One or more of the features that are needed to show you the maps functionality are not available in the web browser that you are using.
Please consider upgrading your browser to the latest version or installing a new browser.
More information about modern web browsers can be found at http://browsehappy.com/
Quinoa plants infected by P. farinosa present a variety of symptoms depending on the cultivar, the growth stage and the environmental conditions. Typical symptoms include pale or yellow chlorotic lesions on the surface of the leaf, which eventually turn necrotic, and grey-violaceous sporulating areas on the lower surface of the leaf. In some cultivars the lesions are small and numerous, whereas in others the lesions are large, diffuse and irregular. In some cultivars, the lesions turn reddish to purple; a hypersensitive type of reaction (small necrotic flecks) has also been observed (Danielsen and Ames, 2000). The degree of sporulation varies considerably among cultivars, most probably due to differences in host/pathogen interactions. Severe infection leads to heavy premature leaf loss (Danielsen et al., 2001). Systemic infection, probably caused by seed-transmitted oospores, may lead to dwarfing and yellowing (Alandia et al., 1979; Frinking and Linders, 1986; Tewari and Boyetchko, 1990).
Introduction
The traditional way of controlling downy mildew in various crops is through application of fungicides such as metalaxyl. However, the use of such fungicides exposes the environment and humans to health hazards, and there is also a risk that the pathogen will develop resistance to metalaxyl, as shown for Bremia lactucae (Crute et al., 1987) and Phytophthora infestans (Davidse et al., 1981). Moreover, the majority of quinoa growers in the Andes are resource-poor farmers who practice low-input farming. Within a scenario of a rapidly spreading, endemic disease and sexually reproducing pathogen populations, these farmers have only two realistic options for disease control: effective cultural practices and the use of durable host plant resistance.
Cultural Practices
Experiences from Bolivia show that cultural practices that minimize the relative humidity in the microclimate may have an effect on the severity of downy mildew. Among these are: adjusting the spacing between rows and individual plants; orienting the field against the predominant wind direction; avoiding excess water in the field (Bonifacio, PROINPA, Fundación para la Promoción e Inverstigación de Productos Andinos, La Paz, Bolivia, unpublished data). However, cultural practices alone are insufficient for controlling downy mildew in quinoa.
Host Plant Resistance
Resistance to downy mildew has been a focus of genetic improvement through recurrent selection for many years at breeding and research stations and at farm level. A wide range of resistance sources are present in quinoa germplasm from the Andes (Otazú et al., 1976; Bonifacio and Saravia, 1999; Danielsen et al., 2001), which makes it possible to select quinoa genotypes for specific agroecological zones according to the downy mildew risk. However, the nature of the resistance has not been characterized, neither has the interaction with pathogen populations of different geographical origin. Danielsen et al. (2001) showed that even in the most resistant cultivars there is a considerable yield penalty due to downy mildew induced defoliation. Susceptibility to downy mildew has shown often to be related to the length of the growing season, as the early cultivars appear to be the most susceptible (Bonifacio and Saravia, 1999; Danielsen et al., 2001). Breeding efforts are concentrated on increasing the level of durable resistance against downy mildew and combining resistance with other desirable traits such as earliness, sweetness and drought tolerance. Additional sources of downy mildew resistance seem to be present in wild Chenopodium species that grow more or less in association with the cultivated crop. There are indications that wild species such as C. hircinum, C. nuttalliae, C. petiolare, C. album, and C. ambrosioides harbour downy mildew resistance genes. These sources may be useful for incorporating resistance into commercial varieties (Bonifacio, PROINPA, Fundación para la Promoción e Inverstigación de Productos Andinos, La Paz, Bolivia, unpublished data).
The vast majority of the world's quinoa cultivation is restricted to the high mountain areas of Bolivia, Peru and Ecuador. However, due to its favourable agronomic and nutritional characteristics, quinoa is increasingly considered as an attractive crop in other continents such as North America and Europe. Downy mildew is well known in the Andes as the main and most important disease of quinoa. In a field trial of eight quinoa cultivars conducted in the humid inter-Andean valley at 3300 m in Peru, where the natural level of downy mildew infection is high, yield losses of 33-99% were recorded (Danielsen et al., 2001). The most susceptible variety, Utusaya, which is usually grown in the very dry salt desert of southern Bolivia, was completely eradicated by downy mildew. The tall, late and more resistant cultivars adapted to valley conditions also suffered considerable yield losses of 33-58%, although the apparent level of downy mildew infection was low. The yield loss is believed to be a consequence of pathogen induced leaf loss. In a similar field trial in the coastal region of Peru, where quinoa is grown experimentally, yield losses were 6-29%. However, the loss was not significant in any cultivar (Solveig Danielsen, Danish Government Institute of Seed Pathology for Developing Countries, Denmark, unpublished data). The natural level of downy mildew in the warm coastal area is much lower than in the highlands and will probably not be a major constraint to quinoa production.
There is potential for downy mildew of quinoa to become an important disease outside of the Andes due to the worldwide presence of P. farinosa f.sp. chenopodii.
