Warm humid weather in northern districts encourages outbreaks of fungous and bacterial diseases. Late blight of potatoes may come at most times of the year at Auckland and Pukekohe, and in all parts of New Zealand if the weather favours it. Frequent spraying is essential in the north. Spraying in the south is done according to seasonal conditions. The main fungous diseases are soil-borne fungi in glasshouses and in seedbeds, late blight of potatoes and tomatoes, botrytis rot of a wide variety of vegetables, and leaf-spotting diseases of celery and lettuce. Bacterial diseases have become more troublesome since fewer copper sprays have been used. Virus diseases are common, particularly in potatoes, tomatoes, brassicas, lettuce, and legumes. Many virus diseases are spread by insects, chiefly aphids, thrips and leaf hoppers. Troublesome insect pests include aphids, caterpillars of the white butterfly, tomato moth, the silver Y moth, and cutworms. All of these cause much damage in some seasons. Soil-borne nematodes occur in many areas (see alsoInsect Pests).