Horticultural Reviews, Volume 49. Группа авторов

cold hardiness and found the maximum hardiness was achieved in December with both accessions surviving –27°C (which corresponds to USDA cold hardiness zone 5a). ‘Alice’ maintained cold hardiness longer into spring than ‘Alison’, suggesting that it may resist deacclimation longer, and therefore tolerate late freezes better. Additionally, this may indicate that genotypic variability for both cold hardiness and deacclimation exists within the species. The full range of variation in midwinter cold tolerance for the species and a subset of cultivars is currently being explored by the University of Minnesota Woody Landscape Plant Breeding and Genetics program. Preliminary results indicate a wide range of variation for deacclimation timing (Sherwood et al. 2021).

      H. quercifolia is susceptible to several vascular pathogens. Phytophthora nicotiana Breda de Haan, Pythium spp., and Rhizoctonia solani Kühn are fungal pathogens that infect the vasculature of oakleaf hydrangea through the roots and cause wilt disease. While no genetic resistance has been reported to exist against these various root rot pathogens, cultural practices can be employed to minimize disease pressure. Avoiding frequent irrigation and using a freely draining medium for container‐grown plants can reduce the severity of infections (Baysal‐Gurel et al. 2016b). Armillaria mellea Kumm. and A. tabescens Emel are also vascular wilt fungi that can infect oakleaf hydrangea and are especially problematic with in‐ground plantings. One possible source of Armillaria is wood mulch infected with the fungus, and therefore care should be used when using such mulching.

Many foliar pathogens have been reported to infect hydrangeas, including Cercospora hydrangeae Atk., Corynespora cassiicola Wei, Phoma exigua Sacc., Glomerella spp., Myrothecium roridum Sacc., Alternaria alternata Keissl., Xanthomonas campestris L., and Botrytis cinerea Pers. (Hagan et al. 2004; Mmbaga et al. 2012; Mmbaga et al. 2015). Among these, Ce. hydrangeae and Co. cassiicola are the most frequently occurring on infected H. macrophylla leaves (Mmbaga et al. 2012). The fungus Ce. hydrangeae and the bacteria X. campestris have been identified on oakleaf hydrangea with foliar leafspot symptoms (Hagan and Mullen 2001; Mmbaga and Oliver 2007; A. Sherwood unpubl.). Leafspots caused by X. campestris on oakleaf hydrangea are typically brown, angular lesions that are contained by the leaf veins (Figure 1.7). Because dead leaves are a source of inoculum, removing fallen leaves and avoiding overhead irrigation are effective control strategies (Baysal‐Gurel et al. 2016a). Further, Mmbaga and Oliver (2007) have shown a kaolin powder based natural pesticide to be as effective in protecting H. quercifolia from X. campestris infection as the broad‐spectrum fungicides that were also tested. Powdery mildew (Golovinomyces orontii syn. Erysiphe polygoni) is a common foliar disease on H. macrophylla, but H. quercifolia seems to be relatively more resistant (Baysal‐Gurel et al. 2016a); this difference may be due to the pubescent leaf surfaces of H. quercifolia in comparison to the glabrous leaves of H. macrophylla.

Oakleaf hydrangea does not have any particular insect pest issues, however generalist nursery and landscape pest insects will sometimes feed on H. quercifolia. Japanese beetle (Popillia japonica Newman) has been noted to be the most abundant insect feeding on H. quercifolia in our studies at the University of Minnesota (A. Sherwood, unpubl.). Mmbaga and Oliver (2007) demonstrated that kaolin powder is as effective at controlling Japanese beetle feeding on H. quercifolia as a broad‐spectrum insecticide. This would be an option for use in nursery or greenhouse settings when additional pest defense is needed. However, kaolin powder may not be suitable for landscape use considering the unsightly appearance of the leaves after application.

Figure 1.7 Photograph showing range of bacterial leafspot (Xanthomonas campestris) severity in oakleaf hydrangea.

