Professional Agriculture

SoilGard® 12G (Gliocladium virens strain GL-21): A Solution for Controlling Lettuce Drop (Sclerotinia minor/S. sclerotiorum) in Conventional and Organic Systems.

S.C. Ockey

Introduction:

Lettuce drop is one of the most frequent and destructive diseases encountered by commercial lettuce growers throughout the US. The causal agents Sclerotinia minor & S. sclerotiorum produce sclerotia requiring long rotations and annual fumigation in fields where the disease has become established. Heavily infested fields have incurred losses of up to 70%.

SoilGard® 12G Microbial Fungicide contains spores of Gliocladium virens strain GL-21 which has proven to be an effective fungicide in laboratory and field studies. Fungicidal activity occurs via four distinct modes of activity:

  1. Antibiosis: As chlamydospores germinate and hyphae begin to grow, GL-21 produces gliotoxin, which kills and inhibits growth of pathogenic fungi.
  2. Mycoparasitism: GL-21 penetrates and consumes hyphae of pathogenic fungi (fig. 1).
  3. Competition: GL-21 grows aggressively and competes against phytopathogenic fungi for nutrients and living space (fig. 2).
  4. Exclusion: GL-21 occupies the rhizosphere and prevents or delays reestablishment of phytopathogenic fungi (fig. 3).
Hyphae and haustoria formation within the large hyphae of  Rhizoctonia solani by the smaller hyphae of  Trichoderm (=Gliocladium) virens (left).  G. virens treated/parasitized and non treated/active sclerotium of Sclerotinia minor (right).

Figure 1: Hyphae and haustoria formation within the large hyphae of Rhizoctonia solani by the smaller hyphae of Trichoderm (=Gliocladium) virens (left). G. virens treated/parasitized and non treated/active sclerotium of Sclerotinia minor (right).

Figure 2: Pure cultures of Rhizoctonia solani (left) and Gliocladium virens strain GL-21 (right). After 3 days the two fungal cultures are growing normally and have met in a typical inhibition zone (top-center). The sporulating GL-21 culture shows its typical deep green color (virens is Latin for “green”). By day 7, the GL-21 culture has overgrown the R. solani culture and has started to sporulate, as indicated by the splotchy green coloration over the R. solani portion of the dish (bottom-center).

Figure 3: Reference petri dishes contain 9-day old, pure cultures of Rhizoctonia solani (top-left) and Trichoderma (=Gliocladium) virens strain GL-21 (top-right). The “challenged” petri dishes also contain 9-day old cultures (bottom), The pure culture of R. solani (bottom-left) was challenged on day 4 by placing a single granule of SoilGard on top of the culture near the center of the Petri dish the same was done with a pure culture of SoilGard with R. solani being placed on top of the culture near the center of the Petri dish (bottom-right). By day 9, GL-21 was taking over the environment already occupied by Rhizoctonia, having overgrown about half of the pathogen culture (bottom-left).
In contrast, Rhizoctonia failed to grow beyond the site of inoculation in the T. virens plate (bottom-right), having been excluded by GL-21 from establishing a new colony.

Materials and Methods:

SoilGard® 12G was tested under field conditions in Greenfield, CA during 2009 in cooperation with Steffi-Saul Ketzler: BioResearch Inc. The trial was set up as a randomized complete block design with 4 replicate plots/treatment each 2 rows x 6m.

Treatments:

  1. Soilgard ® 12G applied at 4.0 lb/acre
  2. Endura ® Fungicide at 11 oz/acre
  3. Certis Experimental product 9090 at 1.0 lb/acre
  4. Certis Experimental product 9090 at 4.0 lb/acre
  5. Contans ®WG applied at 4.0 lb/acre
  6. Untreated Control

Application Information:

SoilGard® 12G was applied at 4lb/a a total of three times:

First application
CO2 pressurized backpack sprayer w/ wand (3 flat fan 8003 nozzles); 75 gal/acre; 40 psi.

Second and third applications
CO2 pressurized backpack sprayer w/wand (3 hollow cone , D-4 spin nozzles); 85 gal/acre; 40 psi.
*All applications were followed by 1hr overhead irrigation.

Application Dates
14 April (1 day post plant), 23 May and 6 June

Evaluation Information:

Disease incidence (fig. 4) was rated on 6 & 23 June and 7 & 16 July.

Figure 4: Early season Sclerotinia sp. infected lettuce plant (left). Resulting dead lettuce plant from infection by Sclerotinia sp. (right). Both plants above were rated as infected an represented as such in disease incidence ratings for this trial.

Results:

At the conclusion of the study, when the season long total of the diseased plants were calculated and the percent control of each product was determined (fig. 5), significant differences between treatments became apparent. The best performer was the Certis Experimental product 9090 at 4 lb/a, which exhibited excellent control at 83%. This was followed by, SoilGard® 12G at 4 lb/a, and the Certis Experimental product 9090 at 1 lb/a, with 67, 65 and 61% control, respectively (fig. 6). Contans showed control of only 51%.

Conclusions:

Soilgard ® 12G provided economically acceptable control of lettuce drop, equivalent to the Endura® Fungicide treatment. Both rates of Certis’ experimental product resulted in economically acceptable control of lettuce drop, with the 4.0lb/acre treatment providing the most effective control.

Figure 5: Season total disease incidence showing effects of trial treatments in controlling Sclerotinia sp. on lettuce, where each treatment was applied three times. Greenfield, CA (July 2009).

Figure 6: Season total percent control of lettuce drop vs untreated control. Greenfield, CA (July 2009). Percent incidence vs untreated control calculated using Abbott’s Formula (1925): [(x-y)/x] *100; where x=number living plants in untreated and y=number of living plants in treatment group.

Credits/Citations:

Figure 1 photo (left): Howell, R., 2003. Plant Disease 87 (1): 4-10.
Figure 1 photo (right): www.mycolog.com/chapter14.htm
Figures 2 & 3 (all): Courtesy of Mike Dimock, Certis USA
Figure 4 photos (both): Courtesy of Steffi Saul-Ketzler, BioResearch Inc.
Abbott’s Formula: Abbott, W.S., 1925. A method for computing the effectiveness of an insecticide. J. Econ. Entomol. 18 (2): 265-267.