CHESTNUT BREEDING, TECHNIQUES AND RESULTS

Abstract

Chestnut breeding, to develop vigorous, blight-resistant timber trees, began in the Division of Forest Pathology, Bureau of Plant Industry, Soils, and Agricultural Engineering, U. S. Department of Agriculture in 1909 and is continuing. In the period 1925–49 more than 6,000 hybrids were grown. About 40 percent of the total possible number of combinations of crosses between the L3 soecies of Casta have been made successfully, indicating the close relationship existing between the species. The period of receptivity of chestnut stigmas was determined as beginning a few days after the staminate catkins began to blossom, reaching its highest degree about 12 days later, and extending nearly three weeks. Pollinations made in the morning were more effective than those made in the afternoon. Attempts to obtain viable seed by self-pollinating pistillate flowers whose styles were clipped off failed; however, viable seed was obtained by cross- pollination. Tests designed to prevent insects from visiting flowering twigs on which all staminate flowers had been emasculated showed that chestnut is wind-pollinated. Exposed pollen on anthers is at first tacky, but air currents soon dry the pollen and, together with the movements of insects feeding on the pollen and nectar of the catkins, it is dislodged and is airborne. Inheritance of blight resistance probably is controlled by two pairs of genes. Early flower ing is dominant over late flowering. At Glenn Dale, Md., temperature departures from the mean for April, and occasionally for April and May, determine when flowering begins. Time of nut drop in chestnut is inherited without dominance. Time of flowering determines the time of nut drop. Size of nuts is inherited with incomplete dominance, tending to approach the nut size of the parent producing the smaller nuts. The everbearing character found in C. seguioii is inherited iii at least a 3 : 1 ratio in the second generation. First-generation hybrids from crossing a selection of C. mollissima with C. dentata appear promising for planting as forest-timber trees. On a poor site they have grown at the rate of 2.8 feet per year for 10 years. Their blight resistance is sufficient to warrant test ing them on forest sites. Scions of Japanese chestnut carrying the mutation “spineless” were grafted and later crossed with other Castauca species. The mutation “cracked-bark,” wherein the bark of chestnut trees begins to crack at the age of three or four years, was obtained in first- generation hybrids from a C. mollissimaC. dcntata cross. Stunted growth, a general drooping of the branches, and an increased susceptibility to blight and to winter injury are associated with cracked bark. An anomaly, “naked chestnut,” was observed in certain hy brids: the burrs stop developing at an early stage while the nuts continue normal develop ment, so that the green nuts are exposed. The anomaly occurs in certain years and probably is a nonheritable mutation. Chestnuts, chinkapins and hybrids graft readily, but are difficult to bud. Extensive investigations have failed to develop an efficient method of rooting cuttings.