Genetic Sequence for Late Blight Resistant Tomatoes Cracked?

Until now not much luck has been had in breeding late blight resistance into tomatoes.  A number of people have tried, but in general either the plants have late blight resistance but don’t produce interesting tomatoes, or they lose their blight resistance as they are bred to produce nicer tomatoes.  A few good quality tomatoes have been developed with some late blight resistance, but their resistance is spotty and generally very geographically localized.  As late blight inevitably evolves, these tomatoes lose their resistance.

Four genes have been identified that seem to be related to late blight resistance: Ph-0, Ph-1, Ph-2 and Ph-3.  It was pretty quickly determined Ph-0 and Ph-1 didn’t really offer much long term resistance, so now people’s attention are on Ph-2 and Ph-3.  Like I just said, until now there’s been some interaction between these and other genes, that seems to prevent a good quality tomato from being developed with late blight resistance.

Last year Tom Wagner bred a tomato with both Ph-2 and Ph-3 genes called Skykomish.  What was unusual about this tomato was the Ph-2 and Ph-3 genes were homozygous.  What this means is since genes are always in pairs, each half being inherited from one parent, the Ph-2 and Ph-3 late blight resistant genes in these tomatoes were inherited from both parents and existed as an identical pair.

The results last year were spectacular!  I will try to find a copy of the picture to link to or post, but late blight came through a field of Tom’s tomatoes,  destroying every one, except leaving his Skykomish plant standing unscathed.  I want to be clear, this is a good tasting high quality tomato.

This year, Tom has been trying to verify that late blight resistance does in fact depend on the genes being homozygous, and the results look very promising so far.  Tom recently posted the following in his discussion forum:

There is one area where I plant tomatoes every year. It is usually the first to come down with late blight and of course, I do not every spray for control. I think the blight is fully entrenched in the soil here and no effort is made to clean up the debris in any year.

It appears that Ph-2 and Ph-3 are still operative in controlling the outbreak.

Make My Day is one that has both the Ph-2 and Ph-3 gene in a homozygous pairing. No blight

Matt’s Wild is homozygous for Ph-2. No blight.

The tomato hybrids that have Ph-2 from one parent have slight resistance to the foliage, but not as much control on the fruit as I would like.

Hybrids with Skykomish which is homozygous for Ph-2 and Ph-3 have slight resistance.

I will check the progress as the blight burns through. It started about three days ago.

With many, many hybrids of these lines being put away as F-2 seed, a fuller evaluation for 2011 will be to look for segregations that are true breeding for the best blight resistance/tolerance. Homozygous traits need to be found in every time of tomato, whether is a red, bi-color, green, striped, black, yellow, etc.
Tom Wagner

There’s still some work to be done to verify the resistance, but it looks pretty good so far.  Next year I will grow the Skykomish in Europe, and see how it stands up to the blight here.