Indoor plants sticky leaves

The lemon tree in my conservatory has developed very sticky leaves which I think is due to an attack by pests or a virus. The plant has small brown blobs, no more than 0.5mm in diameter all up the trunk and under most leaf stems. I can clean the blobs and sticky leaves off with detergent and water but obviously this is not sufficient as the blobs and sticky leaves return quickly. Any idea on how I can treat my lemon tree please.