Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/23/2003
Publication Date: 1/2/2004
Citation: Gronwald, J.W., Plaisance, K.L., Bailey, B.A. 2004. Effects of the fungal protein Nep1 and Pseudomonas syringae on growth of Canada thistle (Cirsium arvense), common ragweed (Ambrosia artemisiifolia), and common dandelion (Taraxacum officinale). Weed Science. 52(1):98-104. Interpretive Summary: Alternative weed control strategies that do not involve using chemical herbicides are needed. Pseudomonas syringae pv. tagetis (Pst) is a naturally occurring bacterium that infects Canada thistle, common ragweed, and dandelion. Pst blocks growth of these weeds by causing yellowing of young leaves. Nep1 is a protein produced by a soil fungus when it's grown in a liquid medium. When Nep1 is sprayed on broadleaf weeds, it causes rapid death of mature leaves. The goal of this research was to evaluate the effectiveness of Pst and Nep1, when sprayed separately or together, in controlling Canada thistle, common ragweed, and dandelion. Pst was grown in liquid medium and Nep1 was isolated from the growth medium of the fungus. Of the three weeds examined, Pst was most effective in controlling common ragweed and least effective on Canada thistle. Shoot growth of common ragweed and Canada thistle was reduced by 82% and 31%, respectively, when sprayed with Pst. When Nep1 was sprayed on plants, shoot growth was reduced about 35% for all three weeds. The limited control of these weeds by Nep1 was due to the inability of this protein to kill young, developing leaves. Spraying the weeds with both Pst and Nep1 did not improve weed control compared to spraying each separately. This research provided new knowledge about the potential for biocontrol of weeds using a naturally occurring plant bacterium and a fungal protein. The results indicate that foliar application of Pst holds promise as a weed control strategy for controlling common ragweed. Spraying Nep1 on weeds suppressed growth, but additional research is needed to increase the weed control potential of this fungal protein. This information will be useful in developing improved methods for biological control of weeds.
Technical Abstract: The effects of the fungal protein Nep1 and Pseudomonas syringae pv. tagetis (Pst) applied separately or in combination on Canada thistle, common ragweed, and dandelion were examined in growth chamber experiments. Experiments examined five treatments: (1) untreated control, (2) Silwet L-77(0.3%,v/v) control, (3) Nep1 (5 µg mL**-1) plus Silwet L-77 (0.3%,v/v), (4) Pst (10**9 cfu mL**-1) plus Silwet L-77 (0.3%, v/v), and (5) Pst (10**9 cfu mL**-1) and Nep1 (5 µg mL**-1) plus Silwet L-77 (0.3%, v/v). Foliar treatments were made at 28, 26, and 21 d after planting for Canada thistle, dandelion, and common ragweed, respectively. For all three species, foliar application of Nep1 alone or in combination with Pst caused rapid dessication and necrosis of leaves with the greatest effect on newly matured leaves. Within 4 to 8 h after treatment, 60 to 80% of newly matured leaves of Canada thistle, dandelion, and common ragweed were necrotic. Pst populations in Canada thistle leaves were reduced when Nep1 and Pst were applied together. Measured 72 h after treatment, Pst populations in leaves treated with Nep1 plus Pst were approximately 10**5 cfu cm**-2 compared to 10**7 cfu cm**-2 for leaves treated with Pst alone. Overall efficacy of the treatments was evaluated by measuring shoot dry wt 2 wk after treatment. For all three weeds, treatment with Silwet L-77 did not have a significant effect on shoot dry wt. Foliar application of Nep1 reduced shoot dry wt of the three weeds by 30 to 41%. Treatment with Pst reduced shoot growth of common ragweed, Canada thistle, and dandelion by 82, 31, and 41%, respectively. The large suppression of common ragweed shoot growth caused by Pst treatment was associated with a high percentage (60%) of leaf area exhibiting chlorosis. Treatment with Pst plus Nep1 did not result in significant decreases in shoot dry wt for Canada thistle and dandelion compared to either treatment alone. For common ragweed, shoot growth reduction caused by applying Pst and Nep1 together was greater than that caused by Nep1 alone, but was not greater than that caused by Pst alone.