High Level of Chestnut Blight Control on Grafted American Chestnut Trees Inoculated with Hypovirulent Strains

Materials and Methods

In May 1980, three American chestnut grafts (TG, RM and TH) were established on three American chestnut rootstocks in an American chestnut plantation at the Lesesne State Forest, Virginia. Large, surviving American chestnut trees [>25 cm diameter breast height (dbh)] growing in Virginia were used as a source of scions. Usually, two scions from each large, surviving American chestnut tree (TG, RM or TH) were bark-grafted into a single rootstock. Natural blight cankers were present on the main stems of the grafts when they were 2 and 3 years old in 1982 and 1983. These cankers were inoculated with a mixture of European and American hypovirulent C. parasitica strains (EP4, EP43, EP47, EP49, EP51, EP60, EP88, EP92, EP171 and EP172) received from J. E. Elliston at the Connecticut Agricultural Experiment Station. The cork-borer method of inoculation (3) was used to inoculate blight cankers with the hypovirulent strains.

A survey of blight on 100 seedling sprout clusters in the chestnut plantation, adjacent to the grafts, was conducted to document the incidence and severity of chestnut blight at this location. The severity of blight and characters of tree health, such as the height, dbh, and visual estimate of percent live crown, were determined for the three grafted chestnut trees. Cankers were rated for severity according to four canker types in decreasing severity of disease: a) sunken, often with exposed wood, b) irregularly swollen and sunken, c) callused and swollen and d) superficial and swollen. Thickness of healthy and necrotic bark in cankers was determined with micro-core (1.7 mm diameter) samples of bark tissue and a ruler.

Results

The survey of 100 American chestnut sprout clusters showed that 100% had chestnut blight, 100% had one or more stems killed by sunken cankers, and for 95% of the sprout clusters, the largest stem had been killed by blight. Active blight cankers with stromata (fungus sporulation structures) were frequent. Low blight severity and a high level of health for the chestnut grafts are indicated in Table 1.

Discussion

The chestnut blight survey indicated that virulent inoculum and chestnut stem death were widespread adjacent to the chestnut grafts at the Lesesne State Forest. The evaluations reported here indicate that a high level and a long period of chestnut blight control were obtained following inoculation of natural blight cankers on the chestnut grafts with a mixture of hypovirulent strains. A high level of blight control is indicated by few blight-killed branches and a large percentage of cankers that were rated as superficial. Further, the swollen, superficial cankers on the three grafts are the most superficial (high ratio of non-necrotic to necrotic tissue) observed during a 20-year period of core sampling of superficial cankers on American chestnut (G. J. Griffin, unpublished).

Some chestnut blight control (abnormal cankers and more live stems per chestnut sprout clump) has been observed 9 years after inoculation of forest American chestnuts with hypovirulent stains (1), but we know of no published reports documenting a high level and long period of blight control, following inoculation with hypovirulent strains, in the natural range of American chestnut.

Although no data are available, we observed an apparent high level of chestnut blight control in Connecticut for orchard American chestnut trees inoculated with a mixture of hypovirulent strains (G. J. Griffin visit to Lockwood Farm with J. E. Elliston). Research is in progress to evaluate factors that may be contributing to the high level of blight control exhibited by these grafted chestnut trees.