nnp interior 11.21 1/9/09 9:25 AM Page 109 Fusarium Species— a British Columbia Perspective in Forest Seedling Production Michael Peterson MICHAEL PETERSON The Genus Fusarium: an Overview President and Principal Scientist Members of the genus Fusarium are among the Applied Forest Science Limited most important plant pathogens in the world. 4417 Bennett Road Fusarium spp. are a widespread cosmopolitan Victoria, British Columbia V9C 3Y3 Tel: 250.478.8358 group of fungi that commonly colonize aerial E-mail: [email protected] and subterranean plant parts, either as primary or secondary invaders. Fungi in this genus cause Peterson M. 2008. Fusarium species—a British Columbia perspective in a huge range of diseases on a wide range of host forest seedling production. In: Dumroese RK, Riley LE, technical coor- dinators. National Proceedings: Forest and Conservation Nursery Asso- plants. The fungus can be soil-, air-, and water- ciations—2007. Fort Collins (CO): USDA Forest Service, Rocky Moun- borne, or carried in or on plant residue or seeds, tain Research Station. Proceedings RMRS-P-57:109-125. Available at: http://www.fs.fed.us/rm/pubs/rmrs_p057.html and can be recovered from any part of a plant: roots, shoots, flowers, fruits, cones, and seeds ABSTRACT (Summerell and others 2003). This review provides a brief biological outline of some Summerell and others (2003) point out that species in the genus Fusarium and how these can be implicated as seedborne organisms leading to conifer Fusarium taxonomy has been plagued by chang- seed and seedling losses in British Columbia. Fusarium ing species concepts, with as few as 9 to over spp. are implicated with pre- and post-emergence 1,000 species being recognized by different taxon- damping-off, seedling wilt, late damping-off, root rot, omists during the past 100 years. Differing opin- and seedling mortality after outplanting. Current ions on species identification has stabilized since understanding of Fusarium spp., with regard to cone the 1980s following publications by Gerlach and and seed pest management in British Columbia, is out- lined. Shortfalls that still exist and how these might be Nirenberg (1982) and Nelson and others (1983) addressed with the development of a vision for better who defined widely accepted morphological understanding this group of fungi and a mission state- species concepts. Since that time, however, the ment of how this might be achieved are presented. application of biological (Leslie 2001) and phylo- genetic (Nirenberg and O’Donnell 1998) species KEYWORDS concepts to new, as well as existing, strain collec- diseases, pest management, tions has resulted in further splitting of many of seedborne diseases, damping-off the previously described species. If changing USDA Forest Service Proceedings :: RMRS-P-57 :: 2008 109 nnp interior 11.21 11/24/08 9:35 PM Page 110 these taxonomic designations were only rare, or duce sharply pointed macroconidia, while others of limited economic importance, they could be produce spores with rounder ends. The shapes of viewed as being merely pedantic. However, many these spores are used to differentiate morpholog- of these species can be important. For example, F. ically between species (Toussoun and Nelson andiyazi and F. thapsinum are major pathogens of 1968). Most Fusarium produce their macroconi- sorghum that differ from one another but had dia on sporodochia, which are cushion-shaped previously been grouped as F. moniliforme (Leslie fruiting structures covered with conidiophores 2001; Marasas and others 2001). Further descrip- (simple or branched hyphae bearing conidia) tion of taxonomy is well beyond the scope of this (Figures 1 and 2). Macro-conidia can also be note. However, its complexity and the recognized found, however, throughout the aerial mycelium. difficulty of rapidly identifying cultures to species Microconidia are 1- or 2-celled, ovoid or oblong, (Summerell and others 2003) has meant that and borne singly or in chains. These spores are research and development of cone and seed pest found scattered throughout the aerial mycelium. management associated with Fusarium spp. in The 1- or 2-celled microconidia are usually British Columbia (BC) has generally been limited smaller than the macroconidia. Macroconida and to genus. microconidia are produced from phialides (a A taxonomic treatment for Fusarium is pre- type of conidiogenous cell). Chlamydospores are sented for completeness: round, 1- or 2-celled, thick-walled spores pro- kingdom: Mycetae (fungi); division: Eumy- duced terminally or intercalary on older myceli- cota; subdivision: Deuteromycotina (the imper- um (Agrios 1988). Chlamydospores generally fect fungi); class: Hyphomycetes; order: function as resting spores, having the ability to Hyphales (Moniliales); genus: Fusarium. survive adverse conditions and enable the fungus Fusarium spp. are grouped in the subdivision to regenerate when favorable conditions for Deuteromycotina, which encompasses the imper- growth are reencountered. This is illustrated by a fect (asexual) fungi. Nelson and others (1983) disease triangle (Figure 3). In