Factors influencing the herbicidal activity of Nep1, a fungal protein thatinduces the hypersensitive response in Centaurea maculosa

The fungal protein Nep1, produced by Fusarium oxysporum f. sp. erythroxyli in liquid culture, caused extensive necrosis to Centaurea maculosa when wat er solutions of Nep1 (5 mug 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 n onionic surfactant or organosilicone surfactant plus unrefined corn oil. Pl ant age, protein concentration, organosilicone surfactant concentration, an d 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 inc rease of 10% or more in foliar necrosis at the 0.313 and 1.25 mug ml(-1) (0 .40 and 1.62 g ai ha(-1)) Nep1 concentrations. Increasing the spray volume from 123 ml m(-2) (1,291.3 L ha(-1)) to 516 ml m(-2) (5,165.2 L ha(-1)) mor e than doubled the amount of foliar necrosis caused by the 0.313 mug ml(-1) (0.40 g ai ha(-1) vs. 1.62 g ai ha(-1)) Nep1 concentration. A maximum necr osis rating of 95% was reached by 1.25 mug 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 herbicide s 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 orga nosilicone surfactant concentration from 1 to 2 ml ai L-1 was required to a chieve levels of Nep1-induced necrosis on Centaurea maculosa acclimated to direct sun comparable to levels achieved on greenhouse-grown plants. Repeat ed 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 ap plied near the middle of the day (80% necrosis at 10:00 A.M. and 85% necros is 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.).