With regard to your lemon plants, generally a plague with aphids and chinchas falls on them, which produce white and cottony scales that cover the leaves and stems, on my farm, when this plague begins in my oranges, lemons, tangerines And especially for guavas, I spray them with laundry detergent, this is super effective and economical. If the plants are tall, use a 1/2 pump with a pressure peak and try to wash the plants well.