GREENHOUSE grower notes: Being keen on clean Many pests and pathogens can survive poor conditions for a long time

摘要

Athorough cleanup between crops is essential if greenhouse vegetable growers are to stand a chance of man-aging pest and disease problems. The reason for this is the ability of many common pests and disease-causing organisms or pathogens to survive unfavourable conditions, and for a long time. Pests, such as the two-spotted spider mite and greenhouse whitefly, have mechanisms for withstanding very low temperatures. Pathogens, too, are particularly resilient because of the protection provided either by the tissue within which they reside, or by the resistant spores they produce. For example, infected air-dried tomato leaves infected with Tobacco mosaic virus can remain infective even after 24 years at laboratory temperatures. Another example is the canker-causing bacteria, Clavibacter michigamnsis, which can survive within infected tissue for at least two years in dry soil, and as long as 36 weeks at -20°C in non-sterile soil. Other examples include Pythium, Botrytis and Didymella bryoniae, the gummy stem blight causing fungus, which all produce thick-walled spores that can withstand long periods of very cold temperatures.