In semi-arid and sub-humid tropical countries in Africa and parts of Asia, {\em

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In semi-arid and sub-humid tropical countries in Africa and parts of Asia, {\em Striga hermonthica} (Del.) Benth. (witchweed) is an economically important parasitic weed, severely attacking cereal crops like pearl millet, sorghum and maize. Pearl millet -- {\em Pennisetum glaucum} (L.) R. Br., is a widely cultivated, drought resistant crop, cultivated mainly on light sandy soils. Striga is in particular a problem in areas with eratic rain fall and on soils that are relatively poor in nutrients. It is recognized as the major biological threat to cereal crops in Africa. It has been estimated that this weed can cause yield losses of greater than 50%, and numerous cases have been documented in which arable fields were abandoned because of high parasite populations.

Due to the withdrawal of water, minerals and organic compounds from their hosts and to phytotoxic effects, parasitic weeds have a very strong negative effect on both the quality and the quantity of the final grain yield.

Recent trends, away from traditional fallowing practices toward continuous monocropping to meet the increasing population pressure, have intensified the Striga problem.

Life cycle of Striga

To better understand the behaviour of Striga in relation to millet, we consider its life cycle in some more detail \ref{fig1}. The seeds of Striga are extremely small and mostly have a long lifespan in the soil. Throughout the dry period the seeds are dormant, but in the first weeks of the wet season they get conditioned under moist conditions at a suitable temperature and become ready for germination. Because of their parasitic character, the conditioned Striga seeds require a germination stimulant for germination that is exuded by the roots of a potential host plant. After they have germinated, the seedlings have to make contact to the host roots within a few days. On contact, the haustorium at the end of the germ tube of the Striga seedling begins to develop and produces enzymes that dissolve the cell wall of the root. Once the haustorium is penetrated into the host stele, direct links are developed between the xylem systems of both organisms. After penetration, the seedling starts to grow towards the soil surface. In this phase, the seedling is completely dependent upon its host for all its organic nutrition, inorganic minerals (assimilates) and water (holoparasite).These substances are withdrawn from the host across the so-called xylem bridge. Four to six weeks after germination, the Striga plant emerges from the soil surface. From this moment on, the parasite is capable of producing assimilates itself. However, its chlorophyll content is relatively low and has a very small photosynthetic capacity. So, even after emergence the parasite is still dependent on its host to a large extent. When the Striga plant is matured, it will flower. Every Striga plant produces vast numbers of seeds ($> 10^5$) that are spread in the vicinity of the plant. By means of rainfall, wind, animal and human activities the seeds are spread over the soil surface and penetrate into the soil. Here the seeds remain in the so-called seed bank until the next growing season.

The parasite has a detrimental effect on its host not only by reason of the materials that it removes. Quite simple measurements show that the reduction of host growth can be enormously greater than can be explained by simple removal of substances. Some phytopathological effects of the parasite on its host are demonstrated, of which the reduction of the hosts photosynthetic capacity, which can be as much as over 50%, is the most definite.






\caption{Life cycle description of striga on millet.}



Striga control measures

Reducing the Striga problem is very difficult for several reasons. The main reason is that initially the parasite develops below the soil surface and becomes visible after much of the damage to the host plant has been done. Using herbicides has several drawbacks. First, they may as well influence the Striga as the host plant. Second, as the farmers on the Striga-infested soils are generally poor, herbicides or other inputs, as fertilizers, are not available at all. Third, the farmers do not immediately observe the result of their actions, as most of the harm already has been done before controle measures could be taken and because many seeds remain dormant in the soil, causing problems in the next growing season. It has been recorded that additional use of nitrate may increase the crop quality and reduce the effects of Striga \cite{BoukHessPayn96}. Striga control requires a form of integrated crop management, where measures are continued during a range of years, focused on the prevention of Striga seed production and exhaustion of the supply of seeds in the soil.

(Dit onderstaande stuk heb ik een andere positie gegeven. Het sluit m.i. nu beter aan op het onderzoek wat hier gepresenteerd wordt)

At the moment it is not yet fully clear what basically determines the amount and distribution of Striga seeds. A positive correlation was found between millet yield and Striga occurrence (Manu et al. 1989?), suggesting that a well fed (?) host may be able to tolerate more Striga attachments than an unhealthy one, so that Striga is more likely to be found at the more fertile parts of a field. High soil fertility may, on the one hand, stimulate crop vitality and as such maintain a high number of Striga plants, causing relatively small effects on host yield. On the other hand, low soil fertility may lead to a crop of a low vitality, and a low number of Striga plants may have a large effect on crop performance. The spread of Striga is to a large extent also dependent upon the effects of rainfall. Rainfall may spread the seeds to larger depths, but in particular large showers may as well spread the Striga laterally towards adjacent fields, and beyond. Seed dispersal between fields is further enhanced by animal and human activities and the use of contaminated crop seeds. However, precise information is lacking at the moment and despite much research, the mechanisms and patterns(?)of the distribution of the weed are still unknown.

\section{This study}
Maps showing Striga abundance are relatively easy to collect. In this paper we explore the possibility of applying modern spatial statistical techniques to a series of such maps, composed between 1985 and 1994 at the ICRISAT Sahelian Center, Niger (?) at various moments in the growing season. The years with multiple observations allow us a glimpse of witchweed distribution. We will also relate these images to various environmental conditions, in particular to the amount and temporal distribution of rainfall. It is hypothesised that in

years with little or very erratic precipitation at the beginning of the growing season, Striga seedling mortality is high (or germination is low).

Een tweede hypothese zou kunnen zijn, dat hevige regenbuien aan het begin van het seizoen (dus voordat de grond bewerkt is), Striga zaden doet afstromen en daarmee ook (dfeels) de patronen verklaart.

[ and {\em hot spots} corresponding to seed banks are the only areas with a seed number sufficiently high that striga may emerge.]?
The use of statistics in agriculture has a rich tradition.


In the current study, our aim is to use spatial statistics to gain a better understanding of the processes governing Striga emergence and abundance. The main benefit for agriculture would be a well directed use of scarcely available nitrate and phosphate, or of even scarcer herbicides, to optimise the yield and minimise damage to the environment.

Opmerking in het algemeen: wanneer we correlative verbanden vinden tussen bodemnutrienten aanwezigheid en Striga verspreiding, dan moeten we wat meer aandacht geven in de inleiding van de rol van nutrienten. Wanneer nutrienten geen rol meer spelen in het verdere stuk, maar bv alleen regenval als verklarende factor, dan zou ik in deze Inleiding het woord nutrienten niet expliciet noemen en toelichten


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