Responses of Strawberry Species and Cultivars to the Root-lesion and Northern Root-knot Nematodes

The relative susceptibility of 44 genotypes of wild Fragaria L. and commercial cultivars of strawberry Fragaria ×ananassa Duch. to Meloidogyne hapla Chitwood and Pratylenchus penetrans (Cobb) Filipjev & Shuurmans Stekhoven was evaluated in the greenhouse. Eleven genotypes were highly resistant to populations of M. hapla from Washington State and Oregon, with Rf values (initial nematode density/final popula- tion density) less than 0.5. However, root growth of most genotypes, including resistant genotypes, was reduced by M. hapla. Thirteen genotypes were ranked more resistant to P. penetrans than F. ×ananassa 'Totem', a susceptible cultivar. Root growth of most geno- types was not affected by P. penetrans under these experimental conditions. We conclude that commercial cultivars and wild Fragaria genotypes can provide a readily exploitable source of resistance to M. hapla. Conversely, sources of resistance to P. penetrans were uncommon in the germplasm evaluated. The F. ×ananassa cultivars, which already have commercially important characteristics, appear to be a better source of resistance for both nematode species than the wild, unimproved germplasm. Plant-parasitic nematodes can affect the growth and development of strawberry resulting in economic loss. Genera of phyto- nematodes modify root growth and function (Meloidogyne, Xiphinena, and Longidorus), induce root necrosis (Pratylenchus), deform and stunt leaves and shoots (Aphelenchoides), vector NEPO viruses (Xiphimena, and Lon- gidorus), and interact with soilborne fungal and bacterial pathogens (Pratylenchus and Meloidogyne) (Brown et al, 1993; Esnard and Zuckerman, 1998). Currently, phytonematodes are controlled by preplant fumigation and/or post-plant applications of nonfumigant nema- ticides. However, these management options may not be available in the future. Methyl bromide, the most effective fumigant, is being phased out of use because of regulations that document its role in depletion of the ozone layer (Clean Air Act, 1990). Methyl bromide will be available in limited quantities after 2005 only for approved critical uses that do not have technically or economically feasible alterna- tives. Registration and use of many nematicides in minor crops also may be limited because of provisions in the Food Quality Protection Act (1996). The introduction of strawberry cultivars that are tolerant and/or resistant to phytonematodes should decrease the reliance on nematicides to maintain the productivity of plants in nematode-infested soils. Meloidogyne hapla Chitwood (northern root-knot nematode) and Pratylenchus pene- trans (Cobb) Filipjev & Shuurmans Stekhoven (root-lesion nematode) are important pests in strawberry production worldwide (Brown et al. 1993; Esnard and Zuckerman, 1998). Meloido- gyne hapla second-stage juveniles penetrate the tips of young roots where they cause small galls and the proliferation of adventitious rootlets (Edwards et al., 1985). The physiology and water relations of infected plants are disrupted and may result in severe stunting in sandy soils. Pratylenchus penetrans is an endoparasite that migrates through and feeds in the root cortex (Townshend, 1963a). These activities kill the surrounding root tissues, which become vis- ible as discrete necrotic lesions. When high P. penetrans population densities are present, the lesions may coalesce and girdle the roots. The above ground symptoms of infected strawberry plants are similar for both nematode species; stunting, reduced runner production, depressed yields, and shortened life of the planting. In addition to the direct damage P. penetrans and M. hapla cause, both species have been implicated in disease complexes affecting strawberry (Abu-Gharbieh et al, 1962; Kurppa and Vrain, 1989; Martin, 1988; Szczygiel and Profic-Alwasiak, 1989). Plant resistance is the ability of a plant to suppress nematode development and reproduc- tion (Roberts, 2002). Plant tolerance is the abil- ity of a plant to withstand nematode infection without the loss of plant growth or productivity (Roberts, 2002). Host plant resistance and/or tolerance have been proven a cost-effective strategy for managing plant-parasitic nematode that affect agronomic and horticultural crops (Young, 1998). This approach has potential