Plant disease epidemics: different dynamic patterns – varying impacts on food security

stem rust CIMMYT
Wheat stem rust, with UG99 seen as an emerging disease (image source: CIMMYT)


by Serge Savary & Laetitia Willocquet

Plant diseases seldom receive attention beyond the agricultural production supply chain. Exceptions are dramatic or threatening events, such as the 19th Century Irish potato famine associated with downy mildew epidemics (potato late blight disease), or the stem rust UG99 races threatening wheat production since their detection in Uganda in 1999.

Tungro, an acute virus disease of rice



Using medical terminology, both UG99 and the potato late blight – at the time when it first hit the European continent – can be called emerging diseases, that is, diseases whose range are expanding to new areas. Since its establishment in Europe, however, the potato downy mildew epidemic became acute – a disease that occurs irregularly, both temporally and spatially with the potential of causing massive disruptions in system performance. Similar in this behaviour is rice tungro in tropical Asia: the viral disease is endemic in some areas, but its range can expand so much in some seasons that it causes important losses. Most farmers (and some policy makers) are concerned by such diseases that can unexpectedly invade and sometimes cause massive damage. Yet, a third type of dynamic pattern exists, in which diseases affect large crop areas, year after year, season after season, causing regular attrition in system performances and yield. Such diseases, which almost never receive the attention they deserve, are chronic diseases.

Emerging diseases typically cause yield losses in a limited area, but are addressed according to the potential threat they may cause if their geographical range expands. Their importance is therefore not related to their current, actual importance, but to the damage they may cause.

Brown spot
Brown spot, a chronic disease of rice

Acute diseases have large variation in intensity over time and space. This variation may be due, for example, to sensitivity of epidemic severity response to climatic conditions, or to changes in the pathogen population which adapt to environmental (e.g., varieties and host plant resistance) shifts. Chronic diseases are often overlooked, because they are commonly seen as part of the production system. They are not spectacular, nor immediately threatening, but regularly take their toll on crop harvests in a discrete way. One good example is rice brown spot, which very often increases towards the end of the crop cycle, and is seen as a

phenomenon associated with crop maturity. Yet, such disease does reduce crop yield, and is the cause of regular, hidden, reduction of food production.

Disease dynamics
A schematic representation of the dynamics of acute, chronic and emerging diseases

Acute, emerging, and chronic diseases have also different impacts on food security. Acute diseases may cause large yield losses over localized areas, therefore destabilizing markets in areas where access to food stocks is poor, thus leading policy-makers to over-react to a spectacular, yet local, outbreak. Reactions such as subsidies for pesticides are part of possible consequences of disease outbreaks. Emerging diseases do not have a direct impact on food security, but represent a threat to it. Chronic diseases do have a regular impact on food security, through the regular losses they cause on crops. Yet, this impact is generally not recognized.

The characterization of disease as acute,

emerging, or chronic can

help designing management strategies adapted to these patterns. They once again remind of the necessity to assess the current, and predict possible future losses caused by diseases, irrespective of perceptions. Perceptions often lead to decisions that are not supported by strong background data and knowledge of the systems at hand.

Further reading

Savary S, Sparks AH, Willocquet L, Duveiller E, Mahuku G, Forbes G, Garrett KA, Hodson D, Padgham J, Pande S, Sharma M, Yuen J, Djurle A. 2011. International agricultural research tackling the effects of global and climate changes on plant diseases in the developing world. Plant Disease 95: 1204-1216.

Zadoks JC, 2008. On the Political Economy of Plant Disease. Wageningen Academic Publishers.

M. K. Barnwal,  A. Kotasthane,  N. Magculia,  P. K. Mukherjee,  S. Savary,  A. K. Sharma,  H. B. Singh,  U. S. Singh,  A. H. Sparks,  M. Variar,  N. Zaidi. 2013. A review on crop losses, epidemiology and disease management of rice brown spot to identify research priorities and knowledge gaps. European Journal of Plant Pathology 136: 443-457.