Fungicides
Sulfur
On all Pacific Northwest grapes, growers applied sulfur to 67,173 acres (14,697 base acres) per year for control of powdery mildew. If sulfur were unavailable, it would be replaced by sterol inhibitors (fenarimol, myclobutanil, triadimefon, and triflumizole) and mineral oil (JMS Stylet-Oil). Change in quantity and quality of yield would be negligible, but resistance development would be a major problem.
The sterol inhibitors have a single site of action, and are therefore susceptible to resistance development. Resistance to sterol inhibitors has been seen in some California vineyards since the mid-1980s. Because the modes of action of sterol inhibitors are very similar, cross-resistance may occur. For example, populations of Uncinula necator that have developed resistance to triadimefon also have been somewhat resistant to myclobutanil (D. Gubler, personal communication). Tank mixing and alternating sulfur with sterol inhibitors is an important means of slowing resistance development. The overall cost (replacement compounds) of $37.53 million ($2,554 per base acre) does not reflect the increased risk of sterol inhibitor resistance development if sulfur becomes unavailable. See also Table 10.
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