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Pythium Root Rot of Common Bean: Biology and Control Methods. a Review

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Pythium Root Rot of Common Bean: Biology and Control Methods. a Review B A S E Biotechnol. Agron. Soc. Environ. 2012 16(3), 405-413 Focus on: Pythium root rot of common bean: biology and control methods. A review John Rusagara Nzungize (1), François Lyumugabe (2), Jean-Pierre Busogoro (3), Jean-Pierre Baudoin (1) (1) Univ. Liege - Gembloux Agro-Bio Tech. Tropical Crop Husbandry and Horticulture. Passage des Déportés, 2. B-5030 Gembloux (Belgium). E-mail: [email protected] (2) National University of Rwanda. Biotechnology Unit. P.O. Box 117. Butare (Rwanda). (3) Belgium Technical Cooperation. IPM project. Kigali (Rwanda). Received on September 2, 2011; accepted on March 20, 2012. Pythium root rot constitutes a highly damaging constraint on the common bean, Phaseolus vulgaris L., grown in several areas of Eastern and Central Africa. Here, this food legume is cultivated intensively under poor conditions of crop rotation due to the exiguity of the land in the region. Yield losses of up to 70% in traditional local bean cultivars have been reported in Kenya and Rwanda. In this study, a detailed analysis of the biology and diversity of the Pythium genus was carried out in order to understand the mechanisms leading to the development of the disease. Various control methods for reducing the damage provoked by this disease were analyzed. Keywords. Phaseolus vulgaris, Pythium, root rots, plant disease control, disease resistance, Africa. Pourriture racinaire du haricot commun causée par Pythium : biologie et méthodes de lutte. La pourriture racinaire causée par Pythium constitue une importante contrainte pour la production du haricot commun (Phaseolus vulgaris L.) dans plusieurs régions de l’Afrique centrale et orientale où la production du haricot est intense dans des conditions de non-respect des schémas de rotation à cause de l’exiguïté des terres. Des pertes de rendement allant jusqu’à 70 % au sein des cultivars locaux traditionnels de haricot ont été rapportées au Kenya et au Rwanda. Dans ce travail, une analyse détaillée de la biologie du genre Pythium et de sa diversité a été conduite pour comprendre les mécanismes qui mènent au développement de la maladie. Pour réduire les dégâts provoqués par ce pathogène, diverses méthodes de contrôle ont été analysées. Mots-clés. Phaseolus vulgaris, Pythium, pourritures des racines, lutte antimaladie des plantes, résistance aux maladies, Afrique. 1. INTRODUCTION increase in the importance of Pythium bean root rots in several countries of Eastern and Central Africa, Common bean (Phaseolus vulgaris L.) is one of the such as Burundi, the Democratic Republic of Congo, most widely cultivated food legume species in the Kenya and Uganda (Otsyula et al., 2003). For example, world (Baudoin et al., 2001). The crop is exploited in a in Western Kenya and in Rwanda, many farmers wide range of areas extending from around 52° North stopped growing beans between 1991 and 1993 due to latitude to 32° South latitude and from sea level to a severe outbreak of root rots, which caused serious 3,000 m above sea level, and it displays great variation food shortages and price increases beyond the reach of in growth habits and length of vegetation (FAO, 2005). many resource-poor households (Nekesa et al., 1998). On a practical level, common bean is a major source of In tropical regions, common bean is characterized by low cost calories, protein, dietary fiber, minerals and low and unstable grain yields due to various ecological vitamins for poor populations (Hillocks et al., 2006). In and agronomic parameters. Among these parameters, Rwanda, this food source provides up to 25% of the total bean root rot and a decline in soil fertility have been calories and 45% of the total dietary protein; this level cited as being among the major causes leading to bean of contribution of common bean to the population’s yield losses (Miklas et al., 2006). In Rwanda, Western diet is considered to be the highest in the world Kenya and South Western Uganda, Pythium spp. are (Pachico, 1993). Pythium species are spread worldwide the fungal pathogens most frequently associated with (Paul, 2004). Over the last 20 years, there has been an severe root rot epidemics (Rusuku et al., 1997). Yield 406 Biotechnol. Agron. Soc. Environ. 2012 16(3), 405-413 losses of up to 70% in commercial bean cultivars have controlling bean Pythium root rot with crop rotation been reported in Rwanda and Kenya (Otsyula et al., practices will be of limited efficiency. In the present 2003). In other studies carried out in these countries study, an investigation was undertaken into the various the following species have been isolated from bean biological characteristics of the Pythium agents present samples affected by root rot symptoms. In view of in Rwanda. The aim was to attain a better understanding understanding the biology of root rot diseases, a of the conditions leading to disease development and study was conducted in Kenya, Rwanda and Uganda to establish appropriate control methods to reduce the (Mukalazi, 2004) to identify the causal