Introduction

The removal or destruction of inoculum of pathogens, especially prior to crop emergence and establishment, may prevent or at least delay the onset and spread of disease. Complete elimination of pathogens is possible in enclosed growing areas such as glasshouses, but is difficult to achieve under field conditions. The sources of inoculum of pathogens of most field crops are so widespread that eradication is simply not a practical proposition.

Disposal of Crop Residues

The debris remaining after growth of protected crops should be taken out of glasshouses and destroyed. This is particularly important with stems of spent cucumber and tomato plants, which may be covered in late season with conidia of Botrytis cinerea. The roots of lettuce plants that have been attacked by B. cinerea should also be removed from glasshouses. Crop remains and other debris should be removed from the nursery and disposed of to green waste recycling sites or incinerated. Efficient windrow or in-vessel composting systems that utilise green waste will kill most plant pathogens.

Stubble and debris remaining after the harvest of cereal and oilseed rape crops may harbour several diseases, and burning was formerly the major (and for farmers the easiest) method of disposal. Stubble burning, however, has frequently been carried out in an irresponsible manner and the practice was banned in the UK in 1992. Cereal and rape stubble is now primarily disposed of by ploughing. Soil bacteria and fungi colonise and degrade ploughed-in crop debris, and in doing so remove the intercrop habitat of some pathogens. The hyphae and resting structures of pathogens may then perish. Consequently the survival of cereal pathogens as, for example, cleistothecia with powdery mildews, pycnidia in the case of Mycosphaerella graminicola, hyphae and spores of net blotch of barley may be reduced. However, some diseases such as eyespot of wheat can survive on straw buried in soil formore than 3 years, and the resting stages of some pathogens may survive even longer. The sclerotia of Sclerotinia sclerotiorumfor example, which form in the stem cavities of oilseed rape (Fig. 2.12), can remain viable in soil for at least 8 years.