Austin K. Hagan, Randy Akridge, and John Olive

Few flowering shrubs can match the beauty, versatility, and popularity of the rose. However, Alabama’s moist and temperate climate is particularly favorable to the development and spread of black spot and other foliar diseases of rose, such as powdery mildew, Cercospora leaf spot, and downy mildew.

As a result, each year 30 or more weekly fungicide sprays are typically required to protect most roses from these diseases and maintain their health and vigor. Such an intensive fungicide spray program limits the use of roses in residential and commercial landscape plantings across Alabama. AAES research has helped identify several groundcover and shrub roses that can thrive in Alabama’s challenging growing conditions.

Typical black spot symptoms on leaves of a shrub rose.

Recently, additional attention has been focused on the use of groundcover and shrubs roses as a means of adding color to residential and commercial landscapes. In response to this, many groundcover and shrub roses are being released by wholesalers to the nursery industry. Although claims of disease resistance have been made, the reaction of these roses to common foliar diseases has not been well documented, particularly under typical Deep South weather conditions.

Of the foliar diseases of rose reported in Alabama, black spot is considered the most common and destructive. Heavy black spot-related leaf spotting and premature leaf shed sharply reduce the size and abundance of flower blossoms, and also damage plant health and vigor. Powdery mildew is also a common disease on some rose cultivars, but unlike black spot rarely causes serious damage. Although downy mildew has on occasion devastated plantings of container-grown roses in nurseries, few damaging outbreaks of this disease in Alabama landscapes have occurred. To date, the occurrence of Cercospora leaf spot has been very sporadic and has not been a cause for concern.

To learn more about disease resistance of groundcover and shrub roses, a study was instituted at the Brewton Experiment Field in the winter of 1998. Bare-root roses were potted and allowed to grow under shade for one month. Potted roses were then transplanted into raised beds at the Experiment Field. The majority of cultivars screened were planted on either January 30 or March 19, 1998. Rosa mutabulis (butterfly rose) was planted on June 4, 1998, while the shrub roses Carefree Wonder, Hansa, and Pink Grootendorst and the hybrid tea rose Double Delight were established on February 11, 1999. The beds were mulched with pine bark, watered as needed by a drip irrigation system, and standard weed control measures were applied to the plots.

The severity of black spot and Cercospora leaf spot was rated on a scale of 1 to 10 (see Table 1 footnote) on April 25, June 3, August 5, October 16, and December 3, 1998, and on March 23, May 6, and June 24, 1999. On selected cultivars, leaf samples were collected on June 24, 1999, to confirm the diagnosis of Cercospora leaf spot and black spot. Data collected on December 3, 1998, and June 24, 1999, are presented in the tables. Observations of foliage quality and plant vigor were recorded at the time disease ratings were taken.

Table 1. Sensitivity of Selected Cultivars of Groundcover and Shrub Roses to Black Spot, 1998 and 1999, Brewton Experiment Field
Cultivar		Rose type1	Black spot rating2		Cultivar	Rose type1	Black spot rating2
			1998	1999			1998	1999
Royal Bonica		S	6.6	6.4	White Flower Carpet	S	4.2	1.0
Betty Prior		S	6.6	6.2	Red Cascade	GC	3.8	3.0
Sevillana		S	6.4	6.0	Rosa mutabulis	S	3.8	5.6
First Light		S	6.4	5.6	Rosa wichuraiana	GC	3.6	1.8
Carefree Delight		S	6.2	1.0	Happy Trails	GC	3.4	1.0
Bonica		S	6.1	5.4	Magic Carpet	GC	3.0	6.0
Cherry Meidiland		S	6.0	6.0	Ralph's Creeper	GC	3.0	6.4
Nearly Wild		S	5.8	6.4	Flower Carpet	S	2.6	1.0
Jeeper's Creeper		GC	5.6	6.6	Petite Pink Scotch3	S	1.0	1.0
Pearl Meidiland		S	5.6	5.8	The Fairy	S	1.0	1.0
Sea Foam		S	4.2	5.2	Double Delight	HT	ND4	6.8
Mystic Meidiland		S	5.2	3.0	Carefree Wonder	S	ND	4.4
Easy Livin'		S	5.2	6.0	Pink Grootendorst	S	ND	3.8
Fushia Meidiland		GC	4.8	1.0	Hansa	S	ND	1.4
Nozomi		S	4.4	3.2
1Rose type: S= shrub rose, GC = groundcover rose, HT = hybrid tea rose. 2Black spot was rated on a 1 to 10 scale where 1 = no disease, 2 = very few spots in lower canopy, 3 = a few spots in lower and upper canopy, 4 = some spots with light defoliation in lower canopy, 5 = spots noticeable with noticeable defoliation, 6 = spots numerous with significant (50% or more) defoliation, 7 = spots numerous with severe (75% or more) defoliation, 8 = upper canopy badly diseased with high level (90% or more) of defoliation, 9 = very few remaining leaves covered with spots, and 10 = plant defoliated. 3Petite Pink Scotch was heavily damaged in 1998 and 1999 by aerial blight caused by Rhizoctonia solani. 4ND = no data, cultivar planted in 1999.

In 1998, weather conditions at the Experiment Field from May through August were unusually hot and dry and did not favor the development of foliar diseases. In September, approximately 30 inches of rainfall from two tropical storms was recorded. Mild and wet weather through the remainder of the growing season stimulated rapid development of Cercospora leaf spot and black spot. From March through May 1999, rainfall totals were below seasonal norms while temperatures were near normal. In June, near normal rainfall patterns again favored black spot and Cercospora leaf spot development.

