FORAGE & GRAZING LANDS Growth and Complexity of White Clover Stolons in Response to Biotic and Abiotic Stress

Fothergill et al., 1997; Wachendorf et al., 2001a). Research in the northeastern USA, however, indicated that White clover (Trifolium repens L.) persists in pastures mainly by white clover may not follow this pattern in a colder stolon growth. Morphologically complex (i.e., highly branched stolons) plants of white clover generally persist longer. We hypothesized climate. White clover plants in grazed mixed-species that biotic and abiotic stresses limit white clover production on grazing pastures in central New York were not smaller and did lands by fragmenting plants into smaller, less competitive individuals. not have a simpler branching order in spring (Karsten We measured changes in the size and structure of plants in a white and Fick, 1999). Rather, white clover plants were clover–orchardgrass (Dactylis glomerata L.) sward during a drought smaller, less complex, and had fewer roots during and (1999) and a favorable growing season (2000) in grazed pastures on for a short time after a drought in mid-summer. Karsten a southeastern Pennsylvania farm. A natural infestation of clover and Fick (1999) speculated that grazing of white clover root curculio [Sitona hispidulus (Fabricus)] and blue clover weevil during the milder winter climate in New Zealand im[Ischnopterapion virens (Herbst)] provided an opportunity to examposed treading and defoliation stress on white clover, ine the interaction of abiotic and biotic stress on stolon structure. whereas in New York white clover remains dormant White clover plants were dug from two orchardgrass (‘Pennlate’)– white clover (‘Will’) pastures during April to November 1999 and and is not grazed during the cold winter. A 30-yr study 2000. Stolon structure and damage from blue weevil and curculio of environmental effects on the persistence of white larvae were determined monthly. Drought in 1999 reduced stolon clover in Australia concluded that late summer moisture production, branching, and rooting in white clover. Stolon length stress was the critical factor limiting white clover persis(cm m 2) in 1999 was 50% of that in 2000. Clover root curculio tence (Hutchinson et al., 1995). Soil type and environdamaged up to 25% of clover roots and 20 to 40% of stolons were ment had major effects on white clover structure in three damaged by weevils. Insect damage was greatest on primary stolons. environments in Pennsylvania (Sanderson and ElWith favorable rainfall during late 1999 and in 2000, white clover winger, 2002). Companion grass cultivars also affected recovered from fragmentation and produced nearly twice the stolon the structure of seedling white clover (Sanderson and length, mass, and density in the next grazing season despite insect Elwinger, 1999) but not established white clover plants damage levels of 10 to 30%. Climate and biotic stresses are the major factors controlling oscillations of white clover stolon density in pas(Sanderson and Elwinger, 2002). tures of the northeastern USA. Biotic stress from insect or disease organisms was not reported to contribute to the seasonal variation in clover structure in the New Zealand, UK, and Wales studies (Brock et al., 1988; Fothergill et al., 1997; Wachendorf W clover is a critical component of pastures in et al., 2001a). In the New York research (Karsten and temperate humid grazing lands and persists via Fick, 1999), however, the authors speculated that fungal clonal growth of stolons and seedling recruitment (Pedpathogens caused white clover plants to fragment. Biotic erson, 1995). Through N fixation, it supplies much of stress, such as fungal and virus diseases along with inthe N needed for growth of itself and other species sects, can reduce significantly the yield and persistence within the sward. Persistence of white clover depends of white clover (Kilpatrick and Dunn, 1958). Abiotic on many factors such as soil type, slope aspect, water, stresses, along with biotic stresses of pathogens, insects, frequency and extent of grazing and cutting, soil fertility, and grazing livestock, may fragment plants into smaller, plant genetics, and insect and pathogen infestation. less competitive individuals and contribute to species Clonally propagated plants such as white clover exdisappearance from grazed swards. ploit new niches by fragmentation of existing plants A Palearctic weevil, Ischnopterapion virens (Herbst) (Brock et al., 1988). Research in New Zealand, the UK, (the blue weevil), a pest of red clover (Trifolium praand Europe has shown that fragmentation of white clotense L.) in Europe, recently was discovered as a potenver growing with perennial ryegrass (Lolium perenne tial pest of white clover in the northeastern USA. The L.) occurs mostly in spring, and the new, small, less weevil was first detected in 1994 in pastures of two complex plants (plants with fewer branches) are particueastern Pennsylvania dairy farms (Hoebeke et al., 2000). larly sensitive to stress at this time (Brock et al., 1988; Surveys during 1997 to 1999 showed the presence of the blue weevil in several northeastern states (Byers and USDA-ARS Pasture Systems and Watershed Management Research Sanderson, 2000). Adults emerge from the host plant Unit, Curtin Road, University Park, PA 16802-3702. Received 28 Oct. 2002. *Corresponding author ([email protected]). in June and feed on clover leaves. The adult weevils aestivate away from fields during the summer months Published in Crop Sci. 43:2197–2205 (2003). and return to clover in the fall to feed on clover leaves  Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA and mate. The adults overwinter in the field near the