If there is a bacterial disease present in the field, wet tires may transport the disease across the field, making the problem worst. Under these conditions, spraying with dew present should be avoided. Research carried out Wolf and Downer at the Lab for Pest Control Application Technology, Wooster, Ohio. Measurements of spray deposits have shown that the amount of spray retained can be affected by wet leaf surfaces as a result of a light rain or dew. Figure 1 shows that wet pigweed leaves retain less spray than dry pigweed leaves.
Wet foxtail leaves, on the other hand, retain more spray than dry foxtail leaves.
In general, plant leaves which are easily wetted will retainless spray when applied onto dew. For the most part when spraying at night, the equipment is the same with slight modifications. The sprayer has to be in excellent working order since the equipment dealer is not open at night. You can't just jump in the truck and go get replacement parts. Similarly, you should have sufficient pesticide on hand for the same reasons.