Blueberry stem blight survey in Alabama

Ayodele Amodu¹, Sejal Patel², Kathy Lawrence³, Jenny Koebernick², Elina Coneva¹, Sushan Ru^1
1Department of Horticulture, ²Department of Crop, Soil, and Environmental Sciences, ³Department of Entomology and Plant Pathology, Auburn University

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Stem blight: a major blueberry disease in the Southeast

Blueberry production in the Southern United States is greatly challenged by diseases like Botryosphaeria stem blight, which is caused by fungi pathogens in the family of Botryosphaeriaceae. Botryosphaeria stem blight can cause sudden wilting and chlorosis of leaves and stems, and in severe cases plant death (Milholland 1972) (Fig. 1). Managing Botryosphaeria stem blight remains challenging in blueberries, as no fungicide, cultural practice, or cultivar alone can effectively prevent or control this disease (Ru et al. 2022). As a result, growers in Florida have identified Botryosphaeria stem blight the most economically costly disease (Wright and Harmon 2010).  Botryosphaeria stem blight is also the top limiting disease in Alabama.

Early studies consider Botryosphaeria dothidea the causal pathogen for stem blight, and therefore the name Botryosphaeria stem blight. However, the use of DNA sequencing and phylogenetic tools in newer studies have revealed many other genera and species associated with Botryosphaeria stem blight (Ru et al. 2022). Identifying the distribution and causal pathogens of Botryosphaeria stem blight is crucial for developing effective management strategies (Babiker et al. 2019). However, such information is limited for Alabama, with the only report from Flor et al. (2022). Our study aims to expand our knowledge on the distribution of stem blight in Alabama by surveying blueberry farms in Alabama and some neighboring states.

Distribution and causal pathogens of blueberry stem blight in Alabama

A total of 47 symptomatic plants were collected between 2021 and 2023, from Alabama, Georgia, and Mississippi (Table1). Small twigs measuring about 1 inch were cut from each sample, surface disinfected with 10% bleach (for 1 minute) and 70% alcohol (for 30 seconds), and then rinsed with sterile water. The twigs were cultured on acidified potato dextrose agar (APDA) and incubated at 28˚C for about 7 days. Pure culture of the pathogens was sub-cultured from the primary culture by transferring a small portion of fungal mycelium into fresh APDA plates and incubated at 28˚C for 7 days (Fig. 2). The DNA from each cultured fungal plate was extracted and PCR was conducted. Three genomic regions: the internal transcribed spacer region (ITS), Beta tubulin gene (BT2), and translation elongation factor (tEF) were sequenced at Eurofins Genomics. The resulting DNA sequences were then compared to similar pathogens previously identified from other regions, using available data on NCBI database. 

Table 1. Summary of disease samples collected in 2021 and 2022.

Location	Time of collection	No. of plant samples	No. of isolates
Auburn, AL	03/11/22-07/26/22	10	10
Brewton, AL	10/06/22-03/20/23	5	5
Chilton, AL	03/01/22-06/10/22	6	7
Fairhope, AL	05/20/22	3	4
Jemison, AL	09/02/21	3	3
Shorter, AL	04/22/22-06/12/22	8	9
Hahira, GA	09/10/21	1	1
Lake Park, GA	04/15/22	8	9
Poplarville, MS	05/09/22	3	4
	TOTAL	47	52

Botryosphaeriaceae (36%) was the most common; however, not the only family associated with stem blight samples. Eleven other fungal families have been identified, among which the Diaporthaceae (13%), Sporocadaceae (11%), and Pleosporaceae (11%) are the major families (Fig. 3). In the Botryospshaeriaceae family, Neofusicoccum is the most common genus. Phylogenetic analysis is being conducted to further classify isolates to the species level.

Pathogenicity test and cultivar screening

Preliminary pathogenicity test was conducted in spring 2023 in a walk-in growth chamber at the Plant Science Research Center of Auburn University, Alabama. Mycelia of isolates within three families: Botryosphaeriaceae, Diaporthaceae and Sporocadaceae were used to artificially inoculate three stems of one Vernon blueberry plants following an attached-stem assay. The growth chamber was conditioned for 12 hours daylight/12 hours night, 60% relative humidity, and 25˚C (Fig. 4). The initial findings of our study revealed that 25 out of a total of 28 isolates tested (89%) caused stem lesions within two weeks of inoculation (Table 2). Botryosphaeriaceae and Diaporthaceae tended to be more aggressive than Sporocadaceae based on the average lesion length across each family. We are currently testing the virulence of pathogenic isolates with three plants and nine stems per isolate to confirm out preliminary findings.

Table 2. Preliminary data on pathogenicity testing of isolates of Botryosphaeriaceae, Diaporthaceae and Sporocadaceae. Only results on pathogenic isolates are presented.

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References

Babiker EM, Stringer SJ, Sakhanokho HF, Smith BJ (2019) Characterization and pathogenicity of stem blight complex isolates associated with stem blight disease on Vaccinium species. HortScience 54(7):1199 – 1203. https://doi.org/10.21273/HORTSCI14033-19.

Wright AF, Harmon PF, (2010) Identification of species in the Botryosphaeriaecae family causing stem blight on Southern highbush blueberry in Florida. Plant Disease:94(8): 966 – 971.

Milholland RD (1972) Histopathology and pathogenicity of Botryosphaeria dothidea on Blueberry stems. Phytopathology 62:654 – 660.

Creswell TC, Milholland RD, (1987) Responses of blueberry genotypes to infection by Botryosphaeria dothidea. Plant Disease 71:710 – 713.

Flor NC, Wright AF, Huguet-Tapia J, Harmon PF, Liberti D (2022) Identification of Fungi in the Botryosphaeriaceae family associated with stem blight of Vaccinium spp. in the Southeastern United States. Fungal Biology 126: 342-355. https://doi.org/10.1016/j.funbio.2022.03.004

Ru S, Ding S, Oliver JE, Amodu A (2023) A Review of Botryosphaeria Stem Blight Disease of Blueberry from the Perspective of Plant Breeding. Agriculture 13(1):100. https://doi.org/10.3390/agriculture13010100

Scarlett KA, Shuttleworth LA, Collins D, Rothwell CT, Guest DI, Daniel R (2018) Botryosphaeriales associated with stem blight and dieback of blueberry (Vaccinium spp.) in New South Wales and Western Australia. Australasian Plant Pathology 2019(48):45 – 57.