Early detection of plant diseases using spectral data

Early detection of crop disease is an essential step in food security. Usually, the detection becomes possible in a stage where disease symptoms are already visible on the aerial part of the plant. However, once the disease has manifested in different parts of the plant, little can be done to salvage the situation. Here, we suggest that the use of visible and near infrared spectral information facilitates disease detection in cassava crops before symptoms can be seen by the human eye. To test this hypothesis, we grow cassava plants in a screen house where they are inoculated with disease viruses. We monitor the plants over time collecting both spectra and plant tissue for wet chemistry analysis. Our results demonstrate that suitably trained classifiers are indeed able to detect cassava diseases. Specifically, we consider Generalized Matrix Relevance Learning Vector Quantization (GMLVQ) applied to original spectra and, alternatively, in combination with dimension reduction by Principal Component Analysis (PCA). We show that successful detection is possible shortly after the infection can be confirmed by wet lab chemistry, several weeks before symptoms manifest on the plants

Control of Plant Virus Diseases in Sub-Saharan Africa: the Possibility and Feasibility of an Integrated Approach

Plant viruses and virus diseases have been studied for more than 100 years and much attention has been given to their control. However, this has been difficult to achieve because of the lack of any effective means of curing virus-infected plants. Chemotherapy, thermotherapy and meristem-tip culture can be successful, but they cannot be used on a large scale. Consequently, the main approach has been to prevent or delay virus infection or to ameliorate its effects. Various means have been used to achieve these objectives, including phytosanitation (involving quarantine measures, crop hygiene, use of virus-free planting material and eradication), changes in cropping practices, use of pesticides to control vectors, mild strain protection and the deployment of resistant or tolerant varieties. These measures can be used singly or in combination so as to exploit synergistic interactions. This paper considers the advantages of an integrated approach and provides selected examples from experience with several important African diseases, including cocoa swollen shoot, cassava mosaic and groundnut rosette. It is emphasised that much detailed research and a thorough knowledge of farming practices is required before an effective integrated programme can be developed and promoted. There are also formidable problems in ensuring adoption because of the generally limited education and resources of farmers in most parts of sub-Saharan Africa and the severe constraints imposed by the cropping practices adopted. Nevertheless, integrated control measures have evident benefits and should be fostered and promoted as a means of enhancing crop productivity to meet the increasing demands of a burgeoning human population.Les plantes agissant comme des virus et les maladies virales ont été étudiées pour plus de 100 ans et beaucoup d'attention a été donnée à leur contrôle. Cependant , ceci a été difficile à accomplir à cause du manque de moyens efficaces de contrôle. La chemothérapie, la thermothérapie et la culture en meristem-tip peuvent être utilisées avec succès, mais elles ne peuvent pas être utilisées à large échelle. En conséquence, la principale approche a été de prevenir ou retarder l'infection virale ou remedier à ces effets. Différents moyens ont été utilisés pour atteindre ces objectifs, incluant la phytosanitation (impliquant des mesures de quarantaine, l'hygiène des plantes, l'usage délibéré du virus sur le matériel de plantation et l'éradiation) ; les changements des pratiques agricoles, l'usage des pesticides pour contrôler les vecteurs, la protection de souches douces et le déployement des variétés résistantes ou tolérantes. Ces mesures peuvent être utilisées singulièrement ou en combinaison, pour exploiter les intéractions synergetiques. Cet article considère les avantage d'une approche intégrée et pourvoit des exemples sélectionnés à partir des expériences avec plusieures importantes maladies africaines, incluant le rejet du cacao gonflé, la mosaïque de manioc et la rosette d'arachide. Il a été souligné que des recherches détaillées et une bonne connaissance des practiques agricoles sont exigées avant de développer et promouvoir un programme effectif et intégré. Il y a aussi des problèmes énormes pour assurer l'adoption à cause de l'éducation et les resources souvent limitées des fermiers dans la plupart des pays de l'Afrique au sud du sahara, et la sévère contrainte imposée par les pratiques culturalles adoptées. Néanmoins, les mesures intégrées de contrôle ont des bénéfices évidentes et devront être encouragées et promises comme un moyen d'améliorer la productivité des plantes pour faire face aux demandes croissantes d'une population humaine en expansion