      (Source: Photo credit: A. Sherwood.)

VII. GENETICS AND BREEDING

      A. Ploidy and Genome Size

      Genome size and ploidy have been well studied in Hydrangea. Although there is some speculation that the ancestral base chromosome number is n = 9 (Cerbah et al. 2001), the current base chromosome number in Hydrangea is n = 18, with most species (including H. quercifolia) having 2n = 2x = 36 chromosomes (Van Laere et al. 2008). Exceptions to this include H. aspera and H. involucrata, which have 2n = 34 and 2n = 30, respectively (Mortreau et al. 2010). Another exception to this is H. platyarguta Y. De Smet & Granados, which has 2n = 2x = 24 chromosomes (Qiu et al. 2009). Cerbah et al. (2001) found that the North and South American hydrangeas had the smallest genome sizes with H. quercifolia being the smallest of all species investigated (1.95 pg 2C DNA; 1.9 Gb). Zonneveld (2004) reported similar findings, noting H. quercifolia with the smallest genome at 2.17 pg 2C DNA (2.1 Gb). However, H. seemannii has been reported to have a similar genome size at 2.09 pg (2.1 Gb) (Cerbah et al. 2001).

      Although most Hydrangea are diploid, higher ploidy levels are not uncommon in the genus. Triploidy has been determined to arise from unreduced gamete production in H. macrophylla and triploid plants were noted to have an increased stem thickness and a decreased number of flowers (Jones et al. 2007; Alexander 2017). While, no naturally occurring tetraploid H. macrophylla have been documented, induced polyploidy has recently been reported in H. macrophylla (Deans et al., 2021) and H. febrifuga (syn. Dichroa febrifuga), which is the one of the most closely related taxa to H. macrophylla, has been determined to exhibit diploid, tetraploid, and hexaploid cytotypes (Rinehart et al. 2010). H. paniculata has also been documented in the diploid, triploid, tetraploid, pentaploid, and hexaploid states with tetraploidy being the most common (Cerbah et al. 2001; Funamoto and Ogawa 2002; Zonneveld 2004; Beck and Ranney 2014). Oakleaf hydrangea has thus far only been documented in the diploid state; however, morphological traits could potentially be enhanced by induced polyploidy. Polyploidy can be induced using antimitotic chemicals or by the strategic use of unreduced gametes, although no H. quercifolia genotypes have been reported to produce 2n gametes.

B. Pollination Biology

      Gametophytic self‐incompatibility seems to be widespread throughout Hydrangea (Reed 2000, 2003, 2004, 2005; Mortreau et al. 2003). Reed (2000, 2004) reported the capacity of self‐pollen to germinate on H. quercifolia stigmas, but not grow long enough to reach the ovaries, except in a very low percentage of cases. Additionally, it was shown that cross‐pollen typically outcompetes self‐pollen when present. The stigmas are receptive to pollen from the day after anthesis until five days post‐anthesis (Reed 2004). The pollen tube grows to the bottom of the style within 48 h after pollination and has fertilized the ovule within 72 h of pollination. It has been shown with H. arborescens and H. macrophylla that pollen is able to be stored at –20°C for at least three months with only a marginal decrease in viability (Kudo and Niimi 1999). However, viability of stored pollen has yet to be confirmed empirically in H. quercifolia.

C. Breeding for Disease Tolerance/Resistance

      Breeding for root rot resistance should be a priority in H. quercifolia, considering the disease is often lethal. No resistance genes have been reported to date, but by screening diverse, wild germplasm, the possibility exists to find tolerant material. Opportunities for high throughput screening for resistance to Phytophthora has been demonstrated in crops such as Nicotiana, Solanum, and Lycopersicon using culture filtrates incorporated into a tissue culture medium (Behnke 1979; van den Bulk 1991). Although promising, more information about the host–pathogen interaction is needed in order to implement such procedures in H. quercifolia. Because Phytophthora is a vascular disease and may