Wednesday, August 3, 2011

Powdery Mildew 101

Hi all!  I am not sure if this post will be helpful to you, but I hope so. 

The past week has been hectic around the Gonzales homestead.  My garden had a late start due to a freeze in late May.  Now we have plentiful zucchini and yellow squash and the cucumbers and even apricots are maturing this week too, so it looks like things are only gonna get busier for a while.  I just bought my first pressure canner online.  I am so excited.

meanwhile, our garden has been under the influence of hot, HUMID, and breezy conditions.  Peculiar because we are at about 4000' elevation in Southern California where July and August are usually dry, very dry.  Our town averages less than 11" rain annually.  We are in the digger pine belt of the Sierra-Nevada foothills.

As you can imagine, humidity + heat = plant disease/insect pest incubator.

Needless to say, we were unceremoniously introduced to a new sort of pestilence this week and have been under attack, but we are fighting back, and so far have been successful at keeping this foe at bay using only homemade organic remedies.  I will keep you posted in case anything changes. 

The University of California Integrated Pest Management website aka (UC IPM) is my favorite agriculture website.  This link gives very detailed information about powdery mildew on vegetables:  UC IPM Pest Notes- Powdery Mildew

Here is a longwinded summary of what has been going on around here...

July 26- Tuesday, my friend Michelle accompanied me into my garden and something unusual caught my eye; what appeared to be a long, white rectangular smudge of powder from a bird overhead on one zucchini leaf.  My heart stopped for an instant as I recounted all of my classroom instruction dedicated to crop science, plant biology, and pest management.  So, I pulled that leaf off and discarded it right away.

My head told me that this could be a fungus, but my heart screamed NO!  It didn't make me feel any better when Michelle calmly told me she had this white stuff last year and it killed her infected plants.  I double checked the entire plant and bed and found no more suspicious foliage.  Certain that I had done everything correct, adequate soil, water and amendments, good sanitation and cultural controls (clearly I was in denial.)  Besides, this patch of white powder looked nothing like powdery mildew.
That fungus develops as white powdery circles on plant vegetation like these.
and as it colonizes and spreads it gets even uglier like this below-
(image taken from
If left unchecked, it wont be long before you see yellowing (chlorotic) tissue below 
followed by brown, hard, dead, (necrotic) tissue and eventually a dead plant. 
Image from Howard F. Schwartz
I have never seen powdery mildew in my garden in 16 years, so why should it happen now, right?

That night I couldn't sleep.  My mind was full of racing thoughts about the garden.  The leaves looked healthy, no chlorosis, no burn, no die back.  There were no small powdery white circles anywhere that I could see.

July 27- I was up at dawn and practically ran outside at first light to check undersides of leaves, stems, and every plant in my garden.

No real surprise, it was Powdery Mildew.  In that short period of time overnight, a few spores had spread to a bed of plants that included eight zucchini, two spaghetti squash, 16 cucumbers and two pumpkin plants.  I knew that I had to remove all infected leaves and relocate them carefully into a trash bag immediately.  Then, I had to use a fungicide and kill it before it spread even worse.  Powdery mildew infestations can kill a plant within days without treatment.  I am determined to maintain an organic garden free of traditional pesticides so I spent much of Day 2 performing internet research and sporadically checking plants and removing leaves with spores.

Eventually, I found a few articles that sounded promising like this one:  Appalachian Feet  Apparently, commercial wine grape growers in New Zealand and farmers of cucurbits (like squash) in Brazil have been experimenting with a fungicide made of 9 parts water to 1 part milk that has been almost as effective as traditional methods of control, especially when used as a preventive measure.  A bit more research and I found out that this milk fungicide works because somehow the sun triggers a chemical reaction that makes the milk toxic to the fungus.  Sounds like a plan! 

Desperate, I quickly mixed up a batch, and violated one of the cardinal rules of spraying.  I sprayed at NIGHT!  In fact I sprayed by flashlight at 10pm.  I sprayed and waited... The recipe can be found on the link above at Appalachian Feet.

Most experienced pest control advisors and plant biologists would probably attempt to dissuade me from spraying anything at night, whether oils or something homemade like this because spraying then would increase humidity overnight (the peak growing time for fungus) and we do NOT want to nurse the fungus, we want to create inhospitable growing conditions. They generally advise to spray during early morning hours, preferably with no wind, early enough for foliage to dry before intense sun to prevent sunburned foliage, and plants should not be sprayed during temperatures of 90 degrees and above because doing so can actually burn and kill the plant.  Oil sprays work by suffocating the spores, but they're also covering plant pores making it harder for plant respiration (cooling) to occur. If you can avoid spraying during hot days when temperatures will get to 90 or higher it is usually recommended to not spray then either, but I was desperate and HAD to spray.  I was going to lose plants if I failed to eradicate the fungus anyway and I figured I could nurse the plants along with a light watering of cool water throughout the heat of the day at root zone only, no foliar watering.

