Potato late blight in developing countries

Potato is the fastest growing major crop in the developing world with important economic impact on many resource-poor farming families. Many factors limit production and profitability, with hundreds of millions of dollars spent yearly on fungicides alone, but little is known about direct losses, with experts agreeing that they are variable and frequently significant. Late blight is most severe in the mountainous areas of developing countries where weather conditions are favorable for disease. Variable topography and continuous production of potato and other late blight hosts, including tomato and in the Andes pear melon, make prediction of disease initiation or severity difficult. New and potentially more aggressive pathogen populations have been introduced into Asia and both mating types are present in a number of Asian countries. There is not yet clear evidence for the role of sexual recombination or oospores in nature in Asia; nor has it been established that new populations have made disease management more difficult. However, this can probably be inferred from what has happened in Europe and the US. New populations exist in Latin America but A1 and A2 populations are generally separated geographically, except in Mexico where sexual recombination is common. Sub-Saharan Africa still has the US-1 population. The social context is an important factor in late blight management in developing countries. Most farmers have no formal training in biology and view disease with a pre germ-theory perspective. Building capacity among farmers for making the right decisions about disease management is most effective if it includes basic information about biology and ecology. Farmers with little education also can not be expected to understand the intricacies of pesticide risk, and late blight control with fungicides is one component of an epidemic of pesticide poisoning and other chronic health problems currently plaguing the developing world. Epidemiological studies in the tropics demonstrate some important differences in the way late blight develops from that of the temperate zone. In the tropics, particularly in the highlands, aerial inoculum is present most of the year from a number of sources. This makes sanitation activities (removal of cull piles and volunteer plants) of little apparent value in some parts of the developing world. In spite of the generalized importance of foliage blight in the developing world, tuber blight is apparently unimportant in many areas. The reasons for this are not clear. Host plant resistance is the primary disease management strategy, and resistance is probably used more in developing countries. Many cultivars released as resistant have “gone down” as virulence was selected in the pathogen population. In contrast, resistance in some cultivars has held for decades. A gene from the wild species Solanum bulbocastanum has provided resistance against a number of pathogen populations and is currently being tested on a larger scale. Fungicides are used almost everywhere that late blight is a problem and fungicide use is increasing. Optimization of fungicides may be one of the best investments for short-term impact with resource poor farmers. Newly