RALEIGH, NC – Christmas trees provide a significant source of revenue in southern Appalachia, resulting in millions of dollars in sales during the holiday buying season. The most popular species in the region is fraser fir, appreciated for its fragrance and consumer-friendly traits such as soft needles, strong branches, exceptional needle retention, and natural Christmas tree shape. Frasers, indigenous to isolated high-elevation mountains in southwestern Virginia, western North Carolina, and eastern Tennessee, are under attack by a pathogen called Phytophthora cinnamom, an insidious adversary that causes root rot, kills seedlings, and threatens serious economic losses for the region's Christmas tree industry. "Once a growing site is infested, the pathogen is nearly impossible to eradicate. Fir seedlings often die within 2 or 3 weeks from infection", noted John Frampton, a professor in the Department of Forestry and Environmental Resources at North Carolina State University. To develop planting stock that is resistant to or tolerant of Phytophthora cinnamomi, some growers in the southern Appalachian Mountains are turning to grafting practices, predominantly grafting fraser fir scions onto rootstocks of resistant momi or turkish fir. To aid growers in the region seeking effective grafting techniques, Frampton and his team designed a study, implemented by graduate student Haley Hibbert-Frey, to compare success rates of the traditional April grafting time with eight summer/early fall grafting dates. The study, published in HortScience, contains important recommendations for tree growers.
Fraser fir is usually grafted in April when the rootstock and scion are dormant. But spring is a busy time for growers, who would welcome the flexibility of performing grafting at other times of the year (e.g., late summer or early fall). The NCSU study compared success and growth of grafting fresh fraser fir scions onto turkish fir rootstocks during the traditional April grafting window with eight biweekly grafting dates from mid-July through mid-October. The scientists also assessed the effect of shade and irrigation treatments on graft success and growth and evaluated grafting during the mid-July through mid-October season using dormant fraser fir scions collected during April and stored at °C.
The team concluded that the effect of grafting date was significant for graft success. Grafting during April when scions were dormant and rootstocks were just becoming active yielded a noteworthy 95% success rate; graft success was significantly lower for the first three summer grafting dates and was unsuccessful from August 24–October 20. "April graft success was 95% but when grafting fresh scions in summer/fall, graft success decreased from 52% in July to 0% in October. To ensure optimal grafting success, grafting should be performed in the late winter or early spring when scions are dormant and the rootstocks are becoming active", recommended Frampton.
The experiment results showed that shade improved summer graft success (52% with shade, 38% without), while irrigation did not significantly affect graft success or subsequent growth. In a supplemental storage study, grafting of stored scion material in summer/early fall was not successful (less than 1%). "Until more successful techniques can be developed, it is prudent to graft fraser fir in early spring with freshly collected dormant scion material", the researchers concluded.
The complete study and abstract are available on the ASHS HortScience electronic journal web site: http://hortsci.ashspublications.org/cgi/content/abstract/45/4/617
Founded in 1903, the American Society for Horticultural Science (ASHS) is the largest organization dedicated to advancing all facets of horticultural research, education, and application. More information at ashs.org
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