The US Forest Service Blister Rust Program in Camino, CA goes through the following procedures to test our candidate seed and determine resistant trees.
Ribes spp. (gooseberries, currants) are infected with WPBR spores in the springtime.
Ribes spp. thrive under ideal growing conditions in the summer.
WPBR uredeniospores develop on the underside of the Ribes spp. leaves.
By autumn, the WPBR telia hairs are well formed and able to infect pines.
The Ribes spp. leaves with their infectious telia and badiospores are arranged on metal screens.
The Ribes leaves are placed above the seedlings being screened for resistance. Samples are taken to ensure the seedlings are fully inoculated.
The inoculated seedlings continue to be cultivated; those that are not resistant to WPBR will develop indications of infection on their needles.
The seedlings are inspected for signs of infection. The yellowing seedling on the left is susceptible and will die. The seedling on the right is healthy and resistant, which indicates that it came from a resistant parent tree!