Cassava virus diseases in Africa

Cassava plays a key role in the food security of sub-Saharan Africa, but as a vegetatively propagated crop, it is particularly vulnerable to the effects of virus diseases and these therefore represent a major threat to the livelihoods of millions of Africans. Nine viruses have been isolated from African cassava, but only cassava mosaic geminiviruses (CMGs) and Cassava brown streak virus (CBSV) cause diseases of major economic signifi cance. In recent years, both CMGs and CBSV have come under increasing research scrutiny, because of the devastating losses they cause. Molecular and fi eld studies of CMGs have revealed a hitherto unrecognized level of complexity in biodiversity, interactions, and epidemiology, most notably in association with the pandemic of severe CMD which has impacted large areas of East and Central Africa. Comprehensive surveys of cassava in coastal East Africa have revealed cassava brown streak virus disease (CBSD) to be the major constraint to production in that zone, with the greatest effects of the disease being reported from northern Mozambique. This review describes recent progress in research on CMGs and CBSV in sub-Saharan Africa, examines the dynamic nature of their status, and provides insights into the major control initiatives that are required to tackle them

Cassava mosaic virus disease in East Africa: a dynamic disease in a changing environment

Cassava mosaic disease (CMD), now known to be caused by cassava mosaic geminiviruses (Family Geminiviridae; Genus Begomovirus), was first reported in East Africa in 1894. Epidemics occurred in Madagascar and Uganda in the 1930s and 1940s, and more localised rapid spread of CMD was observed in parts of coastal Tanzania in the 1930s and coastal Kenya in the 1970s. During the 1990s, a major regional pandemic of an unusually severe form of CMD has expanded to affect parts of at least five countries, causing massive economic losses and destabilising food security. Mechanisms responsible for the development and progress of the pandemic have been described, and comparisons of epidemiological data for varieties grown throughout the period under review suggest that the recent pandemic has been characterised by rapid rates of CMD spread hitherto unknown in East Africa. A key factor in the genesis and spread of the pandemic has been the recombination between two distinct cassava mosaic geminiviruses to produce a novel and more virulent hybrid. Although such events may be common, the known history of CMD in East Africa suggests that the frequency with which they become epidemiologically significant is low. A corollary of this is that resistance, developed originally in Tanzania between 1934 and 1960, and utilized and supplemented at the International Institute of Tropical Agriculture, Nigeria, since 1971, is providing effective CMD control in current pandemic-affected areas of East Africa. Consequently, it is concluded that prospects for managing CMD in the 21st century are good, and that the approach adopted should build on the model of collaborative research and implementation that has been established in tackling the current CMD pandemic

Analysis and Modelling of the Temporal Spread of African Cassava Mosaic Virus and Implications for Disease Control

This paper reports the main conclusions of a series of experiments concluded at Adiopodoume in the forest zone of the Ivory Coast between 1980 and 1990 on the temporal spread of African cassava mosaic virus (ACMV). The experiments sought to gain a better understanding of the epidemiology of the disease it causes and to facilitate the assessment of control measure. The course of ACMV epidemics over time was shown to depend closely on crop age and planting date. These relationships were expressed mathematically and ACMV progress curves were modeled combining a direct interaction between an overall exponential decreasing susceptibility to infection with crop age and a sinusoidal temperature-driven seasonal fluctuation in amount of spread from outside sources. After being validated using 1930s data from Kiwanda in Tanzania, this model was extended to incorporate host plant resistance, spread within plantings and yield losses. Simulation studies showed that when reversion (non-systemicity of the virus) does not occur and when cuttings are not selected preferentially from healthy plants, disease incidence increased in successive plantings of the same clonal stock and ultimately reached 100%. This occurred whatever the degree of host resistance, albeit after different periods. By contrast, with reversion and/or cutting selection, disease incidence may reach equilibrium values below 100% in resistant cultivars. At such equilibria, the effects of reversion and/or cutting selection balance the new virus transmissions by whiteflies. This emphasizes the potential of resistant cultivars to control ACMV by exploiting their ability to revert, as such cultivars not only suffer yield loss when infected, but are less likely to become heavily infected, even after many cycles of crop production