Plant hosts in the nursery industry -
PLANTS MOVE!
How might the movement of plants in the nursery industry contribute to the spread of Phytophthora ramorum to new areas?

by Larry Englander and Paul Tooley

Despite our perception that plants are rooted into the earth for life, many plants travel quite extensively before they reach their final resting place. We shift plants around in our yards and transfer them from the garden center to the landscape, but this just marks the end of the journeys for some plants. Initially, they may have been propagated in another state, possibly from across the country, or even on another continent. In just one small state (Rhode Island), with modest horticultural production, data show that in 1999 and 2000 nurseries brought in plants from 71 suppliers in 16 states, and garden centers and brokers acquired plants from 96 suppliers in 15 states, often with plants traveling coast-to-coast.

Even after death, plants may travel, to the nearby compost pile or to the recycling center where they become chipped into mulch or processed into organic soil amendments or compost, which is then redistributed

There are several reasons for such large-scale movement of nursery plants. First, there are benefits to propagating young plants in those regions with conditions best suited for their propagation and early development, regions with growth-promoting day lengths or favorable rainfall, soil, and temperature characteristics, all factors that favor cost-effective plant production and selling price. These plants may then be shipped near or far, either to be immediately offered for sale or to spend time in a nursery for a period of acclimation. In a few cases, plants brought to substantial size in areas with optimal conditions will be better able to survive the rigors of harsher climates.

Of course, there are some drawbacks to unlimited plant movement as well. The parent (stock) plant material may not have evolved under selective pressure to have the genetic components best suited to a far-removed ecosystem at the plant's final destination. Furthermore, either the plant itself or the associated planting medium may give a ride to an unintended hitchhiker, such as an insect or pathogen. The history of plant disease epidemics provides ample examples of transported agents; one has only to recall the story of Dutch elm disease, or Chestnut blight.

Sudden Oak Death has the potential to attack forest and landscape trees. We know little about the avenues by which the pathogen (Phytophthora ramorum) moves from plant to plant or place to place. It is unknown whether transport of commercially produced plant material will play a role in the dissemination of this pathogen from the region where it now resides (mid-Pacific U.S. coastal area) or from Europe where it has been detected in several countries. Under infected oaks in the U.S., the pathogen has been found on nursery stock such as Rhododendron spp., often causing visual symptoms (leaf spots) that are easily overlooked or that mimic those of several other ubiquitous pathogens. Reports regularly add new plants to the long list of hosts, which already spans many genera and families (Rizzo et al. 2002). Positively identifying suspected infections by P. ramorum requires laboratory culture and microscopic examination or sophisticated DNA tests. Thus, large-scale testing of each nursery crop, shipment, or suspect plant is a daunting task complicated by the routine use of fungicides in nurseries, a practice that may mask the presence of the pathogen without killing it.

Let's examine some of the pathways in commercial horticulture through which P. ramorum may potentially move.

1. Woody plants may be propagated commercially from seed, tissue culture, or vegetative cuttings (Figure 1, high-density commercial Rhododendron culture). Where a variety or species will yield true-to-type progeny from seed that readily germinates, this type of propagation may be employed. Seeds and seedlings are germinated and grown in a medium, often soil-less but nonetheless not sterile. These plants are 'stepped up' by transplantation to larger containers or into the field as they mature.

Figure 1. High-density commercial Rhododendron culture. The potential for P. ramorum to move to new areas on or in infected plants or container planting mix is largely unknown



2. Another means of propagating horticultural plants is by tissue culture. Although initially these tiny plants are propagated under aseptic laboratory conditions and are free of contaminating microorganisms, their subsequent stages of culture in preparation for sale, such as plugs, liners, or container plants, are not in a sterile environment. A very common means of plant propagation is by taking cuttings from stock plants and rooting them, usually under controlled conditions in which high humidity or water mist is provided to prevent desiccation until an adequate root system develops. These plants are 'stepped up' prior to sale as are those following the other schemes, above, and each site change provides opportunity for exposure to a new set of pathogens. Plant material is sold commercially at any of these stages of culture, from tiny tissue-cultured plants, to rooted cuttings, liners, container plants, and balled/burlapped plants of all ages and sizes. These plants may complete all their various phases in the same nursery or may be moved throughout the U.S., for example, from nurseries that specialize in propagation to those that concentrate on growing plants to finished size. Then, these may be sold for installation by the landscape industry or moved to retail garden centers. Each of these transits carries the risk of moving a pathogen.
3. During holiday periods, plants such as florist azaleas (Figure 2) produced on the west coast are shipped in large numbers to states such as Maryland, where large nurseries increase them in size and supply them by the thousands to outlets such as Home Depot and K-Mart.

