Answer: You didn't identify the specific plants so I'm just taking an educated guess here - some plants are quite sensitive to horticultural oil, especially in the presence of sunshine. While oil will suffocate insect pests it can also be phytotoxic - can burn the leaves of sensitive plants and can clog the lenticels within the leaves, essentially stopping the exchange of air. So, while powdery mildew will defoliate some plants, a fungicide should control the disease without the fear of oil residue which could be detrimental to the plant. Pruning to open the plant up to better air circulation and sunshine penetration will help keep fungal diseases at bay.
While insect infestations can cause yellowing leaves due to their feeding habits, the more likely causes of yellowing leaves are due to cultural or disease problems. Again, without knowing exactly what shrubs you are growing, it's difficult to suggest what diseases it might be prone to. In terms of cultural problems, drought, over watering, poor soil drainage, over/under fertilizing can all play a part in the yellowing and browning of leaves.
Scale insects look like little bumps along the stems. Generally you can pick them off or pry them off with a fingernail. Scales are typically controlled with horticultural oils which smother them, but again, some plants are more sensitive to oils than others so if you use oil to kill scales, you run the risk of suffocating the plant, as well.
With 60 feet of hedge, I hope you don't lose them! Try to find out the exact name of the plants you're growing. You might also want to take a sample of the affected plant parts to your local Cooperative Extension office or well staffed nursery. The extension office can send the sample to their plant pathology lab if the local experts cannot pinpoint the problem.
Best wishes with your hedges!
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