BIOLOGICAL IMPROVEMENT OF CHESTNUT AND MANAGEMENT OF THE CHESTNUT PATHOGENS AND PESTS
Investigators: MacDonald, W. L.
Performing Department: PLANT & SOIL SCIENCES -- 1825
Start Date:
10/01/1998
Termination Date:
09/30/2003
Reporting period: 01/01/2000
to 12/31/2000
Progress Report:
A large-scale experiment to deploy hypoviruses as biological control agents
for chestnut blight is underway in a forest stand of American chestnut growing
near West Salem, WI. From 1992-94, a MI hypovirus associated with the blight
fungus, Cryphonectria parasitica, was released in the stand by treating infections.
A second hypovirus, obtained from an Italian C. parasitica strain, was introduced
from 1995-97. No hypovirus introductions have not been made since 1997. Disease
severity and hypovirus transmission have been recorded during the spring of
each growing season by survey and culture of the infecting strains. As of the
2000 growing season, approximately 10% of 5000 trees are infected and harbor
1600 cankers. Two-hundred-twenty newly diseased trees were discovered in 2000.
The European hypovirus is most commonly associated with infections and has been
recovered from 80% of the infections on trees treated with hypovirus. Only 25%
of the infections on non-treated trees contain hypovirus. Another field study
used an orange pigmented virulent strain of C. parasitica to evaluate the effect
of mycelial age on hypovirus transmission in the field. This isolate was used
to initiate cankers that were vegetative compatible with a brown-pigmented strain.
Each month the expansion of the cankers was traced with a permanent marker.
In October of 1998, cankers were challenged at their leading edge with the mycelial
plugs of a hypovirus containing isolate. Control cankers were treated similarly
except the challenge was hypovirus free. Beginning in November 1998, the traced
canker rings were sampled monthly for 7 months using a bone marrow biopsy tool.
The bark plug samples were cultured and the colonies were recorded for their
origin and their virulence was scored based on morphology and pigmentation.
Highest recovery of hypovirulent isolates occurred in the outermost rings of
the cankers with little evidence of hypovirus colonization of portions of the
cankers formed prior to treatment. A third field study examined the influence
of canker age and time of hypovirus introduction on the development of hypoviruses
in C. parasitica cankers. An orange-pigmented strain of C. parasitica was used
to initiate cankers on the American chestnuts. Cankers were initiated in either
May or July of 1999 and outlined with a permanent marker at eight-week intervals
(excluding dormant season) after their initiation. Cankers were treated in July,
September and November, 1999 and April and July 2000 with vegetatively compatible
isolates containing one of two hypoviruses. Each hypovirus was introduced into
the advancing canker margin. Cankers are being sampled for one year following
challenge. The rate of hypovirus colonization is being evaluated by removing
bark samples from all rings and scoring the colonies that arise after culture.
As of this date, the month of hypovirus treatment appears to have no effect
on hypovirus colonization of the cankers.
Publications:
Chaloux, P.H. 2000. Virulence of Cryphonectria hypoviruses from previous release
sites. M.S. Thesis, West Virginia University, Morgantown, WV.
Impact:
The natural biological control of the chestnut blight fungus is known in several
locations in North America and Europe. Understanding the mechanisms by which
hypoviruses successfully regulate the disease in these areas may aid in our
ability to employ hypoviruses as biological control agents for chestnut blight
in eastern North America.