Although none of the groundcover and shrub roses were immune to black spot or Cercospora leaf spot, considerable differences in the severity of both of these diseases were seen (see tables 1 and 2). In both years, black spot was the most common disease recorded, particularly in the fall of 1998. The following spring, Cercospora leaf spot, however, damaged a few selected cultivars as severely as black spot did. In the spring of 1998 and 1999, powdery mildew and downy mildew were noted on one or two cultivars but damage was negligible. Surprisingly, aerial blight caused by the fungus Rhizoctonia solani, a disease rarely found in landscape plantings, damaged the cultivar Petite Pink Scotch. Blighting of the inner canopy, which occurred in June 1998 and 1999, was quite unsightly and extensive.

Table 2. Reaction of Selected Shrub and Groundcover Roses to Cercospora Leaf Spot, 1998 and 1999, Brewton Experiment Field
Cultivar	Cercospora leaf spot rating1		Cultivar	Cercospora leaf spot rating1
	1998	1999		1998	1999
Petite Pink Scotch	5.6	1.0	Cherry Meidiland	1.0	1.0
The Fairy	5.0	6.0	Pearl Meidiland	1.0	1.0
Fushia Meidiland	1.0	6.0	Rosa mutabulis	1.0	1.0
Carefree Delight	1.0	6.0	Rosa wichuraiana	1.0	1.0
White Flower Carpet	1.0	5.7	First Light	1.0	1.0
Happy Trails	1.0	4.6	Bonica	1.0	1.0
Flower Carpet	1.0	4.4	Carefree Wonder	ND2	1.0
Jeepers Creeper	1.0	1.0	Sea Foam	1.0	1.0
Ralph's Creeper	1.0	1.0	Pink Grootendors t	ND	1.0
Royal Bonica	1.0	1.0	Nozomi	1.0	1.0
Nearly Wild	1.0	1.0	Red Cascade	1.0	1.0
Betty Prior	1.0	1.0	Mystic Meidiland	1.0	1.0
Sevillana	1.0	1.0	Hansa	ND	1.0
Magic Carpet	1.0	1.0	Double Delight	ND	1.0
Easy Livin'	1.0	1.0
1See rating scale listed as footnote 2 in Table 1. 2ND = no data, cultivar planted in 1999.

By December 1998, black spot-induced leaf spotting and early leaf loss were seen on all cultivars except Petite Pink Scotch and The Fairy (Table 1). In June of 1999, no black spot-related leaf spotting and premature leaf shed were noted on Carefree Delight, White Flower Carpet, Happy Trails, Flower Carpet, Petite Pink Scotch, and The Fairy (Table 1). Shrub and groundcover roses suffering the heaviest black spot-related damage included Jeeper's Creeper, Royal Bonica, Nearly Wild, Betty Prior, First Light, Cherry Meidiland, Easy Livin', and Pearly Meidiland. So far, the hybrid tea rose Double Delight, which was included as a black spot-susceptible standard, proved to be the most black spot-sensitive cultivar screened in 1999.

In 1998, the occurrence of Cercospora leaf spot was confirmed only on The Fairy and Petite Pink Scotch. On both shrub roses, considerable defoliation in the lower and mid-canopy and spotting of many of the remaining leaves were seen (Table 2). In 1999, significant disease development was seen on six of the 28 cultivars of shrub and groundcover roses screened (Table 2). Overall, the level of spotting and leaf loss attributed to Cercospora leaf spot was similar to that caused by black spot.

As was seen in the previous year, heavy spotting of the leaves and leaf shed were recorded on the shrub rose The Fairy. Other cultivars suffering significant Cercospora-related damage included the groundcover roses Fushia Meidiland and Happy Trails as well as the shrub roses Carefree Delight, White Flower Carpet, and Flower Carpet. Surprisingly, no Cercospora leaf spot was noted in June 1999 on the shrub rose Petite Pink Scotch, a cultivar heavily damaged by this disease the previous year. So far, no mixed outbreaks of black spot and Cercospora leaf spot were observed on any of the cultivars screened.

Over the two-year test period, black spot and Cercospora leaf spot have been the two most prevalent diseases seen. Of the two diseases, black spot is the most common but both extensively damaged selected susceptible rose cultivars. The surprising prevalence of Cercospora leaf spot indicates that this disease may be a more common and damaging disease on roses than previously thought. Although light infections of powdery mildew and downy mildew were seen, neither disease was cause for concern.

A handful of the shrub and groundcover roses screened demonstrated good to excellent resistance to both black spot and Cercospora leaf spot. Over the two-year test period, the groundcover rose R. wichuraiana showed the best resistance to both of these diseases, and this rose appear to tolerate hot, droughty conditions. In 1999, the shrub rose Hansa also suffered from light disease-related leaf spotting and little if any leaf loss. The groundcover rose Red Cascade and shrub rose Nozomi also proved less susceptible to black spot than the majority of other roses. Both cultivars also are highly resistant or immune to Cercospora leaf spot. In its first year in this study, the shrub rose Pink Grootendorst has suffered no more damage from black spot or Cercospora leaf spot than Red Cascade or Nozomi.

These results indicate that plantings of the above groundcover and shrub roses probably can be maintained without the need for protective fungicide sprays and would be excellent additions to a low maintenance landscape. All the remaining roses, however, would require weekly to possibly monthly fungicide sprays to maintain their beauty, health, and vigor.