Epidemiology and Pathology of Strawberry Anthracnose: a North American Perspective Barbara J

Epidemiology and Pathology of Strawberry Anthracnose: A North American Perspective Barbara J. Smith U.S. Department of Agriculture, Agricultural Research Service, Thad Cochran Southern Horticultural Laboratory, Small Fruit Research Unit, P. 0. Box 287, Poplarville, MS 39470 Additional index words. Colletotrichum acutatum, C. fragariae, C. gloeosporioides, Fragaria xananassa, disease control, fungicides Abstract. Three Colletotrichuin species—Colletotrichum acutatum J.H. Simmonds (teleomorph Glomerella acutata J.C. Guerber & J.C. Correll), Colletotrichumfragariae A.N. Brooks, and Colletotrichum gloeosporioides (Penz.) Penz. & Sacc. in Penz. Iteleomorph Glonierella cingulata (Stoneman) Spauld. & H. Schrenkj—are major pathogens of strawberry (Fragaria xananassa). Strawberry anthracnose crown rot has been a destructive disease in commercial strawberry fields in the southeastern United States since the 1930s. The causal fungus, C. fragariae, may infect all aboveground plant parts; however, the disease is most severe when the fungus infects the crown, causing crown rot, wilt, and death. Colletotrichum gloeosporioides was responsible for an epidemic of anthracnose crown rot in strawberry nurseries in Arkansas and North Carolina in the late 1970s. The anthracnose fruit rot pathogen, C. acutatum, was first reported in 1986 on strawberry in the United States. Since the 1980s, increased losses due to anthracnose fruit and crown rots in the United States may be related to changes in cultivars and to widespread use of annual plasticulture production rather than the matted-row production system. Anthracnose investigations in the United States have concentrated on its epidemiology and differences among the three causal Colletotrichum spp. in their cultural, morphological, and molecular characteristics; their infection processes; and their pathogenicity. Results from these studies have resulted in a better understanding of the diseases and have led to better disease control. Strawberries grown in soils with high nitrogen levels are more susceptible to anthracnose than are those grown in soils with lower nitrogen levels or those amended with calcium nitrate. Anthracnose is spread more rapidly in fields that have overhead irrigation and plastic mulch than in fields where drip irrigation and straw mulch are used. Fungicide efficacy has been determined in in-vitro, greenhouse, and field studies, and pathogen resistance to some fungicides has been detected. Anthracnose-resistant cultivars are a major objective of most strawberry breeding programs in the southern United States. Colletotrichum species cause serious dis- 1992; Smith, 1998a, 1988b, 1988c) cause in culture. The growth rate of C. acutatum in eases of many fruit and vegetable crops anthracnose diseases of strawberries. C. fra- culture was slower than the other two species worldwide, and three species, C. fragariae, gariae was assumed to be the causal agent of at all temperatures tested with the greatest C. acutatum, and C. gloeosporio ides, cause strawberry anthracnose in the United States difference being at 32 °C. anthracnose diseases of strawberry (Smith until 1986 when Smith and Black (1986) Colletotrichum fragariae, the "original" and Black, 1990). Since the 1930s, anthrac- reported the presence of C. acutatum on anthracnose fungus, was first identified in nose crown rot caused by C. fragariae and strawberry in the United States. This fungus Florida in 1931 (Brooks, 1931). It spread C. gloeosporioides has been a destructive had previously been reported to cause throughout the southeastern United States disease in strawberry nurseries and fruit- anthracnose fruit rot of strawberry in Queens- and was responsible for crown rot and death production fields in the southeastern United land, Australia (Simmonds, 1965); however, of many plants in strawberry nurseries in the States (Brooks, 1931). In 1986, the presence it had probably been present in the United 1970s (Horn etal., 1972). It has a narrow host of the anthracnose fruit rot pathogen, States for some time under the name of range, infecting only strawberry and a few C. acutatum, was first reported on strawberry Gloeosporium spp. (Maas, 1984). Colletotri- weed hosts, and is rarely found outside the in the United States (Smith and Black, 1986). chum gloeosporioides causes anthracnose southeastern United States. Colletotrichum Increased losses due to anthracnose fruit and crown rot, petiole lesions, and leaf spots fragariae generally causes more severe pet- crown rots in the United States since the indistinguishable from those caused by iole and crown symptoms than C. acutatum, 1980s may be related to the shift from C. [ragariae. and C. fragariae is considered by some to matted-row culture to the annual