On Social and Political Effects of Plant Pest and Disease Epidemics.

Selected historical pest and disease outbreaks in the Old World are discussed in view of their social and political consequences. Large-scale epidemics always caused social unrest, and often hunger, pestilence, and death. When coming on top of deeply rooted and widely spread social unrest such epidemics contributed to political change. Examples are the revolts following epidemics in 1789 and 1846. Epidemics, regardless of causal and target organisms, have elements in common. The notion of a common concept grew into a firmly established discipline: epidemiology.

Pandemics of focal plant disease, a model.

ABSTRACT An analytical model of a pandemic, initiated by a single focus and spreading over a continent, is developed using foci as the smallest units of disease and fields as the smallest units of host. A few generalizing assumptions lead to a parameter-sparse model that may answer general questions on pandemics in a qualitative manner. For pandemic spread of disease during one season, a 'within-season velocity of pandemic spread,' C, is expressed as a set of integral equations. Reduction of inoculum during the off-season is expressed by a 'survival ratio' of inoculum, epsilon. The effect of the off-season is a 'push-back' of the pandemic front over a distance Deltah. It will be shown how Deltah is related to C and epsilon. The mean pandemic spread over successive years is calculated as the 'polyetic velocity of pandemic spread,' V, which depends on C and the push-back distance. The concept of 'pandemic effectiveness' is parameterized. Relations between the two velocities of pandemic spread and several model parameters are studied. Somewhat unexpectedly, velocities of pandemic spread depend only in a very limited way on field density represented by the 'cropping ratio' zeta. This implies that our model and methods will also apply to situations with inhomogeneous field distributions. The effect of parameter values on rates of severity increase are analyzed. Finally, generalizations of the model are developed and their applications discussed.

Exploring Differential Interactions Between Rhizoctonia solani AG 2-t Isolates and Tulip Cultivars.

Experiments were conducted to explore differential interaction of Rhizoctonia solani AG 2-t isolates on tulip cultivars in soil artificially infested under different experimental conditions. Comparison of residual variances obtained by analysis of variance and by analysis of additive main effects and multiplicative interaction effects (AMMI) showed that open-air experiments should be used for interpretation of isolate by cultivar interaction. In open-air experiments, variability was lower than in greenhouse tests. In the biplot, derived after AMMI-analysis over isolates by years and by cultivars, isolates tended to occur in year clusters, indicating a differential effect of year on disease expression. Three isolates occurred in isolate clusters, thus accounting for a significant year by isolate by cultivar interaction. One cluster consisted of three isolates high in aggressiveness on all tested tulip cultivars, and another cluster consisted of three isolates low in aggressiveness. Greenhouse conditions and inoculum carrier significantly influenced quantitative differential interaction patterns. Isolates grown on oat kernels were more aggressive than those grown on autoclaved perlite particles soaked in liquid malt peptone. Differential interaction of AG 2-t isolates on tulip cultivars does occur, although it cannot yet be disentangled completely.

Development of potato late blight epidemics: disease foci, disease gradients, and infection sources.

ABSTRACT Natural potato late blight epidemics were studied to assess the relative impact of various inoculum sources of Phytophthora infestans in Southern Flevoland (the Netherlands) from 1994 through 1996. Disease surveys were combined with characterization of isolates for mating type and DNA fingerprint pattern using probe RG57. Seventy-four percent of the commercial potato fields with early foci were clearly associated with nearby infested refuse piles. Characterization of isolates from refuse piles and fields confirmed the association. Infected seed tubers, volunteer plants, and infested allotment gardens appeared to be of minor importance for late blight development in potato fields. Several foci in refuse piles, potato fields, and allotment gardens contained more than one genotype. Due to favorable weather in August 1994, infested organic potato fields became major inoculum sources, resulting in the spread of P. infestans to adjacent conventional potato fields. Analyses of disease gradients, both at the field and regional levels, confirmed the role of the organic fields as mid-season infection sources. The mean slope of field gradients downwind of refuse piles (point sources) was significantly steeper (100-fold difference) than the mean slope of field gradients downwind of organic fields (area sources). The genotypic composition of the P. infestans populations along the gradient and of the source populations in the organic potato crops did not differ significantly. Analysis of the region gradient revealed genotype-specific disease gradients. Control measures are recommended.

Global climate change: modelling the potential responses of agro-ecosystems with special reference to crop protection.

Pests and diseases reduce yields to lower levels than those that could have been potentially obtained, given the restrictions of climate, nutrients and crop varieties. Climatic change not only affects the potential yield levels, but it may also modify the effects of pests and diseases. Modelling can serve as a tool to integrate these processes, ranging from simple removal of plant material to subtle toxic and hormonal effects. Modelling can help to quantify different modes of action such as on photosynthesis, root activity, assimilate partitioning, morphology, and their interactions. As to climatic change, little is known about pests, diseases and weeds. If climatic change causes a gradual shift of agricultural regions, crops and their associated pests, diseases and weeds will migrate together, though at different rates maybe. To a limited extent, new outbreaks can be foreseen given the changed environmental conditions. Methodology is available, and some interesting results are on record. Specific changes such as an increase in the CO(2) content in the air and in UV radiation are not likely to have large effects. Increasing atmospheric CO(2) reduces crop nitrogen content, which may retard many pests and diseases, and change the composition of the weed flora which accompanies crops. Some cautionary remarks are made to avoid jumping to conclusions.

Crop protection: why and how.

The answer to the question 'why' has proximate and ultimate roots. The proximate answer, 'pests take our harvest', compels one to act. Crops and their pests are products of domestication, the ancestors of the pests still exist in nature. Eradication is impossible and undesirable. What action should be taken? The ultimate answer, how organisms became pests, may tell us how to act. Old cropping systems had man-made ecological sustainability but do not have economic sustainability in modern times. To achieve both we must prevent rather than control pest outbreaks, using old and new ecological tricks, with application of pesticides in emergency cases only. The action will require a move from chemistry to ecology. Optimism on regaining some ecological sustainability is mixed with doubts on economic sustainability. Can farmers be asked to invest in the future at the expense of today's family income? Crop protection faces new technical and moral problems.