Introduction

The main diseases that we are concerned with in this chapter are seed rotting, pre- and post-emergent damping off and various seedling blights. This complex of diseases is mostly caused by a few genera of fungi, especially Rhizoctonia and Pythium, with Phytophthora, Fusarium and Sclerotinia causing less widespread problems. These are unspecialized pathogens (r-strategists) which use exudates from the germinating seeds for saprotrophic growth before they attack the very young plants that have not developed effective mechanical barriers to infection. Rhizoctonia usually attacks the seed, hypocotyl or stem, while Pythium attacks the root tips. Under very wet conditions Rhizoctonia and Phytophthora may grow amongst the tops of the seedlings. The diseases are especially bad if conditions are not favourable to the rapid growth of the seedling. Damping off is characteristic of crops that have been sown too early in the year so that they are germinating slowly in damp soils at low temperatures. Vigorous seedlings getting away to a good start under ideal conditions do not usually suffer from these problems.

However, often there are agricultural reasons for trying to make an early start to the growing season, or seeds sown in a warm period of weather may be overtaken by a cold wet spell. In glasshouses, where many seedlings are raised for horticultural crops, there are less climatic problems, but heating costs are high and some growers may try to manage with just a little less heat than is best, or seeds may be over-watered. Seed rots and damping-off remain a real problem.