Marvin P. Pritts, Dept. of Horticulture, Cornell University, Ithaca, NY 14853

Keywords: biological control, foodborne illness, fruit quality, greenhouses, heat tolerance, trellis, pruning, rhizosphere, shade tolerance, yield

The production of Rubus and Ribes is associated with many expectations from different interest groups. Consumers expect flavorful products with health-enhancing benefits at reasonable costs. They want fresh product at any time, and they expect the quality to be high. Growers want productive plants that yield high quality fruit with minimal labor inputs. Both groups want the production of Rubus and Ribes to have a minimal impact on environment. Over the last century, production systems have evolved incrementally, with progressive modifications to what was done before. However, what may be required to meet ever-increasing demands and expectations is a complete redesign of our plants and production systems. Seven areas of research are suggested for Rubus and Ribes workers, each of which could significantly impact production systems and help retain the strong, positive image that these fruits enjoy. New advances will require the cooperation of breeders, physiologists, agronomists, engineers and ecologists working together at the interface of their overlapping fields.

Researchers and farmers have developed finely-tuned production systems for many of the world's temperate horticultural crops. These systems are highly efficient in the use of labor and chemical inputs. Crops are produced over a very long season, or can be stored for long periods of time. Varieties have been developed for specific growing regions, and pest management practices have become integrated with production systems. Farmers have benefited, at least over the short term, from increased efficiency. Consumers have benefited over the long term from lower prices and a steady supply.

The situation for Rubus and Ribes is quite different in comparison to most other crops. Far fewer researchers are involved with these genera than are involved with Malus, Fragaria or Vitis. As a result, much basic information on the physiology and genetics of Rubus and Ribes remains undiscovered. Breeding programs have released genotypes that are widely adapted rather than outstanding in limited geographic areas. Rubus culture, in particular, requires extensive hand labor for pruning and harvesting for the fresh market. Pest problems are addressed with older technology because it is not economical to use more expensive technologies on low-acreage crops. Fresh fruit is very perishable and cannot easily be shipped around the world without sacrificing quality. Although some may see these as problems to be overcome, others recognize that it is the labor-intensive and perishable nature of the raspberry, in particular, that has allowed local markets to flourish. Few strong markets exist for agricultural products that can be grown with little labor and can be stored for long periods of time.

The question for us to consider today is, given the limited resources available for research into Rubus and Ribes, where should we put our energy? What are the important questions to be asked of those interested in improving the cultivation and marketing of raspberries, blackberries, currants and gooseberries? Should we follow the course set by our colleagues and strive for labor efficiency, shipping attributes, and global sourcing of product, or should we embrace different values that include outstanding eating quality and healthfulness?

The raspberry, in particular, is one of the few remaining horticultural crops that most consumers associate with high quality. One does not hear in the supermarket that "the raspberries tasted like cardboard." One does not see raspberries on any lists of forbidden foods, such as those listing foods high in fats or pesticide residues. Most consumers have an extremely favorable impression of raspberries, and I would argue that it is in our collective interests as raspberry researchers to keep it that way. With this in mind, I would like to present some questions that perhaps should be considered a priority for those working with Rubus and Ribes production systems. Most of these questions exist at the boundaries between disciplines where little work has been done.

A foodborne illness outbreak associated with a particular crop can severely damage the market for that crop worldwide. Perhaps the best example is beef from Britain. Californian strawberry growers lost more than $10,000,000 in sales in 1996 when these fruits were falsely accused of being the source of the Cyclospora protozoan. Strawberry sales in other states suffered as well, and Guatemalan raspberry producers have never recovered from this incident once the source of contamination was traced there.

Foodborne illness on produce is not just a problem in developing countries. In fact, the majority of foodborne illness worldwide is associated with domestic produce. In the United States, over 75% of outbreaks have been traced to domestic produce while only 7.5% have been traced to imported produce (the remainder is unknown) despite the fact that about 1/4 of the produce consumed there is imported (United States Center for Disease Control, Atlanta, Georgia).

