02 March 2009 |

Preventing Powdery Mildew in Your Spring Garden

Written by Jason

 For the last 4 or 5 years we have at least one powdery mildew infection in our vegetable garden every season.  Last year we lost all of our zucchini and squash plants to powdery mildew, and the year before that we lost all of our pumpkins to powdery mildew.  The powdery mildew usually sets in after our plants are well established and fruiting.  We have successfully treated vegetable plants in the past that made a full recovery, and we have also lost many plants when we failed to treat them quick enough.  I expect this year will be no different, but I plan to be prepared to react at the first signs of the white powdery spots.  We may also try some preventive measures this year and of course we will be sharing our results with you. 
It is easy to recognize powdery mildew in your garden.  It usually affects the leaves and shows up as white powdery spots on top of the leaves.  If left untreated it will quickly spread to the surrounding vegetables.  Although the powdery mildew does not usually attack the fruit itself, it can kill the plant or otherwise weaken and diminish fruit production.  We have found in the past that fruit from plants that have recently become infected can be harvested with no problems, but once the powdery mildew is established new fruit production stops and the infected plant soon wilts and dies.   
In my research into methods to prevent and treat powdery mildew, I have found several suggestions for powdery mildew prevention.  The most common suggestions ..........

..........are listed below, followed by comments about how this relates to our experience here at the yardstead:

1.  Select powdery mildew resistant varieties.

This suggestions seems more suited for landscaping purposes as many ladscape plants also have problems with powdery mildew.  We are trying to incorporate more 'heirloom' varieties of vegetables in our garden so we can harvest viable seed as well as fruit from our garden vegetables.  Since we aren't  willing to compromise on plant varieties, we pretty much ignore this suggestion.

2.  Locate plants in direct sunlight with good drainage.

This main purpose of this suggestion is place plants where the leaves are in direct sunlight which seems to inhibit mildew growth and free from excess moisture which promotes mildew growth.  We always plant our cucurbits in direct sunlight and our entire garden can be considered to have good drainage.  This suggestion may help but since we always follow these guidelines and still have powdery mildew problems I can't really say.

3.  Avoid crowding plants together.  Allow space for adequate airflow which helps prevent mildew growth.

We have seen evidence in our garden that seems to validate this suggestion.  We always see the fastest spread of powdery mildew in crowded sections where the vegetation is dense in the garden.  The close proximity of the plants speeds up the spread of the disease which is transmitted by tiny spores which develope in the white patches formed on the leaves. Our squash and zucchini plants usually get quite large and although we provide adequate spacing in the garden, after a few weeks of growth the leaves of the plants in each row are crowded together.  Although it seems fairly evident that spreading out the plants would help prevent powdery mildew, we really don't have the luxury of additional space.  I would recommend following this suggestion if you do have the space.

4.  Avoid excess nitrogen.

Powdery mildew has been shown to thrive on plants treated with high nitrogen fertilizer.  Often these high nitrogen ferilizers produce dense tender leafy growth that is very succeptible to powdery mildew infection.  A better practice is to use an organic fertilizer such as compost which releases nitrogen at a slower rate.  We only use compost for fertilizing so we follow this suggestion by default.

5.  Water plants in the morning so the daytime sunlight will have to dry the excess moisture form the leaves.

This is another suggestion to minimize the excess water on you plants leaves.  Although we usually do follow this suggestion and water our plants in the morning, sometimes our schedule gets messed up or a late afternoon or evening rain wets the plants anyway.  This really seems to make a difference as every outbreak we have seen is usually preceeded by some very wet weather. We have also seen seasons where we watered our plants with a drip system that never sprayed water on the leaves and did not have a single powdery mildew infection.  this seems to be one of the most important suggestions for powdery mildew prevention.
Well as you can see we follow some, but not all of the most common suggestions for powdery mildew prevention.  I believe that all the suggestion are very relevant, but not always an option for us.  This year we will be dilligent about keeping our foilage dry.  If we do see another outbreak of powdery mildew our plan is to respond with treatment as early as possible.  We will be eperimenting with two different treatments for powdery mildew.  One is a milk based spray treatment and the other is a baking soda based spray treatment.  You can find the recipes and details about these treatments here in the vegetable gardening forum.  We will be using both recipes in seperate sections fo the garden, so check back in a couple of months for the results.  I will post another article with more about powdery mildew prevention as soon as we have another outbreak.  In the meantime if you have any questions, comments or suggestions, drop us a quick note in the forum.

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