Increasing temperatures reduce the latency period or generation time, often measured in degree days, and allow a higher number of generations per season in terms of both diseases and pests. This has a major effect on polycyclic diseases and on diseases transmitted by insect vectors. Generation time determines the amplification of plant diseases in two ways: (i) it accelerates and increases the inoculum load in a field or agroecosystem; and, more importantly, (ii) it affects pathogen evolution rates and a pathogen's capacity to adapt to a changing environment often faster than a host can respond. Leaf rust of wheat will be favoured by higher temperatures and might therefore spread to areas where it is not currently important, such as the facultative and winter wheat growing areas of China, parts of Europe, the Pacific north-west region of the USA and the winter facultative wheat areas of Central Asia (FAO, 2008). In the case of potato late blight caused by Phytophthora infestans, a model predicting the date of outbreak in Finland based on thermal time on rainy days suggests that over a range of 1-3°C warming, the period during which the disease needs to be controlled by fungicide applications would be 10-20 days longer per 1°C (Kaukoranta, 1996). In the upper Great Lakes region of the USA, the risk of late blight of potato is increasing because the climatological trends here have resulted in warmer and wetter growing season conditions (Baker et al., 2005). In Africa, higher temperatures and rainfall have led to an increase in the abundance of whitefly, Bemisia tabaci, the vector of cassava mosaic virus and sweet potato chlorotic stunt virus, and of leafhoppers transmitting maize streak disease (Chancellor and Kubiriba, 2006).
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