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 population 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 genotypes 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 phytonematodes 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 Longidorus), 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 nematicides. 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 alternatives. 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 penetrans (Cobb) Filipjev & Shuurmans Stekhoven (root-lesion nematode) are important pests in strawberry production worldwide (Brown et al. 1993; Esnard and Zuckerman, 1998). Meloidogyne 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 visible 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 reproduction (Roberts, 2002). Plant tolerance is the ability 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 for management of nematode damage in strawberry. Orchard and Andrichem (1961) observed differential root galling and egg mass production in eleven Fragaria species and subspecies that were planted in soil infested with M. hapla. Similarly, Dickstein and Krusberg (1978) reported that 33 strawberry cultivars differed considerably in their galling response to M. hapla. Based on the fresh biomass of 46 strawberry cultivars in several greenhouse studies, ‘Earliglow’ (Edwards, et al. 1985) and ‘Glima’ and ‘Senga Sengana’ (Szczygiel, 1981a) were tolerant or highly resistant to M. hapla. Szczygiel (1981b) also found ‘Senga Sengana’ to be the most resistant and tolerant to P. penetrans among 28 cultivars evaluated. Potter and Dale (1994; Dale and Potter, 1998) reported resistance and tolerance to P. penetrans among Fragaria chiloensis (L.) Duch. and F. virginiana Duch. genotypes and F. ×ananassa cultivars. Based on their work with a limited number of genotypes, they concluded that resistance to P. penetrans can be increased in cultivated strawberries by introducing wild germplasm and that further screening of Fragaria germplasm is warranted. Moreover, research data cited above suggests that screening for resistance to M. hapla should be profitable. Hancock et al. (2001a, 2002) have identified a supercore group of native F. virginiana and F. chiloensis clones that represent the broad distribution of the octoploid species in North and South America. Further, they have focused various Fragaria germplasm evaluation projects on this supercore (Hancock et al. 2001b). The objective of this study was to evaluate an elite group of strawberry accessions for resistance and tolerance to M. hapla and P. penetrans. Accessions were primarily from the supercore (Hancock et al, 2001a) of subspecies of F. chiloensis (L.) Miller and F. virginiana Miller along with F. ×ananassa cultivars that are important in production areas of North America Materials and Methods Native octoploid strawberries were propagated vegetatively at the USDA–ARS Horticultural Crops Research Laboratory, Corvallis, Ore. Plants not available at the HCRL were obtained from the USDA–ARS National Clonal Germplasm Repository in Corvallis and Jim Hancock at Michigan State University, East Lansing. Several cultivars were obtained as bare root plants from commercial nurseries. Plants of each genotype were kept in a greenhouse under long day conditions (16-h photoperiod) with 24 °C day and 18 °C night temperatures. In March through April, runners were pegged into four inch pots containing about 800 g of steam pasteurized sandy soil (1:2 by volume, washed sand and Willamette loam). Once established, the rooted daughter plants were cut from the mother plant and grown in the greenhouse. Genotypes varied in initiation of runners and development of daughter plants, with some genotypes requiring an additional month to reach the size desired for the experiment. HORTSCIENCE 40(1):33–38. 2005. Received for publication 25 Sept. 2003. Accepted for publication 20 May 2004. This research was partially funded by a USDA–ARS Germplasm Evaluation Grant. We gratefully acknowledge the assistance of Tim Lair and Ted Mackey. Research plant pathologist. Research geneticist.