agents. In these yield losses caused by Pythium root rot. countries the following species were isolated from bean samples affected by root rot symptoms: Pythium nodosum Bhatn, Pythium echinulatum Matthews, 2. TAXONOMY AND BIOLOGICAL Pythium pachycaule Shtayeh, Pythium oligandrum CHARACTERISTICS OF PYTHIUM SPP. Drechsler, Pythium acanthicum Drechsler, Pythium chamaehyphon Sideris, Pythium folliculosum Paul, The genus Pythium belongs to the family Pythiaceae, Pythium indigoferae Butler, Pythium irregulare order Pythiales, class Oomycetes, Phylum Oomycota Buisman, Pythium lutarium Shtayeh, Pythium and kingdom Chromista (Kirk et al., 2008). macrosporum Vaartaja, Pythium myriotylum Drechsler, Pythium species are fungal microorganisms with a Pythium paroecandrum Drechsler, Pythium torulosum filamentous vegetative body called a mycelium. The Coker, Pythium vexans de Bary, Pythium zingiberis mycelium of Pythium species is colorless, sometimes Takah, Pythium graminicola Subraman, Pythium lustrous, and occasionally slightly yellowish or a spinosum Sawada, Pythium ultimum Trow, Pythium grayish lilac (Owen-Going et al., 2008). The mycelia arrhenomanes Drechsler, Pythium catenulatum in Pythium species branch out apically at right angles Matthews, Pythium diclinum Tokun, Pythium to form structures known as hyphae. These hyphae are dissotocum Drechsler, Pythium rostratum Butler, hyaline, with the main hyphae being mostly 5-7 µm Pythium salpingophorum Drechsler and Pythium wide, occasionally reaching a width of up to 10 µm. deliense Meurs (Nzungize et al., 2011; Buruchara et al., Cross septa are lacking except in old, mostly empty 2007). Root rot diseases are widespread in the world hyphae or where the cross septa delimit the hyphae’s and are often considered as a major constraint to bean reproductive organs (Van der Plaats-Niterink, 1981). production, reducing both yield and quality (Abawi et Protoplasmatic streaming is often clearly visible in al., 2000). Depending on the pathogen(s) involved in the young hyphae. According to Postma et al. (2009), development of the disease, general root rot symptoms hyphal walls are composed of 80-90% polysaccharides, might include any combination of various traits such mainly ß1-6 linked glucans and ß1-3 and ß1-4 as poor seedling establishment, damping-off, uneven (cellulose). It should be noted that Pythium spp. do growth, leaf chlorosis, premature defoliation, death not contain chitin or chitosan in the hyphal walls, but of severely infected plants, and lower yield (Abawi et that they do contain protein and lipid at levels varying al., 2006; Schwartz et al., 2007). Pythium is a complex from 3-8% and from 1-3%, respectively (Postma et al., genus containing over 200 described species with a 2009). Pathogenic Pythium spp. may produce hyphae broad host range and occupying a variety of terrestrial with swollen digitate regions, called appressoria, which and aquatic ecological habitats (Dick, 2001). The enable the fungus to attach and penetrate the host cells presence of the pathogens responsible for producing (Lévesque et al., 2004). root rot and the severity of the disease are associated Pythium spp. can reproduce both asexually and with intensification of land use, inappropriate crop sexually. Asexual reproduction takes place through the rotations and/or reduced fallow periods, leading to a zoosporangia and zoospores. In Pythium the zoospores decline in soil fertility and a build-up of soil pathogen are not formed in the sporangium itself but in a vesicle inoculum (Abawi et al., 2006). outside it (Stanghellini et al., 1971). The sporangium In a study conducted in South Western Uganda, is separated from the rest of the mycelium by a cross seven Pythium species from various crops associated wall. The development of a tube can be observed on with beans were obtained: Pythium macrosporum, the sporangium. The undifferentiated content of the Pythium oligandrum, Pythium spinosum Sawada sporangium moves through this tube and forms a isolated from sorghum, Pythium glomeratum B. Paul vesicle at its end. At this level of development, the isolated from potato, Pythium arrhenomanes isolated zoospores are delimited and start moving (Stanghellini from maize, P. ultimum isolated from peas and et al., 1971). After 10 to 20 minutes, the wall of the Pythium heterothallicum W.A. Campb. & F.F. Hendrix vesicle disappears and the zoospores swim away in isolated from sweet potato (Gichuru, 2008). In cases divergent directions. Zoospores are only liberated where these various Pythium species are identified on under wet conditions. Production of sporangia or plant species intercropped with beans, it is likely that hyphal swellings can be stimulated by Mg, K, and Biology and control of bean Pythium root rot 407 Ca ions (Postma et al., 2009). Exudates of roots and – the character of the oogonial wall (smooth or germinating seeds have a stimulatory effect on the ornamental); germination of sporangia and on mycelial growth – the character of the oospore (plerotic or aplerotic); (Stanghellini et al., 1971). Sexual reproduction in – the characteristics of the antheridia (Matsumoto Pythium spp. takes place through the oogonia and et al., 1999).
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