Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 25, 2000
Publication Date: N/A
Interpretive Summary: Recent studies have demonstrated that the fungal protein Nep1 causes foliar necrosis to dicot plant species including many agricultural weeds. Nep1 acts by inducing a hypersensitive response in leaf tissue that results in death of the leaf. In contrast, some chemical herbicides, although effective, are slow acting taking many days and sometimes weeks to cause defoliation and death of weeds. The physical and environmental constraints on Nep1's effectiveness had not been defined. Also, combining a fast acting phytotoxic protein with slow acting chemical herbicides to accelerate defoliation of treated weeds had never been demonstrated. We determined that Nep1 is fast acting causing significant necrosis of the invasive dicot weed spotted knapweed within 4 hours after treatment. Nep1 is active in very low concentrations but requires relatively large volumes of carrier to be effective. When combined with systemic herbicides such as 2,4-D or glyphosate, Nep1 defoliates spotted knapweed within 24 hrs after treatment after which the chemical herbicides slowly kill the plant. Nep1 has potential for weed control in organic farming operations. Nep1 uses a unique mechanism to damage weeds and when used alone or in combination with traditional chemical herbicides offers a new tool to farmers and others in the battle to control weeds.
The phytotoxic fungal protein Nep1 can be applied as a foliar spray to many dicot weed species where it induces a hypersensitive response resulting in extensive foliar necrosis. Foliar sprays of Nep1 (0.5 microgram mL**-1) were phytotoxic to plants when Silwet-L77 (0.1% or 0.2%), an organosilicone, or Tactic, a latex/organosilicone-based product, were included in the formulations. Nep1 was inactive when Tween 20 (0.1 or 1.0%), Bond (1%), or unrefined corn oil (1 or 10%) was included in the formulation. Plant age, protein concentration, Silwet-L77 concentration and the presence of a dew period influenced the amount of necrosis caused by Nep1. A second application of Nep1 (5 microgram mL**-1) 3 weeks after an initial treatment (5 microgram mL**-1) limited the recovery of spotted knapweed. Nep1 (5 microgram mL**-1) remained active when co-applied to spotted knapweed with the herbicides 2,4-D or glyphosate (0.01 to 0.2% AI) causing foliar necrosis prior to the herbicides killing spotted knapweed. Greenhouse grown plants were more sensitive to Nep1 treatment than plants grown under direct sun, but this effect could be overcome by increasing the Silwet-L77 concentration from 0.1% to 0.2%. Partially purified protein preparations were active against spotted knapweed but higher protein concentrations were required to achieve necrosis ratings similar to the pure protein.