Strawberries are a relatively recently domesticated crop. The most commonly cultivated strawberry, Fragaria x ananassa, is a hybrid of the North American F. virginiana and the South American F. chiloensis (Maas 1998). These parental species are still grown in some areas and F. virginiana is the primary wild, sexually compatible relative to the cultivated strawberry. In addition to F. virginiana, F. vesca and its subspecies are also present in the United States. Fragaria x ananassa and F. virginiana readily cross. Introgression of pest resistance traits into the wild strawberry population is likely, as substantial amounts of crop-weed introgression has already occurred throughout the midwest, northeast, and southeast United States (Jim Hancock pers. comm.). Introgression has occurred to the extent that it is difficult to find "pure" populations of Fragariae virginiana in many areas. Strawberries suffer from several limiting diseases, insects, nematodes, and weed problems (Table 1).

¹Farr et al. 1989; additional information compiled from Maas 1998
² Pest occurs on wild relatives, resistance derived from or identified in wild populations
³ Under consideration or in development

In addition to those listed above, resistance to root-lesion nematode (Potter and Dale 1994), Phytophthora cactorum, Sphaerotheca macularis, and strawberry aphids (Shanks and Moore 1995) have been identified in cultivars of strawberry. Much of the pest resistance that has been incorporated into strawberry breeding programs has been derived from wild relatives. A substantial effort has been targeted specifically towards Phytophthora fragariae resistance (van de Weg et al. 1997), but in most breeding programs, elite types have been screened after selection for other horticulturally important traits. New efforts to breed for resistance to P. fragariae and P. cactorum are underway (Maas et al. 1993).

Strawberry is considered to be relatively amenable to transformation using Agrobacterium (Nehra et al. 1992), and the technique is being used to genetically engineer virus resistance. Strawberry plants have been transformed using the coat protein gene from strawberry mild yellow edge virus (Finstad and Martin 1995), and resulting plants are being evaluated. Other traits that are currently under investigation include glyphosate resistance, broad-spectrum fungal resistance through the use of the stilbene synthase gene and genes for systemic acquired resistance, and nematode resistance through the use of transgenes producing protease inhibitors (Morgan and Gutterson 1998).

Although sexually compatible relatives to strawberry are found in the United States, they are not considered to be a weed problem in strawberry fields. Cultivated strawberries are not capable of persisting outside the area of cultivation in the California production system, but have been found to escape in many areas of the midwestern and southern United States. Strawberries lack significant weedy characteristics and the addition of pest resistance would be unlikely to substantially increase the crop’s ability to persist.

The impact that pest populations have on the spread of wild, sexually compatible relatives is not clear. It is assumed that the environment limits the growth and spread of strawberries more than pest pressure. Little evidence of disease has been noted on the leaves and fruit of natural populations, but the root pathogens of wild strawberry have not been characterized at all; therefore broad-spectrum resistance to fungal plant pathogens may or may not provide an advantage. Information is also lacking on the competitive interactions within natural communities, and we do not know if engineered traits could make the strawberry a more effective competitor within its natural community. For example, even if the insertion of a pest resistance gene did not cause wild strawberry to be a significant weed problem in agriculture, are there endangered plants that might be replaced by a more aggressive strawberry? This information could minimize any concerns that the environmental community might voice about minimizing diversity in native plant populations. It would be helpful to determine if broad-spectrum resistance to fungal plant pathogens already occurs in native species.

Experiments are currently underway in which Frageriae chiloensis and F. virginiana growth is being compared in methyl bromide fumigated and non-fumigated soils. This will provide information concerning general pest resistance in the native species. Lists of endangered species and primary locations are maintained by various government agencies. These could provide information concerning the co-existence of wild strawberry relatives and endangered native species.

Strawberry producers have used pest resistance genes incorporated through conventional breeding for several decades. These resistant cultivars have been grown over large acreages for long periods of time, but increased weediness of strawberry has not been observed. There does not appear to be any evidence that there should be concern about the introduction of pest resistance genes in this crop, particularly those that are specific to a single pathogen.

Cultivated raspberries and blackberries are a diverse group. Most species have perennial root systems and biennial canes; however, some produce perennial canes, and others annual canes. Species producing edible raspberries that are used commercially include R. idaeus subsp. vulgatus, R. idaeus subsp. strigosus, R. occidentalis, and R. glauca. Commercial blackberries are most commonly in the subgenus R. eubatus. Hybrids between blackberries and raspberries are also commonly grown. Several Rubus species are found wild within the United States. Crosses within each subgenus are common and crosses between the subgenera are viable at higher ploidy levels. Diploid hybrids between R. subg. Ideobatus and R. subg. Eubatus are usually sterile.

An extensive literature search on the occurrence of pathogens and pests on native species would contribute significantly to determining the impact of broad-spectrum resistance genes. For example, it would be important to determine if the Himalaya berry (R. porcerus) is sexually compatible with native and commercial Rubus spp.. While the majority of diseases found on cultivated red raspberry (R. idaeus) and blackcap (R. occidentalis) also occur on wild relatives, no information is available that would indicate whether or not these pests are limiting the spread of the wild species.

A survey of the authorities on Rubus spp. could be implemented to determine what "anecdotal" information exists about pest epidemics in native Rubus. When information is lacking, disease surveys could be conducted. Experiments could be conducted using a large number of genotypes of a single potentially weedy species. These plants could be used to screen for levels of resistance that might occur in native populations. Plants could be inoculated with a wide range of potential pathogens to determine if broad-spectrum resistance already exists. Due to the wide range of genetic variability within species and the distribution of native species, small-scale field trials to determine the extent of resistance in natural populations are less applicable than trials that test a wide range of genotypes.

Weed management is extremely important in blueberry production; the plants are not strong competitors with most weeds. Pre-plant weed control is of utmost importance. In established fields, mulching, cultivation, and herbicide application are used in an integrated approach to weed management.

Feral blueberries are not found in agricultural fields and would be unlikely to become weeds due to the introduction of pest resistance traits. Highbush blueberries are very closely related to wild relatives, and lowbush blueberries are undomesticated from a breeding standpoint. Pests occurring on blueberries in commercial areas occur on other native species (Table 3) (Farr et al. 1989). Blueberry viruses, such as shoestring and leaf mottle, have been documented in wild populations.

Table 3. Blueberry pests.

¹ pests known to occur on other Vaccinium spp.

Introgression of pest resistance traits into wild Vaccinium is assumed to be due to their genetic similarity. Numerous hybrid swarms between cultivated and wild species exist in Michigan. However, it is highly unlikely that additional traits would lead to an increase in weed problems with this group.