Inoculation of Carolinas First Backcross Chestnut Orchard

July 21, 2006

Volunteers from the Carolinas Chapter of The American Chestnut Foundation help inoculate 5-year old backcross chestnut trees at Long Branch Orchard under the supervision of Dr. Paul Sisco.   The team helping Dr. Paul Sisco with the inoculation include David Nelson, Brad Stanback, Don Surrette, Carol Namkoong, Zachary Lesch-Huie, David Flood, Ilona Stanback and Dr. Paul Sisco. Each tree is inoculated in two places on the trunk—one with a weak strain of the chestnut bark fungus (Cryphonectria parasitica) and the other with an aggressive form of the fungus. Inoculated trees will be rated later in the year as to how they react to the fungal growth. Resistance to the blight comes mostly from genes inherited from the Chinese chestnut great-grandparent of these trees. A highly resistant tree will form cankers around both inoculation points, walling off and containing the growth of the fungus, enabling the tree to survive. A moderately resistant tree will resist the mild form of the fungus but not be able to contain the aggressive form. Susceptible trees will not canker at all. Uncontained, the fungus grows in concentric circles under the bark, eventually girdling the tree and killing it.

Inoculation Process

  • The area on the trunk of the tree to be inoculated is cleaned using a sterile cloth soaked in 70% ethanol.
  • Next, a cork borer is used to drill a hole into the bark of the chestnut tree to expose the cambial layer. Chestnut bark fungus will be placed in the hole and sealed.
  • Holes are bored in the trees to be inoculated at Long Branch Orchard Number 1. Brad Stanback and his family prepared the land and helped plant the trees in the orchard in November, 2001.
  • The chestnut bark fungus that is used to inoculate trees is grown on a plate of potato dextrose agar in a petri dish. A cork borer is used to cut into the outer edge of the medium, creating plugs that can be removed.
  • A plug from a petri dish is inserted into a hole bored in the chestnut tree bark.
  • A sterile spatula is used to take a plug from the petri dish and to insert the plug into the hole with the fungus-side toward the cambial layer.
  • Masking tape is wrapped around the tree trunk to seal the plug. The tape helps keep the plug moist until the fungus has a chance to grow, normally within 24 hours.
  • The aggressive form of the fungus is inserted at the base of the tree where there is more surface area where the tree can handle the faster growing strain. The milder form of the fungus is inserted higher on the tree trunk.  In a susceptible chestnut tree, the bark fungus will begin growing at the point of inoculation within 24-hours and expand outward concentrically. The fungus can girdle the tree, cutting off nutrients, and kill it.
  • Trees moderately resistant to the chestnut bark fungus grows cankers around the areas inoculated to wall off and stop the growth of the fungus. The ragged edges of the affected bark have been cut away (revealing light colored areas) to show better the cankering around the inoculation area. Some of the backcrossed chestnut trees that were inoculated at Long Branch Orchard are expected to have similar resistance. The level of resistance of each tree will be rated the fall of 2006. (Look for an album later this year which will explain the results.) After rating, trees with desirable resistance and strong American characteristics will be selected for further backcross and intercross breeding.