Response of mint to treatment with Vydate has been found to be dependent on soil type. No significant response was noted when a silty loam soil was treated with rates up to 2 GPA, while yields were more than doubled with rates of 1/4 to 4 GPA in a sandy loam field (Pinkerton et al, 1988). Yield in the nontreated plots was higher in the heavier soil than in the sandy loam field and was not increased by nematicide treatment. Studies in several cropping systems have observed that plants experience less nematode stress in heavier soil. Because soils drain more slowly, the smaller root system is still able to take up adequate water and nutrients even when infested with nematodes.

Increasing the rate of Vydate delivered in a single application generally increases yield in ever smaller increments as the rate of application increases. For example, rates of 1/4, 1/2, 1, and 2 GPA increased hay weight by 200, 226, 234, and 246%, respectively, (Pinkerton et al, 1988). Similarly, rates of 1/2, 1 and 1 1/2 GPA increased the average oil yield for five sites (three in the Willamette Valley, two in Central Oregon) 23, 28, and 18%, respectively. While there was no significant difference in yield between treatments, there was a trend for the highest net return with applications of 1/2 GPA in the Willamette Valley and with 1 GPA in Central Oregon (Ingham, 1992).

Application of Vydate is commonly made in the fall or spring, but all trials which have compared fall vs spring application in fields infested with P. penetrans have observed much greater responses with spring applications. Pinkerton et al (1988) observed that a September 12 application (1 GPA) did not reduce nematode populations present at the next harvest and only increased yield of hay by 61%. An April 7 application reduced populations that remained at harvest by 90% and increased hay yield 175%. In Central Oregon, a 1 1/2 GPA application in September increased oil yield 86%, while an April application at the same rate increased yield 286% (Ingham, 1992).

Spring application in the Willamette Valley appears to be optimal during the first two weeks of April although during mild winters the last week of March may be adequate as well. This corresponds to the flush of root growth in early spring. Root-lesion nematode populations at harvest were least from plots treated on April 2 or April 16 as compared to applications made before or after that period. Populations in plots treated March 15 were already beginning to recover by mid- to late May. Applications in late May slowed population increase but did not decrease populations (Ingham 1990). Pinkerton et al (1988) observed nearly equal results with applications (1 GPA) on April 7 or May 13. Reduced effectiveness with later applications probably occurs because Vydate degrades faster as soils warm. Later applications may also disrupt beneficial predatory mites.

"Split applications" of fall plus early spring or fall plus late spring (1 GPA each application) did not improve yield over spring-only applications (1 GPA), and there was only marginal improvement with an early spring plus late spring treatment (Pinkerton et al 1988). Ingham (1990) also found no improvement with an April 2 plus April 30 treatment (1/2 GPA each application) over a single 1/2 GPA application on April 2.

Most recommendations for treatment of perennial crops with nonfumigant nematicides suggest annual applications. However, in the only study of its kind in mint, the greatest net value (oil value-treatment cost) for a two year period occurred in plots which were treated the first year but not the second year (Ingham 1993). However, the impact of skipping an annual treatment on yield in the third year was not examined. More research on this topic is needed to optimize the cost of nematode management to the grower.