The product label states Fertilome Natural Guard Horticultural Oil RTS is a dormant and growing season insect spray, so the product should be safe to be applied when lemons are in bloom. Keep in mind that this is an oil, so it could kill pollinators such as bees if you spray them with the product. For this reason, we recommend being careful when treating around blooming plants.
If you are dealing with insects that are causing the black leaves and black spots, Fertilome Natural Guard Horticultural Oil RTS may help, but you may be dealing with a disease like peach scab.
Bonide Fruit Tree Spray Concentrate may be a better option as this is a liquid fruit tree spray concentrate containing insecticide, fungicide, aphicide, miticide, scalicide, and spreadersticker all in one solution, and is labeled for peach scab.
We also recommend contacting your local extension office for assistance with a positive identification of what is ailing your trees, and for appropriate treatment recommendations.
