Pathogenic variability in Puccinia Sorghi on maize in South Africa

[摘要] English: Common rust, caused by Puccinia sorghi Schw., is a major disease in maize (Zea mays L.) producing regions throughout the world and can result in high yield losses on susceptible genotypes when the environment is favourable for epidemic development. To understand the principal aspects related to P. sorghi, a literature survey concerning the biology of the pathogen, host range, host-pathogen interaction, and economic importance of the disease, was conducted. Furthermore, different methods of disease management are reviewed, with emphasis on genetic control. To determine pathogenic variability in Puccinia sorghi in South Africa, rustinfected maize leaves were collected during the 1999/2000 season. Isolates collected in the field were increased on susceptible plants and inoculated onto maize differential lines carrying different Rp genes for resistance to P. sorghi. Seven pathotypes, namely, A, B, C, D, E, F and G, were differentiated in the greenhouse. Pathotype B was the most virulent and occurred in Gauteng, KwaZulu-Natal, and Mpumalanga. Pathotype A, virulent only on Rp34 and Rp3B/ was widely distributed, occurring in six provinces. Pathotype C was detected in Mpumalanga and the Northern Province, D in the Northern Province whereas pathotype E was collected from North West, and F and G from KwaZulu-Natal. No virulence was detected for genes Rp1C, Rp1G,Rp1L, Rp3D and Rp3F. All isolates were virulent on Rp3B. The occurrence of virulence for most Rp genes suggests that monogenic resistance is of little value for future protection of maize cultivars against common rust in South Africa. One hundred maize inbred lines from Mozambique, South Africa and Zimbabwe, as well as 58 South African hybrids, were evaluated for resistance to common rust under field conditions over different localities in South Africa during the 1999/2000 season. Inbred lines were planted at Ermelo and Greytown and inoculated by a mixture of spores from previously identified P. sorghi pathotypes. Cultivars were tested under conditions of natural common rust infection at Greytown and Petit. Disease severity was assessed prior to anthesis, at anthesis, and after anthesis at all localities except the Greytown cultivar trial, which was assessed only once after anthesis. Disease was scored on a 0-9 scale converted to percentage leaf diseased area (0-96% scale). The percentage values were log-transformed using the natural logarithm function. Analyses of variance were carried out for each severity parameter (individual rating, mean, sum and AUDPC [area under the disease progress curve]) within, and (mean, sum and AUDPC) across localities. Significant genetic variation existed between genotypes for resistance to P. sorghi at each and across localities. Some inbred lines showed significant levels of partial resistance to the pathogen and may be used in resistance breeding programmes. Most hybrids showed susceptible reaction types to common rust and are thus potentially vulnerable to yield losses under conditions of epidemic occurrence of P. sorghi. In general, similar to other cereal rust pathosystems, it is apparent that monogenic rust resistance in maize will not be durable. Emphasis in breeding should therefore be placed on horizontal resistance, with particular attention to sources of resistance, screening procedures and heritability. This study will hopefully serve as an important source of information for future research of common maize rust in southern Africa.

[发布机构] University of the Free State