the presence of a point out that the perfect (sexual) states of Fusar- suitable host (for example, seedling) and ium are generally unfamiliar to many people pathogen (for example, Fusarium chlamy- working with these fungi. Plant pathologists most dospore), disease of the host will progress when often deal with the imperfect states, as the perfect the environment favors spore germination and states often have little to do with the disease prob- infection over time. lem under study. Some of the most successful Fusarium, for example, F. oxysporum and F. cul- Disease Cycle morum, appear to have lost their sexual ability Fusarium are soil inhabitants that overwinter and have adopted other methods of facilitating between crops in infected plant debris as mycelia genetic adaptations (Booth 1981). and in 3 spore forms. As chlamydospores, Fusari- um can remain in the soil for long time periods. General Characteristics Mycelium can infect healthy plant tissue in the Due to the great variability within this genus, it is same manner as spores do. Healthy plants can one of the most difficult of all fungal groups to become infected through their root tips; either distinguish taxonomically (Alexopoulus and directly, through wounds, or at the point of for- Mims 1979). Conidia (asexual spores) are hyaline mation of lateral roots (Agrios 1988). The fungus and can be divided into 3 groups: macroconidia, can grow as mycelium through the root cortex microconidia, and chlamydospores. Macroconi- intercellularly, ultimately advancing to the vascu- dia are several-celled, crescent or canoe-shaped lar tissue. As the mycelium continues to grow, spores. Their ends vary in that some species pro- usually up toward, and into the stem, it branches 110 USDA Forest Service Proceedings :: RMRS-P-57 :: 2008 nnp interior 11.21 11/24/08 9:35 PM Page 111 Figure 1. Structure of some fungi associated with disease (fungi imperfecti). and produces microconida. The proliferation of growing containers, or within attached extrane- fungal growth in a plant’s vascular tissue can ous root fragments. eventually cause the plant to wilt and die. Conifer Forest seedling nurseries represent artificial seedlings are especially susceptible to this patho- growing environments. In BC, seeds are sown in genic modality when subjected to drought stress soilless, peat-based growing media in Styro- and high transpirational demands. The fungus foam™ (Styroblock™) containers. Styro-block™ can continue to grow on the decaying tissue, containers generally reside on benches over con- where it can sporulate profusely, visibly present- crete or gravel. Seed germination and the early ing salmon to coral-pink colored sporo-dochia part of the growing cycle take place in polyethyl- on the lower portion of seedling stems. At this ene covered greenhouses where temperature, point, the spores can be spread to other plants or light, and moisture are closely controlled, and areas by wind, water, or through the movement of nutrients are applied through overhead irrigation. seedlings themselves (Agrios 1988). Fusarium are considered natural soil inhabi- tants and readily isolated from agricultural and Types of Disease forest soils. Understanding Fusarium-caused dis- In addition to vascular wilting, Fusarium can eases in forest seedling container nurseries, how- infect other plant parts close to the soil to induce ever, requires the recognition that in this growing root and stem rots. When seeds become contam- environment, Fusarium are introduced pests. inated or seedlings are infected with Fusarium, They are introduced to the container nursery via damping-off may occur. The Fusarium that cause seeds, water, and wind, or on old containers or vascular wilts can be spread in soil, dust, and irri- dirty equipment. Fusarium can lead to seed and gation water. Wind, rain, nursery equipment, and seedling losses in several ways: decaying plant tissue can also help to spread the fungus. Additionally, Fusarium can enter nurs- 1) seedborne contamination; eries as seed contaminants, be carried over from 2) pre- and post-emergence damping-off; previous years within surface cracks on dirty 3) seedling wilt; USDA Forest Service Proceedings :: RMRS-P-57 :: 2008 111 nnp interior 11.21 11/24/08 9:35 PM Page 112 4) late damping-off; 5) seedling root rot; 6) seedling mortality after outplanting. Seedborne Contamination Seedborne fungi are defined as those “that are dispersed in association with some kind of dis- persal units of the host (that is, seeds)” (Ingold 1953). This definition includes all seed types and all associated microfungi, and is the one adopted with reference to conifer seedborne fungi in BC. Some authors classify fungi as being either seed- borne or seed-transmitted (Thomsen and Figure 2. Fusarium culmorum showing sporodochia, macro- Schmidt 1999). They define seedborne fungi to conidia, and conidiophores. include all fungal types contaminating the sur- face of seeds or infecting seed tissues. Seed-trans- mitted fungi are those that cause no infection of a seed itself, but infect seedlings in the nursery or field (Neergaard 1979).