Although contamination often occurs during postharvest handling, contamination also can occur during field operations. As scientists, we must work with growers to understand how to lower the risk of using organic sources of nutrients, to identify low-risk sources of irrigation water, to use these sources effectively, and to find ways to minimize handling of the berries to reduce the risk of possible contamination. Very little information is known about the survival of human pathogens in the soil or on the fruit. The aggregated fruit type of raspberries and blackberries would appear to be a particularly good surface for pathogens to reside. Without information on pathogen survivability in the soil and on bramble fruit, it will be difficult to be confident that our current production practices reduce the risk of contamination to acceptable levels.

The extreme perishability of blackberries and raspberries has limited global sourcing of fresh product. This, coupled with increasing transportation costs and high prices for fruit, has led growers to consider producing fresh raspberries for local markets during the off-season. Row covers, high tunnels, and greenhouses are currently in use to extend the season for these high-value fruits. However, once plants are moved indoors, local varieties may no longer be most suitable. New pest problems arise, and pesticides may not be available for indoor use. Entirely new production systems need to be developed. Challenges of indoor culture include: 1) economically maintaining appropriate temperatures, 2) acquiring appropriate plant material (long canes vs. growing from plugs in pots), 3) conditioning plant material for fruiting, 4) managing pest pressure (twospotted spider mites, in particular), 5) optimizing nutrition, humidity, and light, 6) optimizing pollination, 7) plant handling and management, and 8) optimizing the use of space.

Several groups of researchers are working on high tunnel and greenhouse production of floricane-fruiting raspberries. In colder climates, greenhouses are often empty in winter because they are usually too expensive to heat. However, raspberries grow best under cool temperatures, so many growers are considering producing them rather than leaving houses empty (Pritts et al., 1999).

The relationship between day length, chilling and flower bud initiation has remained somewhat of an academic question until recently. However, with interest increasing in off-season production, a better understanding of these relationships could dramatically improve manipulation of fruiting canes. The extremely high value and superb quality of locally-produced off-season raspberries is an opportunity that researchers should nurture.

Solar radiation can limit productivity and fruit quality in Rubus/Ribes. It is reasonable to capture and utilize as much of this free resource as is economically reasonable. Many researchers have studied trellising and pruning systems in raspberries since these practices directly affect interception and conversion into photosynthates. Several general observations have been made over the years: 1)fewer canes result in lower yields of higher quality fruit, 2) increasing cane numbers results in diminishing quality and diminishing gains in yield, 3) yields respond positively to increasing light availability, particularly lower in the canopy, 4) shortening canes increases fruit size and lowers yield, 5) spreading the canopy enhances light availability, increases fruit yield per length of row, but requires more space, 6) suppressing primocanes enhances the current year's crop, but can decrease yield potential in future years.

Several trellising systems have been identified that increase light penetration into the lower canopy and result in higher yields (Oydvin, 1986; Palmer et al., 1987; Goulart and Demchak, 1993; Stiles, 1999). Frequently, though, these trellising systems have been more difficult to harvest - either by hand or with rigid mechanical harvesters. Furthermore, elaborate trellising systems require more pruning and tying. Alternate-row pruning reduces labor costs, but also reduces overall yield and fruit size (Sullivan and Evans, 1992). Worthwhile would be comparisons of entire trellising/pruning systems where all expenses, yield per unit area, fruit quality and price are considered simultaneously. This would provide researchers with some direction in identifying approaches that will optimize profitability.

