, prone to drying out quickly. This year I planted pumpkins at one end of the corn bed and melons at the other. I never grew pumpkins before, because let’s face it, there isn’t much use for a dozen pumpkins in one household unless you’re really into Halloween or fall decorating. I am not. But I wanted to try.

The near-drought arrived in mid-June and with the exception of one massive thunderstorm early in July, barely a drop of rain fell for six weeks. One and 1/4 inches of rain fell on July 20th, accompanied by high winds which almost blew the corn right out of the raised bed, but that was the extent of it. The sweet corn started getting “leggy”, that is thin-looking, and the ears on the stalks looked puny – a desperate cry for water. The melons and the pumpkins were surviving but not thriving, regardless of how many inches of water I dumped on them and the corn. Their normally lush green foliage was beginning to look crisp and brown at the edges. And did I mention our multiple heat waves? With extended bouts of above normal heat, many plants become dormant in order to conserve energy and stay alive. New foliage stops appearing, growth stops, no new fruit appears as the roots dig deeper for water to keep the crown alive. This was the case as July ended.

healthy pumpkin foliage

First week of August – beautiful, healthy green pumpkin foliage. There are a few pumpkins growing in there, shaded by the leaves

Rain finally arrived the first week of August – just passing thunderstorms at first, but my neighborhood caught the bulk of them. Moisture was minimal at best, but enough to let me breathe a little easier. Then this past week, significant rain, lower temperatures (mid-80’s) and high humidity. A fungal paradise.

When you look, you must also see

I’m in the habit of at least making a brief inspection of my vegetable and flower gardens every day, depending on my schedule.  The first week of August the melons were doing very well: rain had returned the green to the foliage and more fruit buds were appearing on the vine. Same with the pumpkins – the gigantic leaves were spreading places I never thought they’d go – like out of the garden bed, across the path and into the bean patch. The sweet corn had also bounced back – the stalks and ears thickened.

But two days ago, I noticed a few white speckled spots on the pumpkin leaves. Fungus isn’t rare around here by any means, considering the humid summers we endure, but I’ve never been troubled with Powdery Mildew, so I didn’t give the blemishes much thought.  In years past, the same blemishes appeared but never led to a wide infection. But within 48 hours, those few spots had turned into sheets of white fungus on most of the pumpkin foliage. The melons were less affected, due to more air flow and more sunlight on that side of the garden bed.

powdery mildew on pumpkin foliage

Second week of August – In only a few days, powdery mildew infected a majority of pumpkin and melon foliage

I did a little research and found this excellent information from the University Of Vermont Extension:

“Once you can easily see signs of powdery mildew… it is too late to apply any treatments for control.”  Damn.

Organically-approved fungicides… work by preventing infection of healthy tissue, so starting treatment early is key to their effectiveness… established powdery mildew colonies not only do not disappear when treated, they continue producing hundreds of spores. These give the spots their powdery appearance.”   Missed my window of opportunity.

“Favorable conditions for the disease include dense plant growth, (guilty)

“low light intensity  (guilty – low light at the bottom of the dense corn patch)

“and high relative humidity  (I hate this weather).

“Not only can powdery mildew reduce yields because of decreased fruit size or number, but the premature senescence (aging) of infected leaves can also result in … fruit becom(ing) sunburned, or ripen(ing) prematurely. Pumpkins with powdery mildew infection often have poor rind color and shriveled handles, and speckling or other imperfections on their rinds.”  Case closed.

pale and dying pumpkin foliage due to powdery mildew

Leaves prematurely age and die from powdery mildew infection

How to combat Powdery Mildew infection

I removed and bagged (do not compost!) as many infected leaves and stems of the pumpkin and melon plants as possible without depriving them of so much foliage that they can’t conduct photosynthesis and ripen the fruit. I also removed any corn stalks that weren’t producing ears and those from which I’d already harvested ears. This should increase air flow around the plants, which will help to keep the foliage dry.

I also mixed a batch of baking powder solution to fight powdery mildew , poured it into a pump sprayer and went to work on all of the foliage -pumpkins and melons alike. While the baking powder solution won’t kill the powdery mildew already on the plants, it will coat the foliage, stems, and fruit not yet infected and will hopefully keep the infection in check. Much will depend on the weather and humidity levels.

I’ll know soon if my efforts worked and will update this post with my progress. Hopefully I’ll be using at least one home grown Jack O’ Lantern this Halloween.

Update, August 24:

Alas, my efforts failed. I used the baking powder solution above, a milk spray solution , and a diluted hydrogen peroxide solution of various strengths over the last two weeks, but the pumpkins are a loss. Once I discovered the powdery mildew, it was too far advanced to control with organic solutions. It appeared to be checked at first, but when more rain fell, the fungus quickly spread to new foliage. I now have two small pumpkins in the garden, attached to vines that are withering.

powdery mildew on pumpkin foliage

Closeup of powdery mildew on pumpkin foliage

All of the information I researched made clear that the solutions I sprayed on the infected plants are to be used for prevention of fungus before symptoms appear. I was holding out hope that I could control it and save the pumpkins and melons. Once powdery mildew or any fungus attacks your garden to this extent, it’s very difficult to kill or control and the best solution is to remove and dispose of all infected plants to keep it from further infecting your garden. Powdery Mildew can also overwinter, so plant resistant varieties in that area the following season.

Buy on Amazon: Seeds of Change Certified Organic Howden Pumpkin Seed

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About Todd Heft

Todd Heft is an organic gardener and freelance garden writer who lives in the Lehigh Valley, PA and has gardened for most of his life. When he isn't writing or reading about organic gardening, he's gardening. His first book, "Homegrown Tomatoes: The Step-By-Step Guide to Growing Delicious Organic Tomatoes In Your Garden" is available on Amazon now. Google

2 Responses to Why Powdery Mildew infected my pumpkins

  1. Jeremy Gonzalez says:

    Powdery Mildew looks like talcum powder, on the leaves and stems of plants. There are actually several types of powdery mildew fungi, but they all look basically the same. You may not notice a problem until the top surfaces of the leaves turn powdery, but powdery mildew can also affect the lower leaf surface, stems, flowers, buds and even the fruit. Vines may also wither and become whitish. It is difficult to control powdery mildew once it begins spreading on the vegetable plant leaves. In advanced stages, however, powdery mildew can cause foliage to yellow, curl or turn brown, and eventually defoliate the plant prematurely. On flowering plants and trees, the fungus can lead to early bud drop or reduced flower quality.

  2. Janet Sullins says:

    Those damn squash beetles are terrible and spread disease….