LIMB DIEBACK OF FLOWERING DOGWOOD
James 0. Strandberg
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Since 1991, a damaging disease of flowering dogwood (Cornus florida L.) has been observed in north and central Florida. Symptoms consist of leaf spots, leaf necrosis, and twig and limb dieback. In nurseries and landscapes, severely-affected trees sometimes die. Others are destroyed or replaced because of unsightly dead limbs and deformed trunks (Fig. 1). Other symptoms consisting of canker-like deformities to limbs and trunks have also been observed. Older affected trees produce flower buds, but the buds do not open.
Many of these symptoms are strikingly similar to those of dogwood anthracnose caused by Discula destructiva which has severely damaged Dogwood in many eastern states but has not yet been reported in Florida.
Figure 1. Common symptoms of limb dieback of flowering dogwood caused by Colletotrichum acutatum. Extensive limb dieback on large, container-grown flowering dogwood tree.
Chellemi et al. (1993) first identified C. acutatum as a pathogen of flowering dogwood in the United States. There are earlier mentions of dogwood leaf anthracnose caused by C. gleospoirioides (Alfieri et al., 1994; Horst, 1990). However, the taxonomy of Colletotrichum is complicated, so it is unclear if these earlier reports actually refer to the disease described here or to C. acutatum. Moreover, the common name "dogwood anthracnose" now refers to an important and entirely different disease (described below). Smith (1993), in Connecticut, reported the infection of mature fruits of cultivated and wild dogwood trees by C. acutatum. Two years later, Britton and Redlin (1995) reported that seed-borne C. acutatum caused damping-off and seedling mortality of dogwood grown from field-collected seeds.
In Florida this apparently new flowering dogwood disease appeared about 1991 and has been since investigated by Chellemi et al. (1993), Strandberg (2001), and more recently by Strandberg and Chelemi (2002). These studies demonstrated that the leaf spot, twig and limb blight, and trunk canker aspects of the disease were all caused by the same fungus Colletotiichum acutatum. The name "Limb dieback of flowering dogwood" was proposed" for this disease.
Figure 2 Two types of leaf damage produced by Colletotrichum acutatum on diseased branches.
An array of symptoms can occur on containerized dogwood plants in the nursery and on trees planted in the landscape. Many diseased cuttings die during the first year of growth, but some diseased trees survive for a few years before symptoms are evident. Leaf damage ranges from irregular, brown leaf spots to complete necrosis, and eventually, abscission. On affected branches, leaves initially droop downward, but did not wilt severely. Later, they become silvery, gray-green and die, but do not immediately drop (Fig. 3). New leaves which develop on diseased trees are often small and deformed. Affected trees produce flower buds, but the buds do not open.
Figure 3. Wilting and death of leaves on small, diseased limb.
Entire limbs can be defoliated. Twig and branch dieback, canker-like deformities, and mortality of all sizes of container-grown plants also occurs (Fig. 1). Branches or limbs of trees, seemingly without symptoms, often die quickly once foliage growth resumes in the spring. Severely-affected trees often produce adventitious shoots at the base of the tree. These shoots are usually affected by the leaf spot phase of the disease (Fig. 4).
In warm, humid weather, the pathogen can produce masses of orange-colored conidia (spores) in clusters of acervuli that form on the bark of dead limbs or just below affected portions of diseased limbs or twigs (Fig. 5).
Figure 4. Adventitious shoots produced at the base of an diseased tree. These shoots are usually affected by the leaf spot phase of the disease.
Figure 5. Masses of orange-colored conidia (spores) produced in acervuli that form on the bark of dead limbs and twigs.
It is important to note the similarities between many of the dogwood limb dieback symptoms and those of dogwood anthracnose caused by the fungus Discula destructiva. Discula. destructiva is a major threat to native Cornus florida trees throughout the eastern and southeastern United States. Its spread and distribution are being extensively monitored, and movement of infected plants has been prohibited in some areas (Click here for additional information). The dogwood anthracnose pathogen has not yet been found in Florida, so for now, diseased dogwood trees in Florida with the symptoms described above are likely damaged by the limb dieback disease caused by C. acutatum. However, to reliably distinguish dogwood anthracnose from dogwood limb dieback requires microscopic examination or other laboratory procedures.
In Florida, the majority of the damage caused by C. acutatum thus far has occurred on the widely-grown cultivar 'Weaver' which is propagated from cuttings. Observations suggest that this disease can be easily spread by propagating material collected from diseased trees. Recently, the disease has also occurred on nursery-grown dogwood propagated from seeds that were collected from landscape trees and from wild C. florida trees growing in northern Florida. The extent of the occurrence of limb dieback on Cornus spp. growing in the wild has not yet been determined.
No effective chemical control measures for dogwood limb dieback been developed. The activities of various fungicides against C. acutatum are well-known and many are effective against other pathovars of C. acutatum that attack other plants, but at present, there is no effective strategy for chemical control of dogwood limb blight. Applications of various fungicides to diseased trees during nursery production have generally been ineffective. The most effective approach to control during nursery production are sanitation and the use of disease-free propagation material.
Alfieri, S. A., K. R. Langdon, J. W. Kimbrough, and C. Wehlburg. 1994. Diseases and disorders of plants in Florida. Florida Dept. of Agriculture and Consumer Services, Division of Plant Industry, Bulletin No. 14, Gainesville, FL. 1114 p.
Anderson, R. L., J. L. Knighten, M. Windham, K. Langdon, F. Hendrix, and R. Roncadori. 1994. Dogwood anthracnose and its spread in the South. Protection Report, Southern Region, USDA Forest Service, Publication No. R8-PR 26.
Britton, F. 0. and S. C. Redlin, 1995. Damping-off of flowering dogwood seedlings caused by Colletotrichum acutatum Plant Dis. 79:1188.
Chellemi, D. O., G. Knox, and M. E. Palm. 1993. Limb dieback of flowering dogwood caused by Colletotrichum acutatum. Plant Dis. 77:100.
Horst, R. K. 1990. Westcott's plant disease handbook. Van Nostrand and Reinhold, New York. 951 p.
Knighten, J. L. and R. L. Anderson. 1993. Distribution of dogwood anthracnose in the Southeastern United States. The United States Department of Agriculture (USDA) Forest Service, Southern Region, Atlanta, GA. Forest. Service FPM Rep. 93-1-30. 11 p.
Smith, V. I. 1993. Infection of dogwood fruit by Collelotrichum acutatum in Connecticut. Plant Dis. 77:536.
Strandberg, J. 0. 2001. A new disease of flowering dogwood caused by Colletotrichum aculatum. Plant Dis. 85:229.Strandberg, J. 0. and Chellemi, D. O. 2002. Limb dieback of flowering dogwood caused by Colletotrichum acutatum. Proc. Fla. State Hort. Soc. 115:259-262.
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