In instances where parasitic nematode infestations are the sole problem in a selected orchard site, nematode-resistant rootstocks may be a viable alternative to pre-plant methyl bromide fumigation.

Nematode resistance is recognized worldwide as an important Prunus rootstock breeding objective. It is fortunate that root systems of Prunus are attacked by relatively few of the many plant parasitic nematodes. Three genera of nematodes are generally recognized as causing economic damage in stone fruit orchards. These include the root knot nematodes (Meloidogyne spp.), root lesion nematodes (Pratylenchus spp.) and ring nematode (Criconemella xenoplax). These nematodes can occur alone, or in mixed populations in the upper soil profile. Symptoms of nematode attack include early defoliation, reduced fruit yield, stunted tree growth and early tree death. Plant parasitic nematodes are a primary reason for pre-plant methyl bromide fumigation of new orchard soils or for spot fumigation of replant sites.

Prunus breeders have been occupied with the development of nematode resistant rootstocks for decades. 'Marianna 2624', a clonally propagated plum rootstock with root knot nematode resistance, was introduced in 1940. 'Nemaguard' and 'Nemared', two seed-propagated root knot nematode resistant peach rootstocks, were introduced in 1959 and 1983, respectively. Numerous other root knot resistant rootstocks, both clonal and seed propagated, have been developed and are being utilized in a variety of orchard environments. Breeding for root knot nematode resistance in Prunus is facilitated by the visually striking symptoms on susceptible Prunus root systems. Furthermore, the resistance is genetically dominant to susceptibility. These two factors allow for relatively easy breeding and selection of new germplasm with resistance to root knot nematode.

The relative ease of breeding for resistance to root knot nematode is a stark contrast to the difficulty involved in the development of resistance to ring nematode. Screening efforts to identify resistance to ring nematode in Prunus have been unsuccessful to date. Over 400 genetically diverse accessions of Prunus have been evaluated and all appear to support ring nematode reproduction. Among these screened accessions were many root knot nematode resistant rootstocks. These evaluations point out that resistance to nematodes in Prunus is both genera and species specific. Therefore, new rootstock accessions must be challenged with each specific nematode species, race, or population for a determination of resistance or susceptibility.

At the USDA/ARS Horticultural Crops Research Laboratory in Fresno, CA, we have screened over 200 diverse Prunus accessions to identify root lesion nematode (Pratylenchus vulnus) resistant germplasm. Resistance to root lesion nematode has been identified in a few genetically diverse Prunus accessions. Whether or not these root lesion nematode resistant accessions will be acceptable as rootstocks remains to be clearly demonstrated. Rootstock acceptability is based on many factors, including rootstock performance in the nursery. The ability to root and growth characteristics of the rooted cuttings may influence a nursery person's choice of whether or not to propagate new rootstock accessions. Rootstock vigor, either lacking or excessive, tree anchorage, water use efficiency and fruit production potential are all critical characteristics which a fruit producer might want knowledge of prior to investing in a newly recommended rootstock. Furthermore, graft compatibility between the rootstock and the fruit-bearing portion of the tree must be demonstrated for all new candidate rootstocks.

We are currently examining some of the important rootstock characteristics of the root lesion nematode resistant germplasm. Since root lesion nematode is known to inhabit the same orchard soils as root knot nematode, we have screened the root lesion nematode resistant candidate rootstocks against root knot nematode. Many of these Prunus candidate rootstocks effectively resist the root knot and root lesion nematode populations endemic to California. Initial graft compatibility of the candidate rootstocks has been demonstrated with apricot and French prune at our Fresno location. We have also evaluated rooting ability and growth characteristics of rooted cuttings. Our results demonstrate that several candidate rootstocks have rooting abilities and growth characteristics similar to the commercial Prunus rootstock 'Marianna 2624'.

We are currently expanding our evaluation efforts through large-scale trials. In 1997, nursery industry personnel will be examining the candidate rootstocks from a commercial perspective. In addition to rooting and nursery performance, our evaluations will include graft compatibility of candidate rootstocks with almond, nectarine and peach. As trees come into bearing, fruit production characteristics will be compared with similar trees grown on standard commercial rootstock cultivars. It is only after these large-scale evaluations that we will be able to make recommendations regarding the use of new root knot and root lesion nematode resistant candidate rootstocks.