Bought four Kieffer pear from Century farms in 2014. Three out of four died first spring from fire blight. I was upset because they where supposed to be resistant to blight. Bought three potted Kieffer in Fall of 2014 from Home Depot on clearance. Last Spring and this year my Century farms tree produced fruit but the HD trees are not. The HD trees look nice and flowered out. What do you think is the problem?