plasticulture In a series of comparative studies of these be a host-specific or con-specific form of production system, as well as to changes in three major causal agents of strawberry C. gloeosporioides (Howard, 1983; Howard cultivars. Anthracnose diseases are increas- anthracnose, Smith and Black (1990) exam- et al., 1992; Sutton, 1992). ing in importance and resulting in major ined the cultural characteristics, conidia, In the late 1970s, C. gloeosporioides was economic losses to strawberry growers appressoria, and setae of 24 Colletotrichum identified as the causal agent on plants worldwide. The objective of this report is to isolates. Colletotri chum fragariae isolates obtained from Arkansas and North Carolina summarize anthracnose related research from developed beige to olive to dark gray colo- nurseries that died from a crown rot identical the United States (excluding research from nies, did not form the ascigerous state in to that caused by C. fragariae (Howard et al., California and Florida) so that we can better culture, and their conidia were typically 1992). It has a wide host and geographic understand these diseases and their control. cylindrical with one end sharply tapered range, causing diseases of many plant hosts and the other end rounded. Colletotrichum worldwide. ANTHRACNOSE PATHOGENS gloeosporioides isolates were very similar The greatest economic losses due to except they formed the Glomerella cingulata anthracnose on strawberry are from fruit rot Colletotrichum fragariae, C. acutatum, ascigerous state in culture and their conidia caused by C. acutatum, which also infects and C. gloeosporioides (Howard et al., usually were rounded on both ends. Isolates many other fruit and vegetable crops, includ- of both C. fragariae and C. gloeosporioides ing apples, tomatoes, peppers, peaches, blue- produced dark black setae, visible with a berries, blackberries, and grapes (Bernstein Mention of trade names or commercial products in hand lens, in acervuli in culture and on et al., 1995; Howard et al., 1992; Smith, of this article is solely for the purpose providing Colletotri- 2002). The presence of the pathogen has been specific information and does not imply recom- petiole, stolon, and fruit lesions. mendation or endorsement by the U.S. Department chum acutatuin isolates produced fusiform reported on strawberries in almost all areas of Agriculture. conidia tapered on both ends; developed of the world where they are grown. Crown To whom correspondence should be addressed; white, pink, orange, rose, or beige colonies; infections of strawberry plants by C. acuta- e-mail Barbara. Smithars.usda.gov and did not form setae or the ascigerous state tum often result in stunted plants rather than A(1\rr,o,v)fluiQ 60 plant death. Infected plants usually do not cause petiole and stolon lesions which are longer periods of wetness than the 4 h thrive after transplantation and produce few dark brown or black and sunken and often required for secondary conidia to form. Col- berries at harvest. girdle the petiole or stolon. Pink masses of letotrichum acutatum survived up to 8 weeks Historically, C. acutatum has been con- conidia are usually visible near the center of on leaves in greenhouse studies (Leandro sidered to be the anthracnose fruit-rotting each lesion. All three species also cause leaf et al., 2003a) and up to 5 weeks on fabric pathogen, and C.fragariae and C. gloeospor- spots (Howard et al., 1992; Maas and Palm, (Norman and Strandberg, 1997). More con- ioides have been associated with petiole and 1997; Smith, 1998c). Black leaf spot, typi- idia formed on leaves when exposed to flower stolon lesions and crown rot; however, all cally caused by C. fragariae and C. gloeo- extracts than when exposed to leaf extracts or three species may cause similar symptoms sporioides, is characterized by gray or light water (Leandro et al., 2003b), suggesting that and may be found to occur on the same plant black spots, usually not necrotic, peppered C. acutatum inoculum levels on strawberry (Howard et al., 1992). Identification of these across the top surface of the strawberry leaf- foliage may increase during flowering. pathogens should be based on classical tax- lets. C. acutatum more typically causes irreg- Rain splash is the primary means by onomic characteristics or molecular techni- ular leaf spot, the primary symptom of which which Colletotrichum spp. conidia are spread ques, not symptoms. is the appearance of necrotic black lesions from plant to plant in the field. Madden Results of several studies suggest that at the tip of the leaflets. All three Colleto- and Boudreau (1997) found that anthracnose C. acutatum may have evolved into a sub- trichum spp. also cause flower blights and fruit rot incidence generally declined as plant group

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