Unfortunately, 'Explorer' has not really been a success. I've seen it twice, both times under tunnels: once in Pennsylvania, where it had virtually no fruit and a powdery mildew problem, and another time in upstate New York, where the plants looked healthier but fruit set was still poor, though better. I was rather disappointed, as I'd been pretty excited about the thing.
Tallman's message today explains a big part of the problem: 'Explorer' is not self-fertile. Apparently his field featured things that flowered and provided adequate pollen at the right time, so the problem was never evident under his conditions. This fits with what I saw: the tunnel at Penn State where I saw it had, if I recall, only one other variety in it, while the one in NY, where it had at least some fruit, had several.
While unfortunate, this isn't entirely shocking, as self-incompatibility is fairly common among wild, diploid Rubus, and 'Explorer' is not far removed from the wild source of the primocane-fruiting trait that Tallman discovered. (Not surprisingly, the trait hasn't persisted very long in most commercial types).
Anyway, all is not lost. Tallman has selected another primocane-fruiting black raspberry, dubbed PT-2A4, which does pass the self-compatibility test, and has other desirable traits compared to 'Explorer'. As he describes it:
"Compared to Explorer, the PT-2A4 berries are larger, higher drupelet count, and smaller seeds. PT-2A4 holds my all-time record for a single primocane black raspberry at 3.82 grams. Admittedly, that's a max berry, not an average, but I gotta track something, and average isn't awfuly interesting. Maybe with a little fertilizer this year I could break 4 grams. Unfortuantely, PT-2A4 hasn't captured the reduced thorniness of Explorer, so there remains further breeding down the road to see if I can tie that trait back in again."
He also included a link to his website, which includes a page for PT-2A4.
