Figure 1Figure 2 Pectolytic Dickeya spp. and Pectobacterium spp. are commercially important seed-borne bacteria of potato (Solanum tuberosum) that cause blackleg, soft rot and aerial stem rot (Potrykus et al., 2014;Stevenson et al., 2001). Dickeya and Pectobacterium spp. have been recovered from potato production fields in parts of the USA (Dickey, 1979;Ma et al., 2007) Five seven-week-old cv. 'Russet Norkotah' potato plants were woundinoculated by inserting a sterile 18-gauge needle just above a central leaf axil at a depth of 1 mm. A 100 μl drop of inoculum (10 6 cfu/ml) was placed on the wound. Plants were exposed to a 24 h leaf wetness period (90 to 100% relative humidity in a mist chamber) until symptom expression (Figs. 1, 2), and lesions were measured. All three inoculated plants exhibited blackening of the stem and in advanced stages, drying and cracking (Figs. 1, 2). Upon desiccation, the lesions became shriveled and turned dark brown to black. Water-inoculated controls were nonsymptomatic (Figs. 1, 2). The reisolated bacteria caused pitting on crystal violet pectate agar (Hélias et al., 2012) and exhibited the same morphology as original cultures on NBY, and were confirmed as D. dianthicola using 16S rRNA and acnA coding sequences, and P. wasabiae using rRNA, acnA and mdh coding sequences, fulfilling Koch's postulates. To our knowledge, this is the first report of D. dianthicola and P. wasabiae causing aerial stem rot of potato in Michigan.
References
Fusarium dry rot of potato (Solanum tuberosum L.) is a postharvest disease caused by several Fusarium spp. Thirteen Fusarium spp. have been implicated in dry rot of potatoes worldwide. Among them, 11 species have been reported causing potato dry rot of seed tubers in the northern United States (1). Historically, Fusarium sambucinum was the predominant species in Michigan potato production (3). Dry rot symptomatic tubers (n = 972) were collected from Michigan commercial potato storage facilities in 2011 and 2012 to determine the composition of Fusarium spp. Sections were cut from the margins of necrotic tissue with a sterile scalpel and surface disinfested in 0.6% sodium hypochlorite for 10 s, rinsed twice in sterile distilled water, and dried on sterile filter paper. The tissue sections were plated on half-strength potato dextrose agar (PDA) amended with 0.5 g/liter of streptomycin sulfate. Dishes were incubated at 23°C in the dark for 7 days. Putative Fusarium isolates were transferred onto water agar and hyphal tips from the margin of actively growing cultures were removed with a sterile scalpel and plated to carnation leaf agar (CLA) and half-strength PDA to generate pure cultures. Seven hundred and thirty Fusarium isolates were collected using these techniques. Preliminary identification of the 730 isolates was based on colony and conidial morphology on PDA and CLA, respectively. While F. oxysporum and F. sambucinum were isolated as expected from prior reports (3), three isolates of F. proliferatum were also identified. On CLA, macroconidia of F. proliferatum were sparse, slender, and mostly straight, with three to five septae (4). Microconidia were abundant, usually single celled, oval or club-shaped in short chains or false heads on monophialides and polyphialides (4), and chlamydospores were absent. On PDA, abundant white mycelium was produced and turned violet with age. Koch's postulates were confirmed through pathogenicity testing on disease-free potato tubers cvs. Atlantic and Russet Norkotah. Tubers were surface disinfested for 10 min in 0.6% sodium hypochlorite and rinsed twice in distilled water. Three tubers of each cultivar per isolate were wounded at the apical end of the tuber to a depth of 4 to 10 mm with a 4 mm diameter cork-borer. Tubers were inoculated by inserting a mycelial plug from a 7-day-old culture grown on PDA into the wound and incubating the tubers at 20°C for 21 days. All Fusarium isolates were tested. Control tubers were inoculated by inserting a water agar plug. Pathogenicity and virulence testing were replicated three times and repeated. Tubers inoculated with F. proliferatum developed typical potato dry rot symptoms but no dry rot symptoms were observed on control tubers. Fusarium proliferatum was re-isolated from symptomatic tubers, confirming Koch's postulates. To our knowledge, this is the first report of F. proliferatum causing potato dry rot in Michigan. References: (1) E. Gachango et al. Plant Dis. 96:1767. (2) D. Geiser et al. Eur. J. Plant Pathol. 110:473, 2004. (3) M. L. Lacy and R. Hammerschmidt. Fusarium dry rot. Extension Bulletin. Retrieved from http://web1.msue.msu.edu/msue/iac/onlinepubs/pubs/E/E2448POT, 23 May 2010. (4) J. F. Leslie and B. A. Summerell. The Fusarium Laboratory Manual. Wiley-Blackwell, Hoboken, NJ, 2006.
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