Seedborne Pathogens and Strategies to Eliminate and Reduce their Presence on Tree Seeds

Stephen W. Fraedrich

Introduction
Seeds are regarded as a highly effective means for transporting plant pathogens over long distances. Numerous examples exist in agricultural literature for the international spread of plant diseases as a result of the importation of seeds that were infected or contaminated with pathogens. Various fungi are commonly associated with seeds of many tree species, and these can include pathogens and saprophytes. Seeds may be infected internally, often resulting in the destruction of endosperm and embryo (Fig. 1), or simply contaminated, whereby the pathogen is associated primarily with the seedcoat. The development of quarantine programs for seeds requires biological and ecological information about seedborne pathogens, the ability to detect their presence, knowledge about the inoculum type and its location on seeds, and effective means for control. With this information, risks associated with the importation of seeds of particular tree species can be evaluated more accurately. Compared to seedborne disease problems of agricultural crops, research has been limited on seedborne pathogens of forest tree species and in the development of disease prevention and control practices.

Figure 1. Internal seed infection of slash pine caused by Lasiodiplodia theobromae.

Types of seedborne pathogens associated with forest tree species
Certain seedborne fungal pathogens primarily cause seed diseases and appear to have minor effects on other developmental stages of trees. Examples include Lasiodiplodia theobromae, which destroys slash pine (Pinus elliottii var. elliottii) seeds in the southern United States and South Africa, and Caloscypha fulgens, which affects seed quality of pines (Pinus spp.), spruce (Picea spp.), and fir (Abies spp.) in Canada and the northern United States.

In contrast, there are several notable pathogens of conifers that can be seedborne and could have severe economic and ecological consequences if they are introduced and become established in regions where they are not native. These pathogens include Sphaeropsis sapinea (syn. Diplodia pinea), Sirococcus conigenus (syn. S. strobilinus) and Fusarium circinatum (syn. F. subglutinans f. sp pini).

S. sapinea causes a blight of pines and has been particularly devastating to pines grown outside their natural range. The fungus has been associated with cones and seeds of various pine species. Although S. sapinea can be isolated from seeds of slash and loblolly (P. taeda) pines in the southern United States, the fungus is not known to cause blight on these pines within their natural range. In contrast, S. sapinea damages many pine species grown in the southern hemisphere, including loblolly, slash, and Monterey pine (P. radiata).

Sirococcus conigenus is responsible for a blight of pine, spruce, hemlock (Tsuga spp.) and fir in Canada, Europe, and the northern and western regions of the United States. Sirococcus blight can affect trees at all ages and has caused severe disease problems in nurseries that have been traced to seedlots with seeds infected by Sirococcus conigenus.

Fusarium circinatum, the pitch canker fungus, can infect various reproductive and vegetative stages of many pine species. The fungus has long been associated with pines indigenous to the southern United States. In recent years, F. circinatum has been associated with disease on native and exotic pines in South Africa, Japan, Spain, and Mexico. In California (western United States), the fungus has had devastating impacts on Monterey pine, and the disease appears much like that caused by an introduced pathogen. Other Fusarium spp. are regularly isolated from seeds of conifers and some, such as F. oxysporum, are recognized pathogens.

Reducing the potential for contamination and infection of seeds by pathogenic fungi through collection practices
Seed collection and processing practices can greatly affect the association of certain pathogens with seeds. In some conifer species, the incidence of seed contamination and infection by pathogenic fungi is increased greatly after cones contact the ground. Cones on trees may not be susceptible or accessible to colonization by specific fungi. For instance, C. fulgens was not recovered from seeds of Sitka spruce (Picea sitchensis) cones picked from trees but was readily isolated from seeds of cones that had been collected from the ground beneath trees. The degree of maturation of cones at the time of collection also may affect the colonization of seeds by fungi. Slash pine seeds from cones that are removed prematurely from trees and left on the ground for short periods may have a much greater level of infection and contamination by L. theobromae than seeds from cones that are close to maturation at the time of collection (Fig. 2).

Figure 2. Mean values (%) of seeds per cone with seedborne pathogens with respect to dates of cone collection, cone handling, and storage treatments. The seedborne pathogens were primarily Lasiodiplodia theobromae and, to a lesser extent, Sphaeropsis sapinea. (Adapted from Fraedrich et al. 1995. Can. J. For. Res. 24:1717-1725.)

The location and management of seed collection areas also may affect the presence of certain pathogens on seeds. Observations in the southeastern United States have suggested that F. circinatum is more likely to be associated with seeds of longleaf pine (P. palustris) produced in intensively managed seed orchards compared to those seeds from seed production areas that are not managed. One cannot assume, however, that a pathogen will not be found in an area because disease symptoms are not evident. F. circinatum has been isolated from rainwater beneath the canopy of asymptomatic pines in the southern United States and from rinse water of Monterey pine cones obtained at locations in California, where disease is not evident. Testing of seeds for the presence of pathogens remains an important consideration in quarantine programs, regardless of the source of the seeds.

