Anthracnose is a group of diseases incited by fungal pathogens including those in the genus Colletotrichum. Colletotrichum is distributed primarily in tropical and subtropical regions, although it can also be found in more temperate regions (Cannon et al., 2012). The disease is characterized by dark, sunken, subcircular, or angular necrotic lesions, as shown in Fig. 1, with salmon or pink conidial masses erupting from the lesions in later stages. Lesions typically enlarge, coalesce, and can result in significant dieback (Freeman et al., 1998; Jeffries et al., 1990). Anthracnose is typically observed as fruit rots, leaf spots, or stem lesions. Stem lesions can be incited by different species of Colletotrichum, and many crops grown throughout the world are susceptible to one or more species. Certain Colletotrichum species have been found to cause stem lesion anthracnose on cassava (Fokunang et al., 2002), mango (Gupta et al., 2015), and dragon fruit (Vijaya et al., 2015), all grown primarily in tropical regions.
There have been a few reported incidences of anthracnose stem lesions on blueberry. Kim et al. (2009) reported C. gloeosporioides infecting stems on highbush blueberry in Gochang, South Korea. The infected stems turned dark brown, then became gray and died. Of five stem isolates taken, all were identified as C. gloeosporioides based on morphological and cultural characteristics. Although no cultivar names were provided, it is assumed the infected plants were northern highbush because of the climate in Gochang, South Korea, which is located at latitude 35° N. In a separate study, stem blight was observed on highbush blueberry in Japan, with the previous year’s shoots turning brown and then blighted, along with the death of adjacent floral buds. In addition, small red or brown leaf spots were observed near the blighted stems. Isolates from stem tissue were identified as Colletotrichum acutatum based solely on morphological and cultural characteristics (Yoshida and Tsukiboshi, 2002).
In addition, in 2010 lesions on green northern highbush blueberry canes were reported in Michigan. The lesions were described as dark brown to black, circular or oval, with light brown to gray centers, and salmon-pink masses of spores. The pathogen was identified as C. acutatum by morphological characteristics. This report also noted that anthracnose stem lesions had been observed on northern highbush blueberry in Ontario, Canada, and Michigan in 2003 and 2004 (Schilder, 2010). Finally, in 2013, stem lesions and leaf spots on highbush blueberry caused by anthracnose were reported in Liaoning, China. It is assumed the symptomatic plants were northern highbush, consistent with the authors’ use of the term “highbush blueberry” (not “southern highbush blueberry”) and the location of Liaoning near latitude lat. 42°N, the same latitude as Michigan. Symptoms were described as yellow to red irregularly shaped lesions on stems and leaves, which expanded and turned dark brown, surrounded by a red halo. Isolates were identified as C. gloeosporioides based on morphological and cultural characteristics, which was later confirmed through molecular methods (Xu et al., 2013).
Until recently, there have been no peer-reviewed reports of anthracnose stem lesions on SHB. ‘Flicker’ is an SHB cultivar frequently selected by growers in Central and South Florida. Characteristics favoring production of ‘Flicker’ in this region are that it has very low chilling requirements, can be grown in an evergreen management system, and tends to ripen early. In 2014, several blueberry farms in Central Florida experienced issues with anthracnose stem lesions and twig dieback on ‘Flicker’, and to a lesser extent on ‘Scintilla’, a progeny of ‘Flicker’ (Harmon, 2014a). Using molecular methods, isolates taken from infected plants on four farms in this area were confirmed to be C. gloeosporioides (Velez-Climent and Harmon, 2016). Further compounding the disease issue, isolates of this pathogen collected on these farms have been found to be resistant to fungicides in the quinone outside inhibitor class (also called strobilurins), including azoxystrobin and pyraclostrobin (Harmon, 2014b). This outbreak of disease has resulted in a reduction in new plantings of ‘Flicker’ and ‘Scintilla’ and the removal of some existing plantings (P.F. Harmon, personal communication). Because of the early success of ‘Flicker’, it has been used as a parent in the University of Florida blueberry breeding program in the past, raising concerns regarding potential susceptibility of offspring from these crosses. In addition, there is a concern about whether other commercial cultivars may be susceptible to this fungicide-resistant form of anthracnose. This could potentially lead to similar problems as those experienced with ‘Flicker’ and ‘Scintilla’, including a possible rejection by growers of further use of such cultivars and the costly removal of existing plantings.
The objective of this study was to determine the level of anthracnose susceptibility of certain commercially available SHB cultivars, so that University of Florida (UF) personnel could use any identified susceptibility to limit the use of susceptible cultivars in the UF blueberry breeding program and communicate the findings to growers for their use in making decisions on which cultivars to plant.
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