[NAFEX] tomato blight

Michael Dossett phainopepla at yahoo.com
Mon Jul 27 11:38:43 EDT 2009

Rivka is right.  Early blight is caused by a much weaker pathogen, Alternaria solani.  It causes foliar symptoms and can infect the fruit but is a very different beast from late blight.  Late blight is caused by Phytopthora infestans and is extremely destructive.  It is the disease that caused the potato crop failures in Ireland back in the mid 1800s.  With good disease conditions, infected plants essentially turn to mush in 2-3 weeks.

I wouldn't say that this year's strain is neccessarily different except that they are all "different" now.  Until the 1990s all the late blight in this country was of a single single mating type and the disease could only perpetuate asexually.  This meant that it changed very little for many decades.  In the 1990s, the A2 mating type was introduced to this country and now late blight reproduces sexually.  This means that potentially any new infection could be of a "different strain."  Unfortunately it also means that the disease has started to evolve at a much faster rate so old resistances have started to break down and old forms of control are no longer as effective.

By the way, roguing plants will do little to curb the disease.  Its reproductive rate is so fast that it doesn't really slow down the infection process, and once you have plants that are showing visible disease symptoms they have already shed spores that are germinating on other nearby plants.  Not to mention that the spores travel so far on the wind that you are likely to just recieve more from the original source of your infection.

When it raises it's ugly head, late blight is a nasty bugger indeed.


Michael Dossett
Corvallis, Oregon
phainopepla at yahoo.com

--- On Mon, 7/27/09, Road's End Farm <organic87 at frontiernet.net> wrote:

> From: Road's End Farm <organic87 at frontiernet.net>
> Subject: Re: [NAFEX] tomato blight
> To: "North American Fruit Explorers" <nafex at lists.ibiblio.org>
> Date: Monday, July 27, 2009, 5:45 AM
> On Jul 27, 2009, at 6:49 AM, Alan Haigh wrote:
> > Yup, the media has been abuzz about tainted tomato
> plants and the blight issue here in the northeast but I
> doubt any scientific connection exists between the big box
> plants (seeds?) and the early late blight this year.  There
> has just been an incredible amount of rain.  I would expect
> my late blight to be early with 2 straight months of rain in
> the double digits with inadequate sun between storms.
> There has indeed been a great deal of rain. However, no
> amount of rain will produce late blight if the spores aren't
> present. The disease generally takes some time to get this
> far north; it does travel, but not so fast that it's
> generally present in New York etc. in June and early July;
> that's why it's called "late" blight. It isn't only the
> media going on about tainted plants, it's Cornell and other
> plant disease experts.
> >  
> > Of course my property has had early blight for well
> over a decade so I'm pretty sanguine at this point although
> my neighbor gardener says the latest blight is a new strain
> Early blight and late blight are two distinctly different
> diseases. Early blight progresses slowly enough that plants
> infected with it will often successfully produce a crop;
> it's much less of a problem.
> --Rivka
> Finger Lakes NY; zone 5 mostly
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