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Project Number: WVA00293 BIOLOGICAL IMPROVEMENT OF CHESTNUT AND MANAGEMENT OF THE CHESTNUT BLIGHT FUNGUS Investigators: MacDonald, W.
L. Termination Date: 09/30/2003 Progress Report: A large-scale experiment to deploy hypoviruses as biological control agents for chestnut blight continues in a forest stand of 2,500 American chestnuts growing near West Salem, WI. As of June 1998, 151 trees were infected with 813 cankers; 142 new infections were discovered in 1998. A European hypovirus, released annually from 1995 to 1997, has become a more commonly associated hypovirus with new and old infections than a North American hypovirus released from 1992 to 1994. Fungi other than Cryphonectria parasitica commonly have been recovered from most infections, with older cankers yielding the greatest number of other organisms. Callus production and disease remission were greatest for previously treated cankers and least for cankers discovered in 1998. In a second field experiment, the poor results obtained when bark patches were used as vehicles for spermatizing cankers withg transgenic strains prompted a study to compare methods and time of conidial application. Cankers on American chestnut were spermatized in August, September and October using a sexually compatible transgenic isolate. Hypovirulent conidia were delivered via bark patches or painting cankers with an agar slurry or liquid peptone solution. Assays of ascopore progeny were used to quantify levels of spermatization. The most successful treatment was an August application of transgenic conidia in the agar slurry. This study was repeated in 1998 with spermatization dates in June, July and August. In other field tests, numerous heterokaryons were recovered from cankers established to test the role of vegetative incompatibility genes on hypovirus transmission. Most heterokaryons were the result of fusion between wild-type C. parasitica and cream or brown strains used to initiate cankers. These results suggest that heterokaryon formation may be more common than previously known. The effect of hypoviruses on growth and sporulation is being evaluated from isolates recovered from sites where release occurred 15 to 20 years ago. Results indicate that the surviving hypoviruses are of North American origin and still retain some ability to depress and isolate's ability to invade chestnut bark. A field study has been initiated to evaluate the influence of mycelial age on hypovirus colonization of cankers. To accomplish this, artificially established cankers were challenged by introducing a vegetatively compatible, hypovirus-infected strain at the leading edge of the canker. To date, hypovirus colonization has occurred only in the advancing canker margin. Publications: Balbalian, C.J., W.L. MacDonald, D.H. Huber, D.W. Fulbright. 1998. Effects of Cryphonectria parasitica vegetative compatibility genes on the transmission of hypoviruses between strains on chestnut trees. Phytopathology 878:S128 (Abstract). Chaloux, P.H., Y. Liu, M.G. Milgroom, and W.L. MacDonald. 1998. The effects of dsRNAs on growth and sporulation of Cryphonectria parasitica isolates recovered from previous hypovirus introduction sites. Phytopathology 88:S129 (Abstract). Double, M.L., W.L. MacDonald, and D.L. Nuss. 1998. Rates of sexual recombination by transgenic hypovirulent strains of Cryphonectria parasitica in a forest setting. Proceedings of the Second International Chestnut Symposium. Bordeaux, France. October 19-23, 1998. Liu, Y.L., W.L. MacDonald, and M.G. Milgroom. 1997. Recovery of dsRNA derived from hypovirulent isolates of Cryphonectria parasitica released in West Virginia. Phytopathology 87 5:59 (Abstract). MacDonald, W.L., M.L. Double, and D.L. Nuss. 1998. Variation in growth and sporulation of Cryphonectria parasitica isolates as influenced by hypovirus infection. Proceedings of the Second International Chestnut Symposium. Bordeaux, France, October 19-23, 1998.
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