Multiple Disease Resistance in Dogwoods (Cornus Spp.) to Foliar Pathogens

MATERIALS AND METHODS

RESULTS

Early powdery mildew symptoms and signs of the causal agent were first observed in the first week of June in 1998 and 2001 and the first week of May in 1999, 2000, and 2002. Disease severity increased over time; the highest disease ratings were recorded in late summer during the last week of August in 1998, 2000, and 2001 and first week of September 1999. Disease severity varied from year to year, with the highest severity recorded in 2002, followed by 2000 (Tables 1 and 2) and the lowest during 1999. Low powdery mildew incidence in 1999 was attributed to the unusually hot and dry conditions during July and August 1999 (data not shown). The use of spreader plants as sources of inoculum improved disease uniformity within cultivars as compared to results obtained from the 1998 evaluation when they were not used.

Most cultivars of C. kousa, C. mas, C. sericea, and C. alternifolia were highly resistant to powdery mildew (Table 1). Two cultivars of C. kousa, ‘Bush Pink’ and ‘Spring Grove’, were moderately susceptible (Table 2). Of the 21 C. florida cultivars, 19 were highly susceptible and two showed some resistance to powdery mildew. They were ‘Fragrant Cloud’ and ‘Cherokee Brave’ (Table 1). Four of the five interspecific hybrids of C. florida × C. kousa were resistant to powdery mildew (‘Celestial’, ‘Stellar Pink’, ‘Constellation’, and ‘Aurora’); ‘Ruth Ellen’ was slightly susceptible (Table 1).

The second most prevalent disease evaluated was spot anthracnose. This disease occurred annually beginning in early June and peaked during July or August. However, disease severity was generally moderate except in a few cultivars (Table 3). During the 1999 growing season, infection was mild because of unfavorable environmental conditions and was highest during 2001 and 2002 (Table 3). Cultivars with more than 10% of foliage showing disease symptoms are presented in Table 3; cultivars that developed insignificant disease levels are not shown. Cultivars of C. kousa that were most susceptible to spot anthracnose were ‘Lustgarten Weeping’, ‘Satomi’, and ‘Wolf Eyes’. Cornus mas ‘Golden Glory’, C. florida ‘Cherokee Sunset’, C. florida ‘Little Princess’, and C. florida ‘Rainbow’ were also very susceptible (Table 3).

Septoria leaf spot was insignificant on all cultivars except on two red-osier (C. sericea) cultivars (‘Cardinal’ and ‘Isante’) that were severely infected (Table 3). Characteristic angular lesions of Septoria leaf spot began in May, and by the end of June, more than 50% of leaves and young twigs were infected. Severe defoliation occurred, and stem lesions that girdled young twigs caused dieback every year. By mid-July, 75% of plants’ foliage had developed brown necrotic lesions covering most of the foliage. The aesthetic appearance of the plants was severely impacted.

DISCUSSION

Dogwood cultivars evaluated in this study are commonly available at most nurseries that specialize in the production of dogwood. Annual variations in climatic conditions during the 5-year study contributed to seasonal variations in disease severity. Hot, dry weather has been associated with low disease incidence, and moderate temperatures with evenly distributed rainfall have been associated with high powdery mildew severity (Aust and Hoyningen-Huene 1986; Mmbaga 2002). The hot, dry weather during July and August 1999 may be responsible for low disease incidence recorded during that season.

Only two cultivars of C. florida—’Fragrant Cloud’ and ‘Cherokee Brave’—were moderately resistant to powdery mildew (Table 1), and four cultivars C. florida—’Little Princess’, ‘World’s Fair’, ‘Pink Beauty’, and ‘Pygmy’—were moderately susceptible (Table 2). ‘Fragrant Cloud’ is susceptible in Alabama and North Carolina (Hagan et al. 1997) and ‘Cherokee Brave’ seems to have more consistent moderate resistance to powdery mildew (Ranney et al. 1994; Windham 1996; Hagan et al. 1995, 1997). Most C. kousa × C. florida interspecific hybrids evaluated were powdery mildew resistant. Similar results have been observed in Alabama (Hagan et al. 1995, 1997). However, in North Carolina, three C. kousa × C. florida hybrids (‘Constellation’, ‘Ruth Ellen’, and ‘Star Dust’) developed powdery mildew on more than 70% of their foliage (Ranney et al. 1994).

Differences in cultivar reactions to powdery mildew suggest that there is a need for a better understanding of the disease and of and host-pathogen relationships. Environmental factors and differences in degree of virulence within pathogenic species are likely sources of variations. Some researchers have reported that the interaction between environmental factors influencing powdery mildew pathogens and subsequent disease severity is complex. Temperatures, timing, and intensity of rainfall influence infection and disease severity (Wheeler 1978; Aust and Hoyningen-Huene 1986). The presence of multiple pathogens in a specific location may also significantly contribute to differences in cultivar susceptibility. In North Carolina, two fungi (E. pulchra and P. guttata) have been associated with powdery mildew in dogwoods (McRitchie 1994; Hagan and Mullen 1995). The effect of the combination of the two pathogens to disease severity is not known. In middle Tennessee, E. pulchra is the most commonly observed fungus and P. guttata is rarely found; thus, the latter is insignificant to the disease complex (Mmbaga 2000, 2002, Mmbaga and Sheng 2002). During this study, E. pulchra was the main powdery mildew pathogen.

Cultivars that were most resistant to powdery mildew, but highly susceptible to spot anthracnose and Septoria leaf spot, should not be grown where all three diseases occur. Four cultivars of C. kousa that were highly resistant to powdery mildew (‘Lustgarten Weeping’, ‘Satomi’, ‘Wolf Eye’, and ‘Golden Glory’) were susceptible to spot anthracnose, and the other 16 cultivars were resistant. Although the two red-osier cultivars (‘Isanti’ and ‘Cardinal’) were highly resistant to powdery mildew, they were highly susceptible to Septoria leaf spot (Tables 1 and 3). Septoria leaf spot is known to have little impact on dogwood growth or its aesthetic value (Daughtrey and Hagan 2001). However, results in this study showed that plant health and growth were severely impacted because of the severe dieback associated with this disease. Red-osier dogwoods are widely used in landscape plantings because their branches have attractive winter color. When red-osier dogwoods are planted in a landscape setting, protective fungicides to avoid this disease should be considered.

Powdery mildew and spot anthracnose cause annual losses in commercial nurseries throughout the southeastern states of the United States. Although C. kousa has high resistance to all three diseases, C. florida is still in high demand because of its impressive spring bloom. To reduce production costs and avoid dependency on the use of fungicides, dogwood cultivars with disease resistance should be chosen over the more susceptible ones. Results from this study suggest that interspecific hybrids between C. kousa and C. florida may have multiple disease resistance. Although D. destructiva is known to be highly destructive on dogwood (Sinclair et al. 1987; Daughtrey and Hagan 2001), it was not observed in our well-drained, full-sun plots during the 5-year study. However, this disease is commonly found in shaded, moist locations in Tennessee.

Information on diseases susceptibility in 51 commercial cultivars to three common foliar pathogens will facilitate informed decisions on the selection of cultivars that are less disease prone. The choice to grow cultivars that are prone to these common diseases necessitates considerations for fungicidal control measures.