Seed treatments to reduce or eliminate the presence of seedborne fungi
Seed treatments to control seedborne inoculum can be based on chemical, physical, mechanical, and biological practices. Chemical and physical methods will be primary considerations for quarantine issues. Although the complete elimination of seedborne inoculum is desirable for quarantine purposes, in practice seed treatments often do not provide this level of control. Few examples exist where seed treatments have reliably and consistently eradicated seedborne inoculum.

Chemical seed treatments. Fungicides have been used routinely to control seedborne pathogens and are often the cheapest and most effective means for control. Fungicides are used to kill or to inhibit growth of seedborne fungi and can be systemic or nonsystemic in their action. Highly selective systemic fungicides have proved to be most useful for the eradication of inoculum in seeds. Fungicides can be used in combination with carriers such as acetone, dimethyl sulfoxide, or dichloromethane to facilitate the infusion of the fungicide into seeds to eliminate fungi located internally. Techniques for the infusion of fungicides into seeds have been developed for numerous agricultural crops, but experiences with seeds of forest tree species are limited. Fungicides such as benomyl and thiobendazole have been used to control seedborne pathogens on conifers with mixed results. Research is needed to better define fungicide rates, duration of exposure, and use of specific carriers that can eliminate seedborne pathogens.

Disinfectants such as sodium hypochlorite and hydrogen peroxide have proved useful for elimination of inoculum associated with the seedcoats of conifer species. Research conducted on the association of the pitch canker fungus with shortleaf (P. echinata), longleaf, and Monterey pine seeds indicates that the fungus is associated primarily with the seedcoat, although some infections of internal tissues may occur. The use of surface-sterilizing agents such as hydrogen peroxide and sodium hypochlorite can reduce much of this contamination (Fig. 3). Combining this practice with the use of selective fungicides may provide effective control of pathogens internally and externally on forest tree seeds.

Figure 3. Association of Fusarium spp. and F. circinatum with longleaf pine seeds treated with sodium hypochlorite (1% for 2 min) and hydrogen peroxide (30% for 55 min). (From Fraedrich, S. W. 1996. pp. 75-81 in National Proceedings, Forest and Conservation Nursery Associations. USDA Forest Service. Pacific Northwest Station.)

Physical seed treatment methods. Heat treatments have been used to control certain seedborne pathogens while maintaining seed viability. Various methods have been used to apply heat treatment, including hot water, hot air, aerated steam, and radiation. These practices have been used to eradicate seedborne fungal pathogens of some agricultural crops, but results are variable. Some attempts have been made to use hot water treatments for control of pathogens in seeds of tree species, but results have not been as good as those with chemical control practices. Use of hot water and other heat treatments may have some limitations. These treatments may not be as effective as fungicides for the elimination of internal seedborne pathogens. In addition, it may be more difficult to achieve consistent and repeatable conditions for pathogen control.

References

Agarwal, V. K. and J. B. Sinclair. 1996. Principles of Seed Pathology. 2^nd edition, CRC Press, Inc., Boca Raton, FL. 539 pp.

Anderson, R. L. 1986. Checklist of Micro-organisms associated with tree seeds in the World, 1985. USDA Forest Service General Technical Report SE-39, 34 pp.

Dumroese, R. K., R. L. James, D. L. Wenny, and C. J. Gilligan. 1988. Douglas-fir seed treatments: Effects on seed germination and seedborne organisms, p. 155-160. In: Proceedings: Combined Meeting of the Western Forest Nursery Council, Intermountain Nursery Association and Forest Nursery Association of British Columbia.

Dwinell, L. D. and S. W. Fraedrich. 2000. Contamination of pine seeds by Fusarium circinatum. pp 75-82 in Proceedings of the Fourth Meeting of the IUFRO Working Party 7.03.04 (Diseases and Insects in Forest Nurseries). Helsinki, Finland.

Fraedrich, S. W. 1996. Seedborne diseases of southern pines and developing strategies for their control. pp 75-81 in National Proceedings, Forest and Conservation Nursery Associations. Landis, T. D. and South, D. B. Technical Coordinators. D. B. USDA Forest Service Pacific Northwest Research Station. Portland, Oregon. General Technical Report - PNW-GTR-389.

Sutherland, J. R., T. Miller, and R. S. Quenard. 1987. Cone and seed disease of North American conifers. North American Forestry Commission. Publication Number 1. Victoria, British Columbia, Canada. 77 pp.