Recently skimmed a paper about hybridization between Eastern White pine and Himalayan to get blister rust resistance. The problem has been that the resulting hybrids don't have sufficient cold hardiness, so they are back crossing into P. strobus. Typical breeding program procedure. I was curious that they chose P. wallinchiana. Which strobus pines can hybridize?
Most of them can hybridise fairly easy, though there's a few exceptions - Pinus lambertiana doesn't cross easily with anything, nor do P. albicaulis, P. cembra and P. sibirica, except with each other. P. wallichiana was likely chosen because of its fast growth (a lot of white pines are very slow as seedlings, but not this one) and ornamental quality (glaucous, drooping needles, and 20-30 cm cones). If you want a hardier blister-rust resistant white pine to cross with P. strobus, then try P. peuce (Macedonian Pine; zone 3-4 hardy) - but it is somewhat slower-growing, particularly as a seedling.
