Defending Soybean: Screening Kenyan Varieties Against Rust Disease | InformativeBD

H.A. Ogot,  S.A. Okoth,  G.O. Obiero, and J.M. Mahasi,  from the  different institute of Kenya. wrote a Research article about, Defending Soybean: Screening Kenyan Varieties Against Rust Disease. Entitled, Screening of selected kenyan soybean varieties for resistance to Phakopsora pachyrhizi (Soybean rust). This research paper published by the International Journal of Biosciences | IJB. an open access scholarly research journal on Biosciences. under the affiliation of the International Network For Natural Sciences| INNSpub. an open access multidisciplinary research journal publisher.

Abstract

Soybean (Glycine max (L.) Merrill.) is a highly nutritious plant which plays an important role in the world’s  economy, however soybean rust  disease caused by the fungus Phakopsora pachyrhizi is a major challange to the soybean industry.  The disease among other constraints  has significatly  affected crop yields in most soybean growing countries.  In this study  Seven  varieties of soybean (Nyala, Bossier, SB19, Hill, SB8, Gazelle and TGx1987-32F) commoly  grown by farmers  in Kenya were tested in the green house for resistance to soybean rust.  The varieties TGx1987- 32F and SB8 showed  resistant reactions  characterized by  red brown lesion with low level of disease severity,  low lesion number,  low sporulation level and low area under disease progress curve (AUDPC) value.  The other five varieties; Nyala, Bossier, SB19, Hill and Gazelle showed susceptible  reactions to  soybean rust producing tan lesion with profuse sporulation and high disease severity level. The Soybean varieties with low lesion densities, low disease severity and low sporulation level may be possible sources of rust resistance genes that can be used in breeding programs to produce rust resistant varieties.

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Introduction

The production of soybean in Kenya is affected by numerous biotic and abiotic factors. Some of the constraints include, low yielding varieties, lack of markets, poor agronomic practices, lack of awareness for its potential, competition with other legumes, drought, water logging, and pest and disease attacks (Hartman et al., 2011). Other factors include lack of varieties which are tolerant midseason moisture stress and high yielding varieties tolerant to low phosphorus (FAO, 2005). Among the biotic factors affecting soybean production diseases are of great concern because of their final impact on yield. There are a number of diseases that infect soybean worldwide the most common disease are Anthracnose, bacterial blight, bacterial pustule, soybean rust, bean pod mottle virus, brown stem rot, charcoal rot , frog eye leaf spot, soybean cyst nematode and soybean mosaic virus among others (Ploper,1997).

Soybean rust caused Phakopsora pachyrhizi as been identified among other diseases as the major challenge to soybean production worldwide. Phakopsora pachyrhizi belongs to the fungal phylum Basidiomycota, class Urediniomycetes and order Uredinales, which produce uredinia, on “dome-like” structures that give rise to asexual urediniospores. Hair-like hyaline hyphae called paraphyses grow inside uredinia. Paraphyses and sporophores are base structures for urediniosopore production (Bromfield, 1984). P. meibomiae is less aggressive while P. pachyrhizi is more aggressive and infects over 95 species of plants from more than 42 genera, including soybean and related Glycine species (Bromfield, 1984). The most susceptible host of P. pachyrhizi is kudzu (Pueraria lobata (Wild.) Ohwi), a weed species that is commonly found in the United States of America. Other common hosts are medic (Medicago arborea L.), lupine (Lupinus hirsutus L.), sweet clover (Melilotus officinalis (L.) Lam), vetch (Vicia dasycarpa Ten), common beans (Phaseolus vulgaris L.), lima and butter beans (Phaseolus lunatus L.), pigeonpea (Cajanus cajan (L.) Millsp), garden peas (Pisum sativum L.) and cowpeas (Vigna unguiculata) (Bromfield, 1984). Soybean rust infection process begins in the low to mid-canopy and moves up the plant. The infection process starts with urediniospores germination to produce a single germ tube that grows across the leaf surface, until an appressorium is formed. Penetration of epidermal cells is direct through the cuticle by an appressorial peg (Miles et al., 2005). During the infection process intracellular invasion of the leaf occurs once hyphae are formed within the mesophyll layer. Within 5 to 7 days volcano shaped uredinia with round ostioles are produced which release urediniospores on the abaxial surface completing the asexual reproduction cycle (Goellner et al., 2010).

The rapid spread of the disease in the continent of Africa has led to major decline in soybean yield (Levy, 2005, Oloka et al., 2008). Losses due to soybean rust can be significantly high. In South Africa losses of 10- 80% have been reported and in areas under monocropping system the losses can be as high as 100%. India has experienced losses of 10-90%, Japan 40% and Taiwan has reported losses of 23-90% in (Hartman et al., 1999). It is therefore important that the major production constraints be addressed so as to improve the crop yield to be able to meet the market demands and sustain the production industries. To control the spread of the rust disease chemical fungicides and cultural practices are used howerever the use fungicides to control the disease commercial plantings significantly increases production costs it is therefore not a feasible option in small scale soybean plantings especially in developing countries (Miles et al., 2003). Furthermore the fungicides are expensive and are not very effective at preventing epidemics as Bonde et al., (2006) noted yield losses of up to 50% under severe rust epidemics with chemical control. Other legumes that also form an integral part of the cropping system such as cowpea, pigeon pea and common beans are functional alternative hosts of P. pachyrhizi which makes control a great challenge (Anon, 2007; Slaminko et al., 2008). Cultural practices like destruction of alternate hosts, timely irrigation, early planting and growing early maturing cultivars can also reduce the incidence of the disease (Akinsanmi et al., 2001). However, the rapid spread by wind-borne urediniospores and the large number of host species increases chances of soybean rust survival making cultural practices relatively ineffective (Hartman et al., 2005).

Planting of disease resistant cultivars is the most viable way to manage soybean rust disease. To identify rust resistant cultivars soybean plants must be screened for resistance to diverse pathogen populations (Twizeyimana et al., 2007). This study therefore aims at screening selected soybean varieties commonly grown in Kenya for resistance to soybean rust isolates under green house conditions.

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