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BLIGHT!! (Part I.)

To the Pacific Northwest home or community gardener, few phrases strike terror to the heart like ‘Tomato Blight’.  After all, to get ripe tomatoes in our climate one must nurture tomato plants through the early season as intensively as newborns, and even then, hope and pray for the warm nights of late July through August that will provide ripening before the fast-shortening days and cooling temps of September spell doom for any still-green fruits.

I had already vaguely known that this pathogen, Phytophthora infestans, was also responsible for the infamous Late Blight or Potato Blight that led to widespread famine in Europe and the Americas in the 1840s, subsequently driving a mass exodus to America by the hungry Irish.  What I hadn’t realized was the vast geographic scale of the 1840s blight, its origins, or the breadth of the political upheaval and economic transformations it wrought – including a financial crisis much like our recent episode, not to mention the introduction of The Economist magazine.  But more on that later.  This is all about me.  At least for now.

Here in western Washington we’ve been experiencing one of our recently-typical long cool wet springs extending well into July.  (You, La Nina, begone!!)   Beginning in late June some of the gardeners in our Community Garden in Redmond Washington (see http://marymoorgarden.org/) began to report wilting, moldy leaves and brown discolorations on stems, classic P. infestans symptomology.

Late blight lesion and spores on tomato stems. From http://www.hort.cornell.edu.

By the first week of July the symptoms had spread across the entire community and some of the tomato plants in our own plots were displaying the telltale stem and petiole lesions.  Only one plant of variety Flamme succumbed entirely, but fully a third of our plants had the lesions, some severe.

Where I could, I broke off and disposed of the infected leaves and stems but it wasn’t possible to remove all the infected tissue.   As the weather has warmed and cleared more consistently through the second half of the month, the stems have flourished above the lesions, some lesions are nearly faded away, and only one small early tomato fruit out of many has shown the brown streaks of blight.

“Healing” late blight lesions on lower stem of otherwise healthy plant. My photo

Some of our potato varieties (Yukon Gold, King Harry and Adirondack Red, described as ‘medium’ for late blight resistance in the catalog) also progressed rapidly through the symptoms of late blight in the same timeframe, not surprising since the same pathogen infects both potatoes and tomatoes (we were able to dig some uninfected potatoes, though yields were light).   This wasn’t the first time we’d been through this in our Marymoor community;  two years ago, during a cool wet September a blight event essentially destroyed the abundant tomato crops that had been nurtured to near-ripeness across the entire community garden.So being a science geek (and proud of it!) I had to seek out more information about this virulent, deadly pathogen in order to treat our current infestation and prevent future ones.  Plenty of fascinating factoids emerged from the search, going well beyond the basics of cause, effect, treatment and prevention into globalization, genocide, mass population changes and the finances and politics of entire nations.  Holy cow!So first for the mundane but practical.

From www.veggiegardener.com I excerpted the following tips for preventing blight diseases in tomatoes and potatoes:

  • First, purchase or grow healthy plants of disease resistant varieties (though not many exist) and avoid those that are especially susceptible.   Look for the capital letters V, F, and N following the cultivar name.  (In our garden, the Flamme and Ananas Noire seemed to suffer particularly badly.  The Big Beef, Carolina Gold and Early Goliath all suffered from stem and leaf infection to an extent.)  Choose a range of varieties that mature at different times.
  • Rotate tomatoes, peppers, eggplants and potatoes to different spots in the garden  every year. Don’t plant tomatoes and potatoes next to each other since they both are susceptible to early and late blight, and don’t retain held-over potato plants that might sprout.  They could be carrying the blight fungus.
  • Space the plants, stake them well and remove suckers and extra leaves to maximize air movement and minimize moisture around the leaves and stems.   While suckering, remove any bottom branches or leaves that might come in contact with the soil.  Cut them with scissors even with the stem; don’t tear them off and create wound      entry points.   
  • Water only at the base of the plant and early in the day. Long periods of moisture on foliage encourage blight.   Mulching is good to reduce the need for watering  (but be sure you’re using clean mulch!)
  • If weather patterns settle in that are conducive to blight, many organic gardening sites (including Penn State Extension, http://extension.psu.edu/vegetable-fruit/blog/2011/organic-late-blight-management-1) recommend regular (every 5 days) treatment with a copper compound, particularly copper hydroxide formulations like Champ.  This is not a cure.  It only prevents the fungal spores from   taking hold on the plant tissue.
  • Monitor leaves and stems for early symptoms of blight.  Remove infected leaves and stems, and begin application of a labeled fungicide if your cropping system allows it – Serenade and Daconil seem to be recommended as effective in some of the blogs.
  • If a plant becomes severely infected with blight in your garden (whether a tomato, potato or eggplant), the best preventive measure (if you can bear it) is to remove the infected plant and dispose of it.  You can burn the plant material, or place it in a plastic garbage bag and throw it in the trash.  Never place infected plant material in a compost pile or community trash heap.   The blight spores can continue to fly until the plant tissue it’s growing on is truly and sincerely dead.    This is a critical step in preventing blight spread.    Communicate with your neighbors and agricultural extension office so that they are aware and taking preventive steps.  These infestations can (and do!) decimate crops in entire regions.
  • Thoroughly wash your hands after touching blight-infected plant tissue.  You could accidentally spread the disease to healthy plants.
  • Remove all plant debris from the garden in the fall.   (This includes ensuring that you’ve dug all the potatoes.)  Many blight organisms overwinter on dried or buried plant matter.

In a subsequent post I will share what I’ve learned about the origins of the blight, how it’s moved around through history, the emergence of new more virulent strains, and the dramatic political, economic and demographic upheavals that this pest has engendered.

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