Andy Nyczepir, Research Nematologist, and Tom Beckman, Research Horticulturist, Southeastern Fruit And Tree Nut Research Laboratory, USDA, ARS, Byron, GA 31008; and Greg Reighard, Associate Professor, Dept. of Horticulture, Clemson University, Clemson, SC 29634.

Guardian^TM rootstock, developed by USDA-ARS and Clemson University, shows promise as an alternative to preplant fumigation with methyl bromide for peach orchards.

The Southeast is a major producer of fresh peaches in the United States. A combination of favorable climate, soil type, and market availability has greatly contributed to the successful production of this stone fruit. Even though the Southeast is noted for being a leader in fresh peach production, the total number of trees has decreased drastically over the past 50 years for a number of reasons, one of which includes the presence of nematode pests. It is estimated that each year Georgia loses about 66,000 bearing trees and South Carolina, 87,000, to peach tree short life alone.

Two nematode pests of concern that are associated with reduced peach tree survival and productivity in the Southeast are the ring (Criconemella xenoplax) and root-knot nematodes (Meloidogyne spp.). The continuous presence of the ring nematode in conjunction with poor management practices makes peach trees more susceptible to cold injury and bacterial canker, two important agents responsible for sudden tree death commonly referred to as the peach tree short life (PTSL) disease complex. Peach trees parasitized by the root-knot nematode, on the other hand, are often stunted in growth during the first 2 years after planting, thus reducing tree vigor. Both nematode pests are widely distributed throughout the peach production areas of the Southeast and continue to be a problem if not aggressively managed. Methyl bromide is one of two preplant nematicides recommended for managing these nematode pests. Furthermore, when both nematodes are present and PTSL is a concern, growers are encouraged to plant trees on Lovell rootstock, even though this rootstock is susceptible to root-knot nematode. Recently it was demonstrated that the interaction between these two nematodes caused a significant reduction in Lovell tree growth as compared to either nematode alone.

At USDA-ARS and Clemson University, scientists are working cooperatively and independently on development of alternatives to preplant fumigation with chemical nematicides. One nonchemical alternative is the development of peach rootstock resistance to nematode pests. A rootstock superior to Lovell that survives longer on PTSL sites and also resists root-knot nematode would be of great value to the peach industry.

A peach seedling rootstock (BY5209), identified in 1991 as providing greater tree survival than Lovell in PTSL sites, is now commercially available under the trade name of Guardian^TM. In early tests, Guardian^TM rootstock also appeared to have some resistance to root-knot nematodes, although industry demand has resulted in commercial availability of Guardian^TM before all root-knot nematode testing was completed. Currently, the USDA and Clemson University are providing commercial nurseries with seed of Guardian^TM collected from a group of BY5209 selections. These selections are all siblings. The mother tree was lost before BY5209's superior field survival was discovered. One unanswered question about commercially available Guardian^TM rootstock from the USDA and Clemson seed sources is whether it is a suitable host for root-knot nematodes, especially the Southern (M. incognita) and Javanese (M. javanica) species.

In 1995-96, studies were initiated to determine the susceptibility of Guardian^TM rootstock to
M. incognita and M. javanica. Results indicate that both commercial seed sources of Guardian^TM are poor hosts to the two root-knot nematode populations tested. For both root-knot nematodes, reproduction was less on both Guardian^TM sources than on Lovell. However, it should be noted that galls form on roots of trees from both Guardian^TM seed sources, and, in fact, M. incognita produced just as many galls on Guardian^TM as on Lovell. The major benefit of Guardian^TM rootstock is that root-knot nematodes do not reproduce well even though galls are formed. Thus, evaluating rootstocks for resistance to root-knot nematode should not solely be based on a root-galling index. Root penetration and development of M. incognita on Guardian^TM and Lovell seedlings at 3, 6, 12, and 24 days after inoculation were also followed. M. incognita infective stage juveniles penetrated Guardian^TM roots, root galls formed, but the majority of the nematodes did not complete their life cycle.

Further investigations are needed to evaluate the host suitability of Guardian^TM rootstock against additional isolates of M. incognita and M. javanica and other root-knot nematode species to determine just how broad and effective its resistance is.