A search for new products for control ofblack shank of tobacco

Steven C. Bost (1) and Steve E. Walker (2)(1) Extension Specialist, The University of Tennessee Extension, 5201 Marchant Dr., Nashville, TN 37211

(2) Extension Agent, The University of Tennessee Extension, Macon County, Lafayette, TN 37083

IntroductionEfforts by tobacco producers to control black shank disease with chemicals concentrate on one product, mefenoxam. The application cost and the tendencyof this material to lose effectiveness due to the development of reduced sensitivity in target pathogens creates a need for alternative control products. Products having potential for black shank control activity include azoxystrobin and the phosphorous acid (PA) products. These products were evaluated for their suitability for control of black shank.

Results of 2005 field trial.

Treatment

Rate/A How

applied**

% Black shank

(P=0.05)At planting 4 WAP*

Untreated -- -- -- 85.1 a

ProPhyt -- 4 pt foliar 59.4 a

AgriFos -- 4 pt foliar 58.6 a

Quadris 18.5 fl oz 18.5 fl oz soil 56.9 a

ProPhyt 4 pt 4 pt soil 56.7 a

Ridomil Gold

+ Quadris

1 pt

18.5 fl oz

1 pt

18.5 fl oz

soil 36.1 b

Ridomil Gold 1 pt 1 pt soil 33.8 b

* WAP = weeks after planting

** Foliar = Directed to the foliage. Soil = A 12-in. band on each side of the row.

Results of 2006 field trial.

TreatmentRate/A and method of applicationa

% Black shank

(P=0.05)

At planting 2 WAPb 6 WAP 6 WAP 10 WAP

Untreated -- -- -- 27.6 a 75.7

Ridomil Gold 1 pt BR -- 1 pt DIR 14.0 ab 68.8

Ridomil Gold 1 pt BR 1 pt DIR 1 pt DIR 19.1 ab 47.8

ProPhyt 4 pt BR 4 pt OTc 4 pt OT 8.0 ab 44.0

Ridomil Gold 1 pt BR 1 pt OT 1 pt OT 7.2 ab 39.9

Ridomil Gold 2 pt BR -- -- 3.9 b 41.5

a BR = broadcast. DIR = directed to soil, 12-in.band on each side of row.

OT = overtop of plants and row middles with boom.b WAP = weeks after planting

c Foliar burn caused by this application.

Results

Cutaway view of tobacco stem with black shank.

Field of tobacco with black shank disease.

Young tobacco plants grown in greenhouseshowing damage from ProPhyt at 1350 ppm(left) and 3375 ppm (right).

Dark stem lesion on greenhouse-growntobacco plant with black shank.

Summary of Greenhouse Trials

Product

Black shank control

Phytotoxicity*Name Conc.

(ppm)

Per-plant rate,

greenhouse:field

Ridomil 26.5 0.5X exc no

53 1X exc no

106 2X exc no

ProPhyt 221 1X slight no

442 2X fair no

663 3X mod no

884 4X good no

1989 9X v good variable

Quadris 31 1X mod variable

62 2X good variable

124 4X v good yes

Black shank is the most damaging disease of tobacco in the southeastern U.S.Chemical control efforts by producers concentrate on one product, mefenoxam.The high cost and the tendency of this material to lose effectiveness due to thedevelopment of reduced sensitivity in target pathogens creates a need for alternative control products. Greenhouse trials indicated black shank control activity by Quadris (azoxystrobin) and the phosphorous acid products ProPhyt and AgriFos. On-farm trials were conducted in Macon County, Tennessee to determine the practical suitability of these materials as black shank control products. At Farm ‘A’ in 2005, only those treatments containing Ridomil (mefenoxam) provided a significant reduction in black shank. All other treat-ments (soil applications of Quadris or ProPhyt and foliar applications of AgriFosor ProPhyt) provided intermediate levels of control. In a trial at Farm ‘A’ in 2006, ProPhyt provided a level of control equal to that of Ridomil, although neither treatment resulted in an acceptable level of control due to the severity of the disease. When applied alone in a 2007 trial on Farm ‘B,’ ProPhyt provided only slight control; however, the control provided by Ridomil was enhanced and yields were increased by about 15% by the addition of ProPhyt to the Ridomil. ProPhyt, if registered for use on tobacco, may provide valuable control of black shank if applied in a manner that maximizes its activity.

Abstract

Greenhouse Trials.Burley tobacco plants growing in 4 in. pots were treated with various rates of azoxystrobin (Quadris), phosphorous acid (ProPhyt), or mefenoxam (Ridomil)1 week prior to inoculation with 5,000 zoospores of the black shank pathogen.Disease ratings were taken each day for 18 days. Four trials were conducted.Field Trials. The tests were conducted in 2005, 2006, and 2007 in fields naturally infested with black shank in Macon County, Tennessee. Plots were two-rows wide and20 ft long, and replicated 4 times. Quadris, phosphorous acid (ProPhyt, AgriFos), and the Ridomil standard were applied with a backpack, CO2 sprayer.

Materials and Methods

• Neither azoxystrobin nor PA products, when used at rates safe to the plant, displayed sufficient activity against black shank to warrant use as a substitute for mefenoxam.

• PA products displayed sufficient plant safety and activity against black shank to warrant further study as a supplement to mefenoxam. Transplant tray drench or transplant water applications are promising methods of delivery.

• Although azoxystrobin was comparable with PA products in efficacy, the tendency of azoxystrobin to cause phytotoxicity reduces its potential for black shank control.

Conclusions

Results of 2007 field trial.

Rate/A (all soil-applied) % Black shank

(P=0.05)

Yield (lb/plot)

(P=0.05)Treatment At planting 4 WAP* 7 WAP

Untreated -- -- -- 85.5 a 22.8 c

ProPhyt 4 pt 4 pt 4 pt 74.9 a 29.4 bc

ProPhyt 6 pt 6 pt 6 pt 70.4 a 31.2 bc

Ridomil Gold 2 pt -- -- 32.0 b 50.9 abc

Ridomil Gold 1 pt 1 pt 1 pt 21.7 b 56.8 ab

Ridomil Gold

+ ProPhyt

2 pt

4 pt

--

4 pt

--

4 pt

21.6 b 58.3 ab

Ridomil Gold

+ ProPhyt

1 pt

4 pt

1 pt

4 pt

1 pt

4 pt

16.8 b 65.9 a

* WAP = weeks after planting