Abstract
Twenty taxa of dogwood including cultivars of kousa dogwood (Cornus kousa) and cultivars of hybrids between flowering (Cornus florida) and kousa dogwoods were evaluated for susceptibility to dogwood anthracnose (Discula destructiva) and powdery mildew (Microsphaera pulchra). Trees were grown under the canopy of a native stand of flowering dogwood that was infected with both diseases and were also inoculated artificially with spores of D. destructiva. None of the taxa were found to be immune to dogwood anthracnose, yet there was considerable variation in resistance to the disease. Cornus kousa ‘Steeple’, a clone of C. kousa ‘Milky Way’, C. × Stardust®, C. × Stellar Pink®, and C. × Celestial™ were found to be resistant to dogwood anthracnose as indicated by ≤ 1 % of total leaf area affected, 100% survival through the following spring, and no detection of D. destructiva in woody tissue. The C. kousa cultivars ‘Wolf Eyes’, ‘Moonbeam’, and ‘Autumn Rose’ and seedlings of C. florida were highly susceptible with 100% of the total leaf area affected and 100% mortality by the following spring. The other taxa were intermediate in susceptibility to D. destructiva. Only five of the taxa: C. kousa ‘Elizabeth Lustgarten’ and ‘Steeple’ and the hybrids C. × Stardust®, C. × Constellation®, and C. × Ruth Ellen® were infected by M. pulchra.
Under favorable disease conditions, D. destructiva can be an aggressive pathogen of flowering and mountain (C. nuttallii) dogwoods (5,6,13,14,21). Since early accounts of the disease in the late 1970s (3), dogwood anthracnose has spread throughout much of the Northeastern and Northwestern United States and has caused extensive mortality in some areas (7,17,20).
Evaluation of several species of dogwood has shown considerable variation in susceptibility to D. destructiva (22). Kousa dogwood has generally been found to be more resistant to dogwood anthracnose than is flowering dogwood (8,16); however, variation in resistance among taxa of kousa dogwood has been noted. For example, Windham and Trigiano (22) reported that one selection of kousa dogwood (C. kousa var. chinensis) was relatively susceptible to D. destructiva while another unnamed selection was resistant. Identification of taxa of kousa dogwood and related hybrids with greater resistance to dogwood anthracnose would provide information for selecting disease resistant dogwoods for planting in areas where the disease is prevalent.
Powdery mildews are also common diseases on dogwoods (12). Although powdery mildews generally occur late in the growing season and seldom cause mortality, powdery mildew fungi are obligate parasites that can cause stunted, distorted growth and can be undesirable aesthetically (18). Identification and selection of taxa with natural resistance to powdery mildews would minimize this problem in the landscape.
Kousa dogwood is a popular, large-bracted dogwood valued as a landscape plant (9). There are currently over 80 cultivars of kousa dogwood that have been selected primarily for ornamental characteristics (9,15,19). Selections of hybrids between C. kousa and C. florida have also been made (11). Little information, however, is available on variations in disease resistance among these plants. The objective of this research was to evaluate 20 different taxa of dogwood including cultivars of C. kousa and hybrids of C. kousa × florida for resistance to dogwood anthracnose and powdery mildew.
Materials and Methods
Plants were propagated in 1992. Kousa dogwoods and hybrids between flowering and kousa dogwood were grafted onto seedling rootstocks of kousa dogwood. Flowering dogwoods were grown from seed. Cultivars of kousa dogwood included: ‘Autumn Rose’, ‘Big Apple’, ‘China Girl’, ‘Elizabeth Lustgarten’, ‘Gay Head’, ‘Greensleeves’, ‘Julian’, ‘Milky Way’, ‘Milky Way Select’, ‘Moonbeam’ (P.P. No. 3482), ‘Steeple’, ‘Temple Jewel’, and ‘Wolf Eyes’ (syn. ‘Princeton Varigated’). See Jaynes et al. (9), Orton (11), and Santamour and McArdle (15) for cultivar descriptions. The cultivar designation ‘Milky Way’ does not necessarily represent an individual clone (10). ‘Milky Way’ is a cultivar name given to a group of seedlings grown from open pollinated sources. ‘Milky Way’ has since become a common plant in the nursery industry and is propagated by sexual and asexual means resulting in an ambiguous cultivar designation describing variable genotypes. The ‘Milky Way’ plants included in this experiment were clonal and were propagated from a single ‘Milky Way’ tree. Similarly, ‘Milky Way Select’ is a clonal selection originating from a seedling of ‘Milky Way’ (9).
