Volume 43 Number 4 Winter 1996

Figure 1. (left) Indian hawthorn cultivar Olivia proved to be highly resistant to Entomosporium leaf spot. Figure 2. (right) By early spring, several cultivars, including Harbinger of Spring, suffered complete defoliation.

Austin Hagan, Ken Tilt, Randy Akridge, and John Olive

Indian hawthorn is an evergreen shrub that offers dense foliage, a mounded canopy, and dwarf-type growth habit, all of which make it a popular choice for residential and commercial landscapes in the southern half of Alabama. But it has one major foe, the disease Entomosporium leaf spot, which is caused by the fungus Entomosporium mespili. To combat this disease, AAES research has been identifying cultivars that are leaf spot resistant.

Entomosporium leaf spot, which occurs not only in indian hawthorn but also in other wood ornamentals of the rose family, is characterized by heavy spotting of the leaves followed by premature defoliation (leaf drop). The humid, mild weather patterns in South Alabama and neighboring states favor development of this disease and spread of the pathogen.

Fungicides provide good protection from this disease and can be used in some production nurseries. Due to health and environmental concerns, the intensive spray program needed to control this disease is not a practical option for residential and commercial landscapes. The best defense against this disease in landscape settings is to use disease-resistant cultivars; however little information has been available about which cultivars of indian hawthorn are most resistant. Recent AAES variety trials have identified cultivars of indian hawthorn with good resistance to Entomosporium leaf spot.

In March 1994, 21 cultivars of indian hawthorn were established in a simulated landscape planting at the Brewton Experiment Field. Two additional cultivars, Snow White and Rosalinda, were added to the study in March 1995. The cultivars are listed in the table.

Prior to planting, soil fertility and pH were adjusted according to the results of a soil test. The plants were grown on beds mulched with aged pine bark and watered as needed with a trickle irrigation system, which is a management system that should help control the disease. Twice a year, the beds were top-dressed with a slow-release fertilizer. A visual rating of Entomosporium leaf spot damage was made on May 28, 1995, and May 29, 1996, using a scale of 1 to 5 (1 = no disease, 5 = 76-100% leaves damaged or lost due to disease).

Spread of Entomosporium leaf spot on indian hawthorn occurs in the Brewton area primarily during the winter and early spring months. Frequent showers coupled with persistent cloud cover and mild temperatures often intensify disease on this shrub. By early summer, few fresh leaf spot symptoms appear on the leaves of any of the cultivars screened and their disease ratings declined.

Over the two-year test period, considerable differences in leaf spot prevalence among the various cultivars were seen (see the table). Although none of the cultivars remained completely free of leaf spot year-round, severity of symptoms was consistently lower on several cultivars. Overall, disease ratings for many cultivars were slightly higher in 1996 than in 1995.

Indian hawthorn cultivars that consistently exhibited the best resistance to Entomosporium leaf spot were Dwarf Yedda, Indian Princess, Olivia, and Fl. In 1995, all the leaves on three of the four above cultivars remained almost spot-free. Although disease ratings were higher in 1996 for three of four cultivars than those recorded in 1995, the level of Entomosporium leaf spotting on the leaves generally remained low and disease-related damage unobtrusive. Typically, leaf spot symptoms were confined to a handful of leaves on each plant. With the notable exception of Dwarf Yedda in 1996, disease-related defoliation on these four cultivars was very light.

In 1995, the cultivar R. x delacourii also suffered very little leaf spot damage. In the months after hurricane Opal, however, nearly all the R. x delacourii died. Although the roots of the affected plants were rotted, no plant pathogens were found. Apparently, R. x delacourii is more sensitive than other cultivars of indian hawthorn to waterlogged or flooded soils. No other cultivars suffered these decline symptoms.

Nine additional cultivars demonstrated low to moderate levels of resistance to Entomosporium leaf spot. In one or both years, light to moderate leaf spot and some defoliation was seen on the cultivars Snow White, Janice, Eleanor Tabor, Majestic Beauty, Jack Evans, F2, Clara, F3, and Rosalinda. In 1996, symptoms were severe enough, particularly on the cultivars F3 and Rosalinda, that overall plant aesthetics were adversely affected.

The remaining eight cultivars of indian hawthorn were highly susceptible to Entomosporium leaf spot. In both years, heavy spotting of the leaves and severe defoliation was seen on the cultivars Pinkie, Harbinger of Spring, Enchantress, Heather, White Enchantress, Spring Rapture, F6, and Springtime. By April 1996, several cultivars had shed nearly all their leaves. Although all the above cultivars leafed-out during the late spring and early summer, they never developed the attractive dense, spreading, dark-green canopy that was characteristic of the leaf spot resistant cultivars of indian hawthorn.

Results of this study indicate that numerous indian hawthorn cultivars exist that can be maintained in home or commercial landscape settings with little or no need for fungicide applications. Based on these results, careful selection of cultivars for this resistance will help ensure indian hawthorn is an attractive, low maintenance addition to landscapes.

Hagan is a Professor of Plant Pathology, Tilt is an Associate Professor of Horticulture, and Akridge and Olive are both Superintendents of the Brewton Experiment Field and Ornamental Horticulture Substation, respectively.

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