4. Can we breed raspberries that are more tolerant of warm temperatures?

Researchers have recently documented the actual temperature response curves of some major cultivars, and have found some surprising results. The cultivars studied have exhibited maximum photosynthetic rates under light saturation at 20 - 22C, a much cooler temperature optimum than for many other crops (Fernandez and Pritts, 1994; Percival, Proctor and Tsujita, 1996). This helps to explain why raspberry productivity is greatest where summer temperatures are cool (i.e. British Columbia, Scotland, southern Chile). More heat-tolerant raspberries would likely enhance yield potential and quality, even in cool temperate regions where summer temperatures often exceed 25C. Germplasm for heat tolerance likely exists as many species can be found in the subtropics of southeast Asia. The lack of heat tolerance, perhaps more than any other factor, limits the areas where raspberries can be grown.

5. Can we breed raspberries that are better optimized physiologically and morphologically for agricultural production?

Raspberries have a very low harvest index of 2 - 5%, much lower than for most other crops (Fernandez and Pritts, 1994). For example, as much as 70% of the current year's dry matter production in apples can be allocated to fruit (Forshey and Elving, 1989). However, raspberries store large amounts of carbohydrate in their root systems (Fernandez and Pritts, 1993). This provides the plant the option of remobilizing carbon to floricanes during fruiting, or storing them for use the following year. Consequently, yields tend to be low in any one year, but variation in yield from year-to-year is buffered by root storage capacity (Fernandez and Pritts, 1996). This storage capacity is useful if resource levels are unpredictable over time as they would be in the natural environment, but is likely detrimental in agricultural situations where resource supply is high and consistently available. A raspberry plant that stores less carbohydrate in the root would likely have more photosynthate available for current fruit production.

The rooting pattern of raspberry is also relatively undescribed, and there exist few good estimates of carbon loss through respiration or exudation (Percival et al., 2001). Variation in root architecture can affect the amount of carbon in structural biomass, the amount of exudation, and the amount respired (Nielsen et al., 1994). This, in turn, can differentially affect soil microorganisms and mineralization of organic matter. Raspberry roots appear to have a higher temperature optimum for growth than shoots (Percival et al., 1996), but it is unclear if temperature is acting directly on roots, or is influencing activity in the rhizophere. It is also unknown how soil compaction affects raspberry growth (MacConnell, 1984). Much useful information could be gained by a comprehensive study of the raspberry root system, and of which components are under genetic control.

In addition to an apparently large root storage capacity, raspberries produce an excessive number of primocanes than can lead to a low production efficiency (Braun et al., 1989). These primocanes often are suppressed or removed to allow light to penetrate into the canopy and to reduce interference with floricane harvest. This removal or suppression of excessive primocanes is labor intensive and uses plant resources that might otherwise be diverted to fruiting (Freeman et al., 1989).

The raspberry plant is also fairly shade-tolerant (Fernandez and Pritts, 1996), despite the observation that few commercial raspberry plantings exist under a canopy. Shade-tolerant plants tend to have lower maximum photosynthetic rates than other plants, so again, yield potential may low in our current raspberry ideotype.

Plant breeders and physiologists, working together, could develop a new physiological type of raspberry that could dramatically impact productivity. Such a raspberry would be more heat tolerant, have less root storage capacity, have a higher net photosynthetic rate, produce a limited number of primocanes, and have a higher allocation of photosynthate to fruit. Incremental increases in yield with the current ideotype are unlikely to significantly increase harvest index beyond the currently low levels.

We are just beginning to understand the complex interactions that occur within the soil involving bacteria, fungi, nematodes, protozoa, macrofauna, carbon sources, nitrogen supply and the rhizosphere. For example, managing the nitrogen supply or carbon source influences populations of bacteria and fungi, which, in turn, can influence populations of pathogenic organisms. Certain rhizobacteria, when provided with organic matter, keep weed seeds from germinating. By producing toxins and excessive concentrations of plant growth hormones, root cells of weeds rupture and leak, replenishing the organic matter for the rhizobacteria. Once weakened by the bacteria, weeds are less able to compete with other plants, and they become more vulnerable to other control measures. Bacteria have received little attention in perennial fruit systems but likely play an important role. How might suppressiveness be managed in Rubus/Ribes plantings?

