The fungal protein Nep1, produced by Fusarium oxysporum f. sp. erythroxyli in liquid culture, caused extensive necrosis to Centaurea maculosa when water solutions of Nep1 (5 µg ml⁻¹) and an organosilicone surfactant (1,1,1,3,5,5,5-heptamethyltrisiloxanyl propyl-methoxy-poly[ethylene oxide]) were applied as foliar sprays. Nep1 did not cause necrosis when applied with a nonionic surfactant or organosilicone surfactant plus unrefined corn oil. Plant age, protein concentration, organosilicone surfactant concentration, and the presence of a dew period influenced the amount of necrosis caused by Nep1. The addition of an 18-h dew period after treatment resulted in an increase of 10% or more in foliar necrosis at the 0.313 and 1.25 µg ml⁻¹ (0.40 and 1.62 g ai ha⁻¹) Nep1 concentrations. Increasing the spray volume from 129 ml m⁻² (1,291.3 L ha⁻¹) to 516 ml m⁻² (5,165.2 L ha⁻¹) more than doubled the amount of foliar necrosis caused by the 0.313 µg ml⁻¹ (0.40 g ai ha⁻¹ vs. 1.62 g ai ha⁻¹) Nep1 concentration. A maximum necrosis rating of 95% was reached by 1.25 µg ml⁻¹ Nep1 applied at 516 ml m⁻² (6.46 g ai ha⁻¹) followed by an 18-h dew period. Nep1 (6.46 g ai ha⁻¹) remained active when coapplied to Centaurea maculosa with the herbicides 2,4-D or glyphosate (0.13 to 2.58 kg ai ha⁻¹), causing foliar necrosis prior to the herbicides killing Centaurea maculosa. An increase in the organosilicone surfactant concentration from 1 to 2 ml ai L⁻¹ was required to achieve levels of Nep1-induced necrosis on Centaurea maculosa acclimated to direct sun comparable to levels achieved on greenhouse-grown plants. Repeated application of Nep1 (6.48 g ai ha⁻¹) 3 wk after an initial treatment (6.48 g ai ha⁻¹) prevented the recovery of acclimated Centaurea maculosa. Greater damage was caused to acclimated Centaurea maculosa when Nep1 was applied near the middle of the day (80% necrosis at 10:00 a.m. and 85% necrosis at 2:00 p.m.) compared to early or late in the day (25% necrosis at 6:00 a.m. and 10% necrosis at 6:00 p.m.).

Nomenclature: Glyphosate, monoisopropylamine salt of glyphosate; 2,4-D, sodium salt of 2,4-D; Centaurea maculosa Lam. CENMA, spotted knapweed; Fusarium oxysporum; Nep1.