Grafting Eggplant for Verticillium Resistance
Posted: June 3, 2012
Michael D. Orzolek (firstname.lastname@example.org) and Guoyang Lin, Penn State Horticulture and Extension
For many decades there have been reports of the improved characteristics of horticultural crops on grafted rootstocks. Grafted rootstocks can increase yields, fruit quality, and tolerance to abiotic and biotic stresses. Costs may even go down by producing ‘somatic’ hybrids where rootstocks and cultivars can be evaluated and selected for independently versus breeding for traditional hybrids. Grafts are also an important tool to meet the challenges from new and constant mutations of soil borne disease pathogens.
Grafting used to be limited to perennial fruit trees, forest trees and ornamental plants. In the last two decades a grafted vegetable crop movement started in Asian and Europe. By 1990 Japan utilized grafted vegetables in 59% of their production area of watermelon, cucumber, melon, tomato and eggplant. The percentage has gone up to 92% for watermelon in 1999. Asian researchers have demonstrated success in overcoming tissue damage and/or plant mortality caused to vegetable crops by the soil borne diseases Fusarium wilt (there are some varieties resistance to Race 0 and 1, but there is not commercial variety resistance to Race 2 and 3), Verticillium wilt and bacterial wilt as well as nematodes. In recent years there is an emerging interest in the United States in grafting vegetables. King, S.R., Davis, A.R. (2007) discussed results on “hot topics for watermelon research: A survey of the industry” in which growers prioritized grafting as the second priority just below Gummy Stem Blight on the importance list.
There is currently no commercial eggplant variety that is resistant to Verticillium Wilt caused by the fungus Verticillium spp. in the United States. Verticillium wilt causes stunting of eggplant, interveinal chlorosis, wilting of generally half the eggplant and in some cases plant mortality. Even if Verticillium does not kill the eggplant, it will significantly reduce marketable fruit yield. This soil borne disease is able to persist in the soil for many years. Even growing eggplant on raised beds with plastic mulch and drip irrigation will not prevent Verticillium wilt from reducing marketable fruit yield by 25% to 30% per acre. Typically plants become infected within 3 to 5 weeks after being transplanted in the field. The earlier an eggplant is infected with Verticillium wilt, the greater potential for yield loss and reduction in fruit quality. Because of the severity of this disease, a crop rotation interval of 5-7 years is required before eggplant can be grown again in a previously infected field of Verticillium.
In 2007, fifty eggplant scions were grafted onto tomato rootstock that was resistant to Verticillium Wilt. The cleft grafting method was used for this trial conducted at the Department of Horticulture’s greenhouses in early spring. Instead of using clips to secure the scion/rootstock of the grafted plant, a natural rubber latex tube was used to secure the graft. In cleft grafting, the stem of the scion and the rootstock are cut at right angles, each with 2-3 leaves remaining on the stem. The tube is placed to the cleft cut rootstock. The stem of the scion is cut in a wedge, and the tapered end fitted into a cleft cut in the end of the rootstock. The graft is held firm with the tubing. This method of grafting also reduces water lose. After the eggplant scion has been grafted onto the tomato rootstock (almost all tomato hybrids are resistant to Verticillium Wilt-V), the grafted plant is than placed in grafting chamber to heal and acclimate to environmental conditions and increase the potential for the graft to take (about 10-14 days). The grafting tunnel is covered with materials which provide shade and maintain inside humidity: silver/white cheese-cloth (outside) and transparent film (inside). During acclimatization, it is recommended to keep light levels low and humidity high (90%).