Application of suppressive organisms to the soil may be one approach to reducing disease incidence. For example, we have found that Trichoderma applied to raspberry roots at planting provides short-term suppression of Phytophthora, but maintaining high levels for long periods of time is difficult (Raines, 1997). We have found that calcium amendments, particularly calcium sulfate, can suppress Phytophthora root rot in raspberries (Maloney, 2001). Despite the disease-suppressive properties of compost, we do not yet know how it can be effectively used in Rubus and Ribes production systems. Regardless, managing soil pathogens through application of a mulch, soil fungus, gypsum or compost is probably more environmentally friendly than multiple applications of fosetyl-Al, metalaxyl or methyl bromide.

7. Can biologicals be incorporated into our production systems to reduce reliance on biocides and energy?

Since Rubus/Ribes are minor crops, it is unlikely that an abundance of new pesticidal products will be registered for them. Researchers must be imaginative in identifying methods of pest management that do not involve the direct poisoning of a pathogen, insect or mite.

The use of pheromones for mating disruption and moving pests off-site is a promising approach to management, as is the use of plants that attract beneficial insects. Feeding-attractants and stimulants can be used to enhance the efficacy of pesticides. The release of sterile insects or insects containing lethal genes also holds promise. Scientists at Oxford University in England have bred an insect with a dominant gene that kills females when they are not exposed to tetracycline. When the antibiotic is removed from the diet, females die. In nature, all of these females and their female offspring eventually die and the population is severely curtailed (Thomas et al., 2000).

It has been known for many years that straw mulch at planting can dramatically enhance primocane production in an establishing planting (Darrow and Magness, 1939; Trinka and Pritts, 1992), although applying it can be labor intensive. An alternative may be to grow mulch in place and plant raspberries into the mulch residue (Raines, 1997). Vrain, et al. (1996) found that nematode-suppressive cover crops in row middles were not effective at controlling pathogenic nematodes within the row; however, most cover crops were developed for agronomic or ornamental purposes, not for their suppressive effects on harmful organisms. What could we achieve if cover crops were bred specifically for their suppressive effects? Could we develop a Brassica species sufficiently high in glucosinolates that nematodes and soil-borne pathogens would be eliminated after growing them on a site? Could we develop a cultivar of rye that would inhibit nearly all germination of weed seeds? Might we discover that certain undomesticated species have broad spectrum pest control properties with potential use in raspberry production?

We have been experimenting with "harpin protein" on raspberries - a protein derived from the bacteria Erwinia amylovora. This protein, when sprayed on plants, stimulates the production of secondary chemicals within the plant involved with pest resistance (Wei et al., 1992). Crops appear to improve their pest resistance when treated regularly with the harpin protein. Such materials are not directly toxic to fungi or insects, and they degrade rapidly in the environment. Also, races of fungi that are antagonistic to Botrytis cinerea have been selected and introduced onto flowers using honey bees as a vector to obtain acceptable levels of gray mold control (Sutton, 1994).

Once promising interactions have been identified, the agronomist must ensure that the planting is maintained in such a way that pest suppression is effective and sustained. The use of these low-risk products has an important place in the production of Rubus/Ribes because they help retain their image as healthful, wholesome products. Undoubtedly there are creative ways to accomplish production and quality goals without purchasing expensive inputs or increasing environmental impacts.

Many questions exist regarding Rubus/Ribes production and marketing; enough to keep scientists busy for years to come. Many problems of local importance exist, particularly those that involve pests or unique climatic factors. The 7 questions I have presented in this paper are not intended to minimize the importance of local problems, but rather to identify questions that have wide implications for enhancing the reputation of the raspberry with the public, and perhaps fundamentally changing the production systems we currently use. With so few scientists working with Rubus/Ribes, it would be useful to focus our research on problems that can make a major impact. Significant advances are possible in the near future if breeders, physiologists, agronomists, engineers and ecologists work together at the interface of their overlapping fields.

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