We have blackberries that came with the house along a block wall about 15 feet long. Trellis consists of wires screwed into the wall. The first picking (about 2 weeks ago was a little early). The fruit we are now getting is smaller, some to a point where the fruit is all seeds and very little flesh. What is causing this? We also planted some thornless blackberries 2 years ago at one end. That fruit is longer, more like a mulberry. Fruit on the newer plants also appear smaller than last year. What could be causing this? If I can feed to rectify it now, we have about 1/2 of the fruit that is ripening left on the canes. Otherwise, we will have to try your suggestion for next growing season. Thanks.
Environmental effects, lack of pollination are possible reasons.
Here are some links with more information.