Cornus x rutgersensis Mattera, T. Molnar & Struwe, 2015

Mattera, Robert, Molnar, Thomas & Struwe, Lena, 2015, Cornus x elwinortonii and Cornus x rutgersensis (Cornaceae), new names for two artificially produced hybrids of big-bracted dogwoods, PhytoKeys 55, pp. 93-111 : 97-102

Cornus x rutgersensis Mattera, T. Molnar & Struwe hybr. nov. Rutgers’ dogwood Figs 3, 4, 5

Diagnosis.

Cornus x rutgersensis is similar to Cornus kousa and Cornus florida , but differs in its intermediate leaf size and fruit aggregation and size. Cornus x rutgersensis has leaves 9.0-16.8 × 4.2-9.1 cm, whereas the leaves of Cornus kousa are 5.1-10.2 × 2-5 cm and for Cornus florida 7.6-15.2 × 2-7 cm). Cornus x rutgersensis forms many single-seeded parthenocarpic drupes 0.5 × 0.25 mm wide, but does not form a multiple fruit as in Cornus kousa . Cornus florida has larger, fertile drupes 13-18 × 6-9 mm.

Type.

USA: New Jersey: New Brunswick, Middlesex County, Ryders Lane, Rutgers Gardens, original tree (ramet) of ‘Rutgan’ Stellar Pink®, cultivated plant in open grass field behind Rutgers Ornamental Horticultural Field lab, adjacent to a pine tree windscreen, GPS (WSG84) 40.4732N, -74.4238E, 22 m, 25 May 2014, R. Mattera 34, holotype (NY), isotypes (CHR, JEPS, MO, US, to be distributed).

Description.

Trees with upright or rounded habit, F1 hybrids cultivated at Rutgers range from 3 -10 m in height at maturity. Bark smooth when young, light gray to brown older bark exfoliating; lenticels on young bark abundant, 0.5-0.7 × 0.3-0.4 mm. Leaves opposite, simple, ovate to elliptic, 9.0-16.8 × 4.2-9.1cm; base attenuate, cuneate-crenate to oblique; margin entire to moderately wavy; apex apiculate or acuminate; with 5 pairs of secondary veins; abaxial surface smooth; indumentum of many white trichomes on both surfaces, abaxial margin with many white trichomes, with dark tufts of trichomes along midrib and veins. Overwintering inflorescence buds intermediate in size and developmental structure between the parents. Outermost vegetative bracts barely covering the inflorescence; inner two pairs of floral bracts enclosing flower head; unlike in either parent, floral bracts covering only 10-45% of the flower head. Inflorescence capitate, globose, with 30-50 flowers per head, surrounded by 4 floral bracts; floral bracts sessile, entire, in decussate pairs, petaloid at anthesis, ovate to lanceolate, sometimes wider than long, overlapping or not; 4.0 –6.5× 3-6 cm, white or pink; base tapering to point of attachment, apex acuminate to cuspidate. Peduncle 3.5-7.5 cm long at time of flowering. Flowers actinomorphic, bisexual; 4-merous. Calyx lobes ovate, acute. Corolla lobes obovate, slightly acute. Stamens 4, exserted, inserted in corolla lobe sinuses; filaments 2.7-4.5 mm long, 0.2-0.3 mm wide; anthers longitudinally dehiscent, 0.4-2.0 × 0.5-0.8 mm; pollen yellowish brown. Gynoecium epigynous, with nectar disc; ovary syncarpous; style 1, inserted to exserted from corolla mouth, 1.5-1.9 × 0.3-0.5 mm; stigma slightly capitate, ca. 0.25 mm long. Fruit single drupes, rarely fused into a multiple fruit; fruits often formed without proper seed development (i.e., sterile fruits), if fertile, then 1-seeded.

Parent source materials.

The parents of the described type F1 hybrid ( ‘Rutgan’ Stellar Pink®) are Cornus kousa K2 (female) grown at Rutgers Gardens from a seedling received from Ben C. Blackburn, Willowwood Arboretum (Gladstone, NJ) in May of 1949, and Cornus florida 'Sweetwater Red’ (male), received from Boyd Nursery (McMinnville, TN) and planted at Rutgers Gardens.

Ecology and phenology.

Cornus x rutgersensis flowers in New Jersey (USA) in May; the fruits mature from September to October. Adrenid and halictid bees and cerambycid beetles pollinate the flowers of Cornus kousa while only adrenid and halictid bees pollinate Cornus florida ( Rhoades et al. 2011). It is believed that the same insects visit the flowers of the hybrid. All cultivars released to the public, except ‘KF111-1’ Hyperion® (first backcross to Cornus kousa ), are sterile. Sterile specimens produce very little pulp in the fruit and no fully formed seeds. It is unlikely that these aborted fruits serve as a significant food source for insects or birds. Hyperion® produces fruits that are more similar to Cornus kousa and likely serve as a food source for wild animals, although there are no studies to substantiate this assumption.

Etymology.

The epithet rutgersensis is based on Rutgers University, The State University of New Jersey, the academic home of Dr. Elwin Orton’s dogwood breeding program, which is now continued by co-author Thomas Molnar. Rutgers University was founded in 1766 in New Brunswick, NJ, and was named in 1825 after Colonel Henry Rutgers, a US Revolutionary War veteran ( Rutgers University 2014). We suggest the common name Rutgers’ dogwood for this hybrid.

Distribution.

Cornus x rutgersensis is known only from cultivation. One of the parent species, Cornus florida , an understory tree in mesic forests ( Fulcher et al. 2012, Hillier Nurseries 2002, Porter 1903, Schwartz 1994, Wennerberg 2006), ranges from southern Maine to Florida, and as far west as Texas in the USA ( Mohlenrock 2006, Schwartz 1994, Wennerberg 2006). Cornus kousa occurs in mesic forests in Japan, Korea, and China (Flint 1997, Xiang and Boufford 2005). No formal studies have been done to determine climate range for Cornus x rutgersensis ; however, it is generally believed that its range is similar and intermediate between the two parent species Cornus florida and Cornus kousa .

Horticulture.

Cornus x rutgersensis is grown as a landscape ornamental and, in general, can be cultivated wherever Cornus florida or Cornus kousa can be grown. Cornus x rutgersensis is typically propagated asexually through budding and grafting on seedling rootstocks of Cornus kousa or Cornus florida . The cultivars ‘KF1-1’ Saturn®, ‘Rutban’ Au rora ®, ‘Rutcan’ Constellation®, ‘Rutdan’ Celestial,®, ‘Rutfan’ Stardust®, ‘Rutgan’ Stellar Pink®, and ‘Rutlan’ Ruth Ellen® are all direct F1 hybrids of Cornus florida and Cornus kousa , and all produce sterile fruit. ‘KF111-1’ Hyperion® is a first backcross to Cornus kousa and produces some fertile fruit. We know of no other commercially available plants of Cornus x rutgersensis .

Disease response.

Cornus x rutgersensis shows resistance to dogwood anthracnose and resistance or high levels of tolerance to powdery mildew ( Erysiphe pulchra and Phyllactinia guttata ; Li et al. 2009, Ranney et al. 1995, Trigiano et al. 2005).

Additional material examined.

Additional collections from the same individual from which the holotype was collected, but at other dates: R. Mattera 26, R. Mattera 28, R. Mattera 30, R. Mattera 32, will all be deposited at CHR, NY, and MO).