The relevance of nursery crop transport in dissemination of Phytophthora ramorum is, as yet, unknown. Nurseries can control the Sudden Oak Death pathogen with use of a common fungicide. But, if infected plants make their way to oak forests in the East, the fungus might spread to nearby trees and cause as yet unknown damage.

While some things are known about dispersal of P. ramorum (Davidson et al. 2002), much remains unknown. Patterns of spread observed in the field suggest spread due to wind-blown rain and/or rainsplash. Abundant sporangia and chlamydospores are produced on leaves of understory species such as California bay laurel (Umbellularia californica) and Rhododendron spp.? (Davidson et al. 2002). And the pathogen has been recovered from recycled irrigation water (Werres et al. 2001) as have other Phytophthora species (Themann et al. 2002).

The potential for nursery crop transport must be considered as new, much-needed research results become available. For example, if infected plants are shipped to new areas, will conditions permit the pathogen to sporulate well enough to spread to new hosts? Will fungicides, if used, suppress the pathogen so that spread is unlikely, or merely mask symptoms so that the pathogen appears disease-free when shipped but later is able to sporulate and move to new hosts. Will the pathogen survive in a commercial planting mix? Also, the role of other potential carriers needs to be assessed, such as commercially distributed foliage, cut flowers, hothouse plants and edible plant products; consumer mail-order purchases, insects, and itinerant wildlife (including people).

To the good, several factors are working against the spread of P. ramorum to new areas. State and Federal agencies regulate and critically examine both crops in the nursery and crops destined to be shipped. State surveys will be initiated soon to see if P. ramorum already exists in new areas and thus might be eradicated. The susceptibility of eastern nursery species to P. ramorum is being examined (Tooley et al. 2002) so that surveyors will have an idea of which plant species might be the most likely to show symptoms (Figure 3). Furthermore, nursery growers routinely examine their plants, treating or roguing those that are not in an acceptable condition, and these folks are known to be very discriminating.

The California problem threatens to become a nationwide problem if we don't learn how P. ramorum moves and where it might or might not relocate successfully.

References:

Davidson, J. M., Rizzo, D.M., and Garbelotto, M. 2002. Phytophthora ramorum and Sudden Oak Death in California: II. Pathogen transmission and survival. Pp. 741-749 In: 5th Symposium on California Oak Woodlands, R. Standiford and D. McCreary, eds., USDA Forest Service, Gen. Tech. Rep. PSW-GTR-184.

Rizzo, D. M., Garbelotto, M., Davidson, J.M., Slaughter, G.W., and Koike, S.T. 2002. Phytophthora ramorum and Sudden Oak Death in California: I. Host Relationships. Pp. 733-740 In: 5th Symposium on California Oak Woodlands, R. Standiford and D. McCreary, eds., USDA Forest Service, Gen. Tech. Rep. PSW-GTR-184.

Themann, K., Werres S., Diener H.-A., and R. Lüttmann (2002). Epidemiology of Phytophthora spp. in water recycling systems of commercial nurseries. European Journal for Plant Pathology 108:337-343.

Tooley, P. W., Kyde, K. L., and Englander, L. 2002. Infectivity of Phytophthora ramorum on selected Ericaceous host species. Phytopathology 92:S81 (Abstr.)

Werres S., Marwitz, R., Man in 't Veld, W.A., De Cock, A.W.A.M., Bonants, P.J.M., De Weerdt, M., Themann, K., Ilieva, E. and Baayen, R.P. (2001). Phytophthora ramorum sp. nov., a new pathogen on Rhododendron and Viburnum. Mycological Research 105:1155-1165