Austin K. Hagan, Ken Tilt, David Williams, and Randy Akridge

The combination of spectacular floral displays, brilliant fall color, and brightly colored fruit have made the flowering crabapple a fixture in many residential and commercial landscapes across the Northeast and Midwest. In Alabama and surrounding states, however, concerns about their adaptability and disease susceptibility have greatly limited the use of this versatile, small, flowering tree.

Diseases are a significant threat to the health and beauty of crabapple, particularly in the hot and wet regions of the Deep South. Typically, the most destructive diseases on crabapple and its close relatives, the apples, are fireblight (Erwinia amylovora) and cedar-apple rust (Gymnosporangium juniperi-virginiana). Farther north, apple scab (Venturia inequalis) and frogeye leaf spot (Botryosphaeria obtusa) also are recognized as damaging diseases on crabapple.

The release of a number of new cultivars and selections has fueled interest in adding crabapples to residential and commercial landscapes in Alabama and other southern states. However, the adaptability of these new releases to growing conditions in Alabama had not been determined nor had their sensitivity to disease. To answer these questions, a planting of 60 crabapple selections was established in 1992 at the Brewton Experiment Field (Plant Hardiness Zone 8) as part of the National Crabapple Evaluation Program.

In May 1992, bare-root crabapple liners were planted in a Benndale (A) fine sandy loam on 18-foot centers in rows spaced 10 feet apart. Before planting, these liners, which had been healed-in due to planting delays, had begun to leaf-out. Each spring, approximately three pounds of 5-10-15 fertilizer was evenly spread around the base of each tree. The trees were irrigated. A vegetation-free zone was maintained with directed applications of a tank mix of one pound of Gallery DF and two quarts of Surflan T/O per treated acre and directed applications of recommended rates of Roundup. During the growing season, alleys between the rows of trees were mown about every two weeks. Fireblight was rated on a scale of 0 to 4 on May 28, 1993, May 24, 1994, and May 29, 1996. (Fireblight severity was assessed on a scale of 0 to 4 where 0 = no disease, 1= one or a few blighted spurs or shoot tips, 2 = numerous blighted spurs or shoot tips, 3 = several major branches damaged, 4 = major portion of tree damaged or tree died.) Apple scab severity was recorded on a scale of 0 to 5 on the same dates as fireblight in 1993, 1994, and 1996. Tree survival was noted at each disease rating date. In April 1997, the landscape value of each crabapple selection was assessed (Landscape value ratings range from 1 to 5 where 1 = not suitable for landscape use and 5 = highly desirable for landscape use.)

Over the test period, fireblight was the most common and damaging foliar disease seen on crabapple. Other foliar diseases noted on selected crabapples included apple scab, frogeye leaf spot, and cedar-quince rust. Powdery mildew, which is a common disease of crabapple in other regions of the United States, was never found.

In each year, considerable differences in the level of fireblight-related blossom blight and shoot dieback was noted among the 60 crabapple selections (see Table 1, “Reaction of Selected Crabapples to Fireblight”). The only crabapple that remained free of fireblight in 1993, 1994, and 1996 was M. baccata cv. Jackii. Other selections that consistently suffered little if any noticeable fireblight damage were Coral Burst, Pink Princess, Robinson dwarf, Dolgo, Spring Snow dwarf, Adams dwarf, Radiant Pink Spires, Adams, Liset, Jewelberry, Velvet Pillar bush, Profusion, Bob White, and White Angel. During at least one year, noticeable and usually unsightly blossom blight and shoot dieback were seen on the remaining 45 crabapple selections. Over the test period, the heaviest damage was found most often on the Mary Potter, Silver Moon, Brandywine, and Klehm's Improved Bechtel crabapples.

Fireblight on Klehm's Improved Bechtel.

Apple scab and cedar-quince rust caused far less damage on fewer crabapples than did fireblight (data not shown). On the majority of selections, no scabbing of the leaves was seen. In only one of three years, very low apple scab levels were found only on Indian Magic, Royalty dwarf, Red Barron, Strawberry Parfait, Liset dwarf, Dolgo, Pink Spires, and Robinson dwarf. The Radiant crabapple was lightly damaged in 1993 and 1994 but not in 1996. Severe scabbing of the leaves and early scab-related defoliation occurred each year on only the Eleyi crabapple. Symptoms of cedar-quince rust were found on the fruit of 10 crabapple selections. The fruit of M. floribunda and M. floribunda dwarf were colonized most often by the cedar-quince rust fungus, but damage was light.

Frogeye leaf spot on Spring Snow dwarf.

In 1994 and 1996, light to moderate outbreaks of frogeye leaf spot were seen on a few select crabapples. Heaviest spotting of the leaves was seen in both years on Spring Snow dwarf, Louisa, David, and Ormiston Roy. In either 1994 or 1996, typical but minor leaf spot damage was noted on six additional crabapple selections.

In the first year, the planting delay and lack of irrigation contributed to the death of many trees, particularly M. baccata cv. Jackii, Donald Wyman, Golden Raindrops, Red Splendor dwarf, Sinai Fire, Red Barron, and Red Jewel (data not shown). With the notable exception of Radiant, one or more trees of the remaining 52 crabapples were lost due to the above factors in the first year after establishment. With the exception of Adams dwarf, Eleyi, Sentinel, Spring Snow dwarf, Velvet Pillar shrub, and Radiant, the numbers of survivors for many crabapple selections recorded between 1993 and 1996 sharply declined. Other causes of tree loss may have included drought, a sudden hard freeze, inadequate chill hours, and a root and collar rot caused by the fungus Oxysporum populinus. Tree loss due to this disease appeared to have been concentrated in wet, poorly drained areas behind terraces and was less common on the higher, drier portions of the site. The identity and numbers of individual crabapple selections lost due to Oxysporum root and collar rot was not determined.

After eliminating trees susceptible to fireblight and a poor survival rate, crabapples with the highest landscape ratings include Eleyi, Radiant Adams, Velvet Pillar tree form, and Tea (see Table 2, “Landscape Value of Selected Crabapples, 1997”). Red Splendor dwarf and M. baccata cv. Jackii also had very high landscape values (4.0), but many of these trees died shortly after establishment. Profusion, Red Jade, Snowdrift, Pink Spires, Liset, Lousia, and Brandywine crabapples also had good landscape values; however, Oxyporus root and collar rot or another stress-related disorder killed many trees.

In summary, poor survival due to prevalence of Oxysporus collar and root rot indicates that the majority of crabapple selections tested are not well adapted to the Coastal Plain of Alabama. Of the crabapples screened, Eleyi, Radiant Adams dwarf, Velvet Pillar tree, and Tea exhibited good landscape ratings, high survival rates, and generally good fireblight and frogeye leaf spot resistance, indicating they would be acceptable choices for Alabama landscapes. Red Splendor dwarf and M. baccata cv. Jackii, which both had good overall disease resistance (particularly to fireblight) and high landscape ratings, should also make good landscape trees.