Dark tobacco variety evaluation for angular leaf spot susceptibility Zachariah Hansen1, Eric Walker2, Andy Bailey2,3, & Robert Ellis4

1Department of Entomology and Plant Pathology, University of Tennessee; 2Department of Plant Sciences, University of Tennessee; 3Department of Plant and Soil Sciences, University of Kentucky; 4Director, Highland

Rim AgResearch and Education Center, University of Tennessee

IntroductionAngular leaf spot (ALS), caused by the bacteria Pseudomonas syringae pv. tabaci, is a disease that has become increasingly destructive in dark tobacco production in recent years and has become the most important foliar dark tobacco disease in Tennessee (Figures 1 & 3). Reliance on agricultural streptomycin for ALS control can lead to streptomycin resistance in the pathogen, which could be contributing to unsatisfactory control. With limited chemical options available for ALS management, growers are left with few options for disease management. The goal of this project was to evaluate dark tobacco varieties for their susceptibility to ALS, and to use this knowledge to inform Extension recommendations.

Objectives• Evaluate nineteen dark tobacco varieties for susceptibility to angular leaf spot

in an inoculated field trial

Materials and MethodsField trial establishment• Trial conducted at the Highland Rim AgResearch and Education Center

(HRREC) in Springfield, TN (Figure 2)• Plots were single rows, 35 ft long, 14 plants per row, 4 replicates per variety• One row of ‘KT D8’ between each variety• Seedlings transplanted on Jun. 10, 2019• Trial inoculated on Jul. 30, 2019

Pseudomonas syringae pv. tabaci isolation and inoculation• P. syringae pv. tabaci isolated from symptomatic dark tobacco leaves

collected from Robertson County, TN on Jun. 26, 2019• Lesions surface disinfested with 0.5% sodium hypochlorite• Tissue homogenized in sterile DI H2O and spread on LB plates• Discrete colonies spread on plates containing King’s B media

• Isolate pssytatn19001 identified as P. syringae pv. tabaci by fluorescence on King’s B media and by sequencing using L1G1 primers

• P. syringae pv. tabaci isolate pssytatn19001 was grown in KB broth and quantified by dilution plating for field inoculation• 320 ml of 1X108 CFU/ml solution with 0.01% tween-20 applied to each

plot (approximately 23 ml per plant) using hand-pump sprayer on Jul. 30, 2019 (Figure 2)

Disease rating• Plots were rated for ALS severity on Aug. 20, 2019 (21 days post inoculation)

• One representative mid-canopy leaf per plant rated as % ALS severity• Means were analyzed in R and separated using Tukey’s HSD (P<0.05)

• Black shank incidence was also recorded (Figure 3)

Results

Conclusions• ‘PD 7309’ and ‘DT 538’ were significantly more resistant to ALS compared

to ‘TN D950’ (P<0.05).• ‘KY 171’, ‘Lit Crit’, and ‘TR Madole’ were also more resistant than ‘TN

D950’, but with less statistical confidence (P<0.1).• ALS ratings in this test appear to be inversely correlated with wildfire

susceptibility, but more testing is needed to support this association.• These results indicate an opportunity to incorporate genetic resistance

into ALS management programs, and ALS resistance may be a valuable target for future breeding efforts.

• Black shank occurrence was inconsistent throughout the trial, which is probably why statistical differences were not observed among varieties.

Future work• This trial will be repeated in 2020.• If consistent results are obtained, they will be shared through Extension

meetings and publications.

Acknowledgements• We thank Altria Client Services for supporting this

work• Thanks to Elgin Akin, Brinklie Thomas, and the

HRREC crew for technical support

zMeans followed by the same letter are not significantly different (Tukey’s HSD, P<0.05).yRatings taken from the 2019-2020 Burley and Dark Tobacco Production Guide. Wildfire: S=susceptible; R=resistant. Maturity: E=early; M=medium; L=late. “.” means rating not available. xBlack shank incidence did not differ significantly among varieties (P=0.1).wSignificantly different from TN D950 at P<0.1 (Tukey’s HSD).

Table 1. Angular leaf spot ratings and black shank incidence, dark tobacco trial at HRREC, Springfield, TN, 2019.

Variety Angular leaf spot

severity

(% disease)z

Wildfire

susceptibilityy

Maturityy Black shank

incidence (%)x

PD 7309 0.8 a . M 3.3DT 538 0.8 a . M 1.5

KY 171w 0.9 a b S M 3.3Lit Critw 0.9 a b S M-L 1.0

TR Madolew 1.1 a b . M 0PD 7318 1.1 a b S E-M 1.8

NL Madole 1.2 a b S M-L 1.8PD 7312 1.7 a b S M 3.3Shiree 1.9 a b . . 0.8DT 558 1.9 a b . M 0

PD 7319 2.0 a b . M 0.3PD 7305 2.2 a b R E 0

KT D8 2.2 a b S M 0KT D6 2.5 a b R E-M 0

KT D14 2.5 a b R M 0DFH 1404 3.3 a b . . 0

VA 309 5.8 a b . E-M 3.3KT D17 6.0 a b R M 0

TN D950 7.0 b R E 0

Figure 2. (A) Student Elgin Akin inoculates a dark tobacco plot with P. syringae pv. tabaci on Jul. 30, 2019. (B) Dark tobacco trial at Highland Rim AgResearch and Education Center in Springfield on Jul. 30, 2019, just prior to inoculation.

A CB

Figure 1. Dark tobacco leaves showing mild (A), moderate (B), and severe (C) ALS symptoms. Photos taken the same day disease ratings were taken (Aug. 20, 2019).

Figure 3. (A) Tobacco plant showing ALS symptoms at the time of rating on Aug. 20, 2019. (B) Severe black shank observed in some plots, but incidence was inconsistent across replicates. Variety ‘KY 171’ shown in panel B.

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