Hybrids between kousa and flowering dogwoods included: Aurora® (C. × ‘Rutban’, P.P. No. 7205), Constellation® (C. × ‘Rutcan’, P.P. No. 7210), Celestial™ (C. × ‘Rutdan’, P.P. No. 7204; formerly called Galaxy), Ruth Ellen® (C. × ‘Rutlan’, P.P. No. 7732), Stardust® (C. × ‘Rutfan’, P.P. No. 7206), and Stellar Pink® (C. × ‘Rutgan’, P.P. No. 7207). See Orton (11) for descriptions. In addition, seedlings of flowering dogwood were included for comparison.
In the spring of 1993 trees were transplanted into 19 liter (5 gal) containers with a 10:1 (v:v) pine bark: river sand substrate, amended with 4 kg/m3 (7 lb/yd3) dolomitic limestone and 3 kg/m3 (5 lb/yd3) of 13N - 2.6P - 5K fertilizer (Pro-Start, 13-66, Sta-Green Plant Food Co., Inc., Sylacauga, AL). At the time of transplanting, plants ranged in height from 45 - 90 cm (18-35 in).
On May 11,1993 plants were moved to the U.S. Forest Service, Bent Creek Experimental Forest, Asheville, NC and placed in a mixed hardwood forest beneath the canopy of a grove of native flowering dogwoods that were infected with D. destructiva and powdery mildew. The containerized plants were arranged in a completely randomized design, with 3 - 4 replicate trees per taxa. Photosynthetically active radiation was approximately 10-15% of full sunlight as measured with a quantum sensor (LI-COR, Inc., Lincoln, Neb.). Plants were fertilized with an additional 13 g (0.46 oz) / tree of 12N - 2.6P - 5K (Nursery Special, 12-6-6, Sta-Green Plant Food Co., Inc.) in mid May 1993 and irrigated as needed.
In addition to naturally occurring inoculum, plants were artificially inoculated with a suspension of D. destructiva spores (conidia) in water applied with a hand held sprayer on May 27 and June 24,1993 with 10,000 and 20,000 spores/ml, respectively. Spores were obtained from infected flowering dogwoods growing at the Bent Creek Experimental Forest.
Plants were periodically rated for disease severity throughout the growing season based on a rating system used by Anderson et al. (1). Ratings included visual estimates of 1) the percentage of leaves showing symptoms per plant (P) and 2) average percent of leaf area affected on diseased leaves (L). The product of these two factors (P × L) was also calculated to provide an estimate of the percent of total leaf area affected per plant.
Infection by D. destructiva was confirmed on symptomatic tissue based on morphology of acervuli and conidia (13). Leaf samples were examined during the 1993 growing season while twig samples were examined in the spring of 1994. If acervuli were not present on symptomatic tissue, samples were placed on wet towels in plastic boxes in an incubator at 20°C to induce sporulation. Identification of powdery mildew was confirmed based on morphology of cleistothecia, conidia, and host specificity (2,4). Symptomatic leaves were collected and examined periodically during Oct. and Nov. 1993.
In Nov. of 1993, plants were moved to an overwintering structure covered with polypropylene fabric for the winter. On Jan. 31, 1994 the plants were moved into a heated greenhouse at day / night temperatures of 24°C (75°F) / 18°C (65°F), with natural photoperiod, to force growth and to evaluate survival and shoot infection for dogwood anthracnose.
