A number of pests attack cacti during its growth – pests that include mites and mealy bugs. Leaves of cacti tend to appear waxier, which is the primary symptom of these pest attacks. Appearance of grey spots is a symptom of mite attack. An antidote for this is the pesticide, Malathion. Two milliliters of Malathion mixed in one liter of water can be sprayed on the leaves. Grey spots can also appear if the cactus is over exposed to sunlight. If so, move the plant to a shadier place. These are some of the most common diseases affecting the cactus.