Screening of Kenyan Soybean Cultivars for Resistance to Different Races of Phakopsora Pachyrhizi (Soybean Rust) and Determination of Molecular Basis for Rust Resistance

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. High yield losses associated with P.pachyrhizi, have been reported worldwide. The first report of the pathogen in Kenya was made in 1996 and it has continued spreading agrressively and affecting soybean bean industry in the country. Studies to determine resistance of soybean to rust disease have only been done on commercial varieties but no such studies have been done on the local varieties of soybean. Similarly no significant data has been collected on soybean rust disease severity and incidence in the country. In this study, farmers’ participatory research was used to collect data on soybean varieties preferred by farmers and the major constraint facing soybean farming in Western Kenya region. Soybean rust disease incidence and severity levels were also established in Khwisero, Butere, Mumias and Teso sub counties of Western Kenya. The presence of the rust fungi was confirmed through microscopy and polymerase chain reaction (PCR) analysis. Seven local varieties of soybean (Nyala, Bossier, SB19, Hill, SB8, Gazelle and TGx1987-32F) were tested in the green house for resistance to soybean rust. To determine presence of rust resistance genes, 12 Simple sequence repeat (SSR) markers previously mapped on linkage groups of soybean were chosen and tested on the resistant varieties. The study revealed that the farmers in Western Kenya region grow mainly the local varieties of soybean and the area under soybean cultivation is <30% of the total land available for crop production. It was further noted that soybean rust disease is present in all the subcounties with the percent disease index (PDI) ranging from 40.37% to 74.8%. The disease severity level in all the four sub-counties did not vary significantly (P<0.05). The disease incidences per farm ranged from 22%-87% with Teso Sub-county recording the highest average disease incidence (50.55%). Microscopy and PCR analysis identified the pathogen causing soybean rust disease as P. pachyrhizi and not P. meibomiae. Screening for rust resistance in the green house showed that the varieties TGx1987- 32F and SB8 were resistant producing Red brown lesion with low level of 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 were susceptible to rust producing Tan lesion with profuse sporulation and high disease severity level. Simple Sequence repeats (SSR) markers analysis of the resistant varieties showed that they both contained Rpp1, Rpp2, Rpp3 and Rpp4 genes while the susceptible varieties Rpp1, Rpp2 and Rpp4 genes. The results of this study clearly indicate that adequate preventive measures have not been put in place to deal with soybean rust disease and other constraints of soybean production. It has also been demonstrated that the local varieties contain the rust resistance genes, however the resistance can be overcome by certain rust pathotypes. The varieties that have shown resistance to diverse rust isolates are possible sources of rust resistance genes that could be used in breeding programs.