Results and Discussion
Dogwood anthracnose
Disease symptoms of dogwood anthracnose were first observed in May of 1993 on three taxa (Table 1). From May to the end of October, all plants eventually developed symptoms of dogwood anthracnose. The kousa dogwood cultivars ‘Wolf Eyes’, ‘Moonbeam’, and ‘Autumn Rose’ and the flowering dogwood seedlings were found to be highly susceptible with 100% of the total leaf area affected per plant by Oct. 20, 1993. These susceptible plants were typically infected early and suffered from rapid spread of the disease. Plant survival in the fall of 1993 for’Wolf Eyes’, ‘Moonbeam’, ‘Autumn Rose’, and C. florida was 0, 0, 33, and 0%, respectively (Table 2). Dogwood anthracnose was confirmed on leaves of all of these taxa (Table 2). Following overwintering, there were no surviving plants of these four taxa and dogwood anthracnose was confirmed in stems of all these taxa except flowering dogwood which was severely decomposed. In contrast, the taxa ‘Steeple’, Stardust®, Stellar Pink®, ‘Milky Way’, and Celestial™ were more resistant to dogwood anthracnose as indicated by ≤ 1% of total leaf area affected, 100% survival through the following spring, and no detection of the D. destructiva in stem tissue. The remaining cultivars including ‘Milky Way Select’, ‘Gay Head’, Constellation®, ‘Julian’, ‘Temple Jewel’, ‘Elizabeth Lustgarten’,’Big Apple’, Aurora®, ‘China Girl’, Ruth Ellen®, and ‘Greensleeves’ were intermediate in resistance.
Disease severity ratings including: percent of leaves infected per plant (P), average percent of area affected on infected leaves (L), and percent of total leaf area affected per plant (P × L) for Discula destructive evaluated on 20 taxa of dogwoods during a 4 month period in 1993.
Percent plant survival and confirmation of Discula destructiva from leaves (1993) and stems (1994).
Disease pressure in this experiment was relatively heavy as indicated by rapid infection, disease progression, and death of flowering dogwood and some cultivars of kousa dogwood. Although none of the taxa were immune to the disease, infection on the more resistant cultivars was confined to localized lesions and did not appear to invade woody tissue.
Many trees died during the winter period suggesting that the disease continued to progress during this time. This is supported by confirmation of D. destructiva in stem tissue of the majority of dead trees. Hibben and Daughtrey (6) observed that the rate of canker progression accelerated during winter and may account for the increased mortality during this time. In addition to D. destructiva, fungi in the genera Colletotrichum, Phomopsis, and Alternaria were routinely found on dead twigs.
Powdery mildew
Powdery mildew was first observed in early September on some cultivars and appeared to result from natural inoculation from native flowering dogwood infected with powdery mildew in the experimental area. By late October, five taxa including ‘Elizabeth Lustgarten’, ‘Steeple’, Stardust®, Constellation®, and Ruth Ellen® were infected (Table 3). Three of the hybrid cultivars (Stardust®, Constellation®, and Ruth Ellen®) were most heavily infected with over 70% of the total leaf area affected per plant. No data were collected for ‘Autumn Rose’, ‘Moonbeam’, ‘Wolf Eyes’, or flowering dogwood because they were severely defoliated or dead by that time.
Disease severity ratings for powdery mildew evaluated on dogwood taxa during 1993 including: percent of leaves infected per plant (P), average percent of leaf area affected on infected leaves (L), and percent of total leaf area affected per plant (P × L).
Despite regular inspection of leaves infected with powdery mildew, cleistothecia were only observed on one plant of Constellation®. Several asci, but no ascospores, were present in the cleistothecia. The disease organism was identified as Microsphaere pulchra.
Summary
Results from this study demonstrate considerable variation in resistance of kousa dogwood cultivars and hybrids to both dogwood anthracnose and powdery mildew. Where these diseases are prevalent, selection of resistant taxa is recommended. Taxa found to be resistant to dogwood anthracnose include: C. kousa ‘Steeple’, a clone of C. kousa ‘Milky Way’ used in this study, C. × Stardust®, C. × Stellar Pink®, and C. × Celestial™. Taxa found to be resistant to powdery mildew included the kousa dogwood cultivars ‘Milky Way Select’, the clone of ‘Milky Way’ used in this experiment, ‘Gay Head’, ‘Julian’, ‘Temple Jewel’, ‘Big Apple’,’China Girl’, and’Greensleeves’ as well as the hybrids Stellar Pink®, Celestial™, and Aurora®.
Acknowledgments
This research was supported in part by the North Carolina Agricultural Research Service, Raleigh, N.C. Technical assistance of Everett Whitman and personnel at the Mountain Horticultural Crops Research Station and the USDA Forest Service, Forest Pest Management Unit is gratefully acknowledged.
- © 1995, International Society of Arboriculture. All rights reserved.
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