Effect of Fenamiphos Placement on Tylenchulus semipenetrans and Yield in a Florida Citrus Orchard

Grapefruit trees on sour orange rootstock on the east coast of Florida were treated with 22.3 kg a.i./ha fenamiphos (broadcast equivalent) in 1.52-m bands extending from the dripline to beneath the canopy, in 1.52-m bands extending from the dripline toward the row middle, or left as untreated controls. During the course of the experiment, mean density of feeder roots and Tylenchulus semipenetrans in control plots was 3.8 and 5.8 times greater, respectively, in samples from beneath tree canopies than from 90 cm beyond the dripline on the top of beds. Population densities of T. semipenetrans in each zone (under canopy vs. row middle) were smallest when treatment occurred in that zone. Nematode levels beneath the canopy were 11% of control levels 19 weeks following treatment when fenamiphos was applied beneath the canopy and 52% of controls when treated at the dripline. The number of female T. semipenetrans per gram root weight was also reduced under the canopy by the under-canopy treatment. Fruit yield 5 months following nematicide application was not affected by treatment. Percentage change in yield between that harvest and a harvest 16 months after treatment was + 17% in the under-canopy treatment, - 1% in the dripline treatment, and - 17% in the untreated controls.