Late blight (Phytophthora infestans) on tomato in the tropics

Of all the pathogens of economically important plant species, Phytophthora infestans (Mont.) de Bary can rightly claim its place as the most notorious. Its occurrence in Ireland during the last century caused both the potato famine of 1846-49 and the establishment ofplant pathology as a scientific discipline by Rev. M. J. Berkeley and others. Even today, late blight frequently decimates potato crops during wet summers, despite significant advances in disease prediction, crop protection (usually with phenyl amide or dithiocarbamate fungicides) and the introduction of more resistant cultivars. Although' late blight has traditionally been associated with potatoes, in tropical climates with periods of high rainfall the same pathogen can be a significant factor limiting yields from tomato crops. In many developing countries, tomato represents an important food crop, not only because of the high price it commands compared to staples such as rice, but also because it provides a valuable source of vitamins when fresh and improves the flavour of meals when cooked. The requirement of tomatoes for moderately low night-time temperatures for efficient fruit-set limits production in the lowland tropics to the cooler seasons. Consequently summer production is centred upon cooler upland areas (e.g. in the Cameron Highlands of Malaysia and the Central African massif), where high rainfall levels are optimal for late blight infection. In such areas, late blight represents a serious problem, since application of costly imported fungicide is often ineffective due to dilution by rainfall and the potential occurrence offungicide-resistant strains of the pathogen. Research at the University of Wales, Bangor being funded by the UK Government Overseas Development Administration and in collaboration with the Asian Vegetable Research and Development Centre (AVRDC) in Taiwan has two objectives. Firstly, we are identifying promising sources of resistance to late blight by screening material from tomato germplasm collections; promising accessions are currently being field-tested in Taiwan, Tanzania and Costa Rica prior to the initiation of a breeding programme. Secondly, we are examining levels of genetic variability within populations of the pathogen from tropical areas; it is vital to know the extent of geographical variation in the aggressiveness and virulence within pathogen populations to maximize the possibility that resistance factors introduced into planting material will provide durable disease protection over a wide geographical range. An important component of our research therefore involves the acquisition of samples of the pathogen from a range of countries, both temperate and tropical. These will permit testing the resistance of promising tomato accessions against pathogens from the countries where the resistant cultivars will ultimately be grown. A further reason for wishing to assess P. infestans populations is the increase over recent decades in the international potato trade. The concomitant migration of the fungus in diseased tubers is believed to have led to significant population changes. We hope that by obtaining isolates ofthe fungus from around the world it will be possible to more accurately monitor these changes. Wherever tomatoes are grown without irrigation and without heavy use of fungicides, it is likely that late blight is present unless conditions are very dry. The symptoms include blackening of the leaves (Fig Ia; often spreading lesions originating from the tip of the leaf) and death of whole branches of the plant (Fig Ib). Infected green fruit become marbled brown in appearance (Fig Ic) without becoming soft (not to be confused with nutritional disorders such as blossom end rot, which usually affect the area around the tip of the fruit). Sporulation of the fungus (only under