Introduction

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

Disposal of Crop Residues

The debris remaining after growth of protected crops should be taken out ofglasshouses and destroyed. This is particularly important with stems of spentcucumber and tomato plants, which may be covered in late season with conidia ofBotrytis cinerea. The roots of lettuce plants that havebeen attacked by B. cinerea should also be removed fromglasshouses. Crop remains and other debris should be removed from the nurseryand disposed of to green waste recycling sites or incinerated. Efficient windrowor in-vessel composting systems that utilise green waste will kill most plantpathogens.

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