July 28- Next, I started a log recording dates, what I applied, time of application, daily high and low temperatures and relative humidity.  Then I checked in on my plants regularly inspecting foliage for new outbreaks and any other new symptoms that I could attribute to either the fungus or chemical sunburn and documented this information.  I soon realized that the disease was spreading much slower than it had the during the two previous days, before the milk solution was sprayed, but I needed more time to know for sure.

July 29- The sun disappeared behind clouds and a thunderstorm kicked off somewhere in the distance.  My milk fungicide works because of a chemical reaction created by direct sun exposure.  No sun = no fungicide.  I had noticed that the disease was slowing down movement among plants before the sun took a vacation.  That was encouraging.  I continued to remove infected foliage and carefully watered lightly at root zone, no foliar water applications.  I also added new mulch to base of each plant in bed to help absorb excess humidity.

July 30- Still, no sunshine, and although I was still getting new outbreaks, there were much fewer leaves affected daily.  I went from 2 trash bags full of leaves and stems during the first 48 hours to less than a handful of leaves and stems during the next 48 hours.  However, I was just slowing the disease down, I hadn't yet been successful at stopping it from spreading to new foliage so I went online again.  On another organic gardening website I saw a recipe for a baking soda and Dawn dish soap concoction for Powdery Mildew.  Baking Soda Remedy Recipe I mixed it up and sprayed it.  Doing my regular inspections I found that this concoction was even more effective at slowing progress of powdery mildew than the milk mixture.

After 24 hours on foliage I saw no new leaves or stems demonstrating new powdery mildew infection.

July 31- Early morning.  A new powdery mildew outbreak, (one leaf) other side of garden, on giant climbing pumpkin.  This was my fault because I was so worried about causing severe damage to plants by using too manny sprays too close together that I only sprayed the garden bed that was affected with the baking soda mixture.  I believe that had I sprayed the entire cucurbit family of plants with the baking soda initially, the disease would have been eradicated then. 

In my Integrated Pest Management class, I was taught to change products/methods of control frequently to prevent or slow down an organism's ability to become immune or tolerant to one particular method of control so after I removed infected foliage from this one pumpkin plant, I mixed up the slightly less effective milk mixture again and resprayed the entire garden this time.

August 1- I check frequently during the day, but at this point, I am crossing my fingers.  It appears that I have escaped this scrape with fungus with only aesthetic losses to plants ie: three trash bags of photosynthesizing leaves and stalks gone, but plants intact.  There has been a little evidence of sunburn on a few leaves.  I expected that because of the high temperatures and frequency of spraying that I put them through.

Conclusion- I was very lucky.  I attacked the fungus immediately utilizing several strategies.
  1. Inspections and removal of infected material- critical to successful eradication of any disease.  Removal of affected leaves and stalks to plant base ( as close as can be done safely) is essential
  2. Removing the affected debris from the yard is very important; spores are airborne, had I put the foliage in my compost pile it would be everywhere by now.
  3. Adapt routine ie: how and when to water and the addition of new mulch to absorb more humidity.  Think about ways to minimize optimal growing conditions for disease.
  4. If cultural controls above are not enough, research and select an acceptable fungicide.  I have been pleasantly surprised by the efficacy of the milk and baking soda concoctions I found online.  They definitely slowed down the spread of the fungus after application.  They seemm to work as well as Neem Oil or Horticultural Oil, which I do have experience with in an orchard settings.  Maybe I just got very lucky, but it was a huge surprise to me that the plants didn't all burn up.  Can these homemade remedies really be less harsh?  I am not certain, but I sure hope so. 
  5. It's also important to note that if I ultimately fail to eradicate this fungus, the next decision will be to pull out all infected plants.
I am happy to report that Day 8 is over and I am still Powdery Mildew FREE!  Just checked at 7:30pm.  Yippee!  I also want to point out that I was a little concerned about how often I could safely treat my plants with either method because any time we spray something on the leaves of our plants we are stressing the plant, some. But so far, I feel confident that I am not seeing widespread sunburn damage so I am going to follow advisory of Appalachian Feet article and continue to use the milk and baking soda concoctions alternatively as preventative measures for the rest of this humid season by applying them in 7-10 day intervals.  If next year is as humid and hot as this season I will use them before any symptoms, to help prevent a welcome mat from developing.  

If you have any gardening questions, please feel free to contact me and even if I do not know the answer  I will endeavor to find it for you.  The UC IPM website is an invaluable resource for all of us within this region. Navigate it.  There are so many additional sites linked to it that are written exclusively for backyard (small scale) farmers, and orchardists.  It's a gem.

Amy over at Homestead Revival is responsible for my participation in this Barn Hop.  Please check her blog, if you haven't already.  It's amazing.  Here's a quicky link: Homestead Revival

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