In reviewing some of the internet sources I discovered the following:

Indeed this outbreak this year can and has struck fear, perhaps even panic, into the hearts of many gardeners and agriculturists in the Northeast and elsewhere. According to a report published two days ago – http://www.post-gazette.com/pg/09228/991203-455.stm – the outbreak now stretches all along the eastern seaboard from Florida to Maine, and a quick Google also shows it in Canada and many Midwestern states, all confirmed by the respective State University Pathologists. The news accounts repeatedly use the terms “disaster” and “devastate” to describe the effect on the tomato harvest.

It is now, and may continue to be, a significant challenge in the coming years, and you are right to look for very specific information in your attempt to meet this challenge, some of which follows.

The specific answers to some of your questions, I think, can be found at the Cornell site listed above and relisted here.http://vegetablemdonline.ppath.cornell.edu/factsheets/Potato_LateBlt.htm

Cornell Pathologist, William E. Fry writes, “Phytophthora infestans affects several different plant species and has the potential to cause devastating disease almost everywhere potatoes are grown. It is also a serious pathogen on tomatoes in cool, wet climates. In central Mexico it is a parasite or pathogen of many different wild Solanum species. In Canada and the United States P. infestans has been reported to infect hairy nightshade (Solanum sarachioides), bittersweet (S. dulcamara) and Petunia (Petunia hybride) in addition to potatoes and tomatoes. In South America it has been reported as an important pathogen of pear melon (S. muticatum).”

Rereading this, in light of your comment, I was ashamed to realize that I confused Pansies with Petunia, so little do I know about these little flowers. My mistake. Thanks for calling my attention to this and clarifying the record. Boy, am I going to catch heck from Leslie! (update: I didn’t give him heck but I did correct the error. LL)

Whereas the pear melon (S. muticans), is a Solanaceous plant which has become a host of P. infestans, it is other species of Phytophthora (P. capcisi and paracitica) which affect true melons: According to plant scientists at Cornell, “Phytophthora blight is a disease that should be a concern to all cucurbit growers. It has been described as the ‘most destructive disease of cucurbits’ because ‘nothing causes greater loss’. Total crop loss has occurred in some fields …While all cucurbits are susceptible, squash, cucumber and pumpkin seem to be affected more commonly than cantaloupe. It has been increasing in importance in the U.S.” http://vegetablemdonline.ppath.cornell.edu/factsheets/Cucurbit_Phytoph2.htm

You also wondered how a pathogen specific to one plant might spread to another, especially one from another family. Part of the process seems to involve the ecological conditions that favor transmission of the pathogens. Consider the following:

The same weather conditions that favor the spread of P. infestans also favors the spread of capcisi and parasitica. For the latter two species, it appears that both mating types may currently be available, at least in some fields in the Northeast, therefore allowing for genetic shuffling of the genes and overwintering oospores, factors which in turn facilitate rapid adaptation, and evolution. In addition, according to the Cornell site, “The fungus that affects cucurbits [capcisi] can also cause blight in pepper, fruit rot in eggplant, and buckeye rot in tomato. Buckeye rot can also be caused by Phytophthora parasitica”.

Geneticists warn that when two or more pathogens inhabit the same cell at the same time quite large jumps in the presence, strength, and expression of the pathogens are possible. The jump is facilitated via a mechanism called ‘plasmid exchange’ and can leap not only from species to species, but also from genus to genus, family to family and even kingdom to kingdom. It is evolution on the move. If my memory is correct Jonathan Weiner describes the process well in the last chapter(s) of his Pulitzer Prize winning book, The Beak of the Finch. http://en.wikipedia.org/wiki/The_Beak_of_the_Finch By this process, a bacterium can develop resistance to an antibiotic via normal evolutionary pathways and then ‘confer’ that resistance to entirely unrelated organisms (bacteria, viruses, and parasites) that have never been exposed to the antibiotic.

A related process involving ‘jumping genes’ (transposons) can also shift genes around very quickly. About half the genes in corn (Zea maize) for example seem to have resulted from this process. Once transferred, these transposons can force their way through the entire population quickly. See http://en.wikipedia.org/wiki/Transposon for an overview and examples.

All of this does get complicated. In my haste to put this article out, I omitted some of the some of this documentation and exposition, and did collapse some of the material. My statement about Pansies and Melons was in error. Yours is correct.

You are also right that “Nightshades are the common name for the Solanaceous family”, and this would be redundant if read without the modifier “certain” in my statement, the intent of which was to stress primarily that rapid adaptation appears to be underway allowing Phytophthora species to infect other plants within (and outside of) their respective ‘obligate’ families (and secondarily to remind us that effective tactics of control may also need to include rooting out these other hosts which also may be infected.)

Thanks for your interest in Leslie’s Blog and your thoughtful comment. I hope your Solanaceous plants have escaped this Blight this year, but if not, consider that even Martha Stewart, with her crisp management style, teams of dedicated workers, and buckets of money couldn’t escape it! http://www.themarthablog.com/2009/08/the-tomato-blight-in-my-garden.html

As I said before, Yikes!

Nightshades are the common name for the Solanaceous family, so it is redundant to say that it also infects them. When you say bittersweets, do you mean bittersweet nightshade, a weed in the Solanaceous family, or the bittersweet vine? And could you tell me your information source for melons and pansies being infected by P. infestans? I had read that petunias could be infected & transmit to tomatoes, which makes sense as petunias are in the same Solanaceous family. But it seems strange to me that melons and pansies would be infected.

I don’t mean to be nit-picky, but when dealing with such a destructive pathogen, any bit of misinformation can cause a lot of trouble, even panic.