Host-pathogen studies of common rust of maize in South Africa

English: Maize (Zea mays L.) is one of South Africa's most important grain crops because of its significance as a staple diet among a large proportion of the population. Maize is also an important component of livestock feed and is used in the manufacturing of several industrial products. Observations during the past few seasons showed an increased occurrence of common rust, caused by the fungus Puccinia sorghi Schwein. This gave rise to questions from maize producers and researchers regarding the epidemiology, economic impact and control measures of the disease. The spore stages of this macro-cyclic fungus, as they occur on both the primary (maize) and alternate (Oxalis corniculata, creeping sorrel) hosts in South Africa, were described. Detailed schematic illustrations as well as microscopic records of the five spore stages were prepared. A survey was done to determine the occurrence of the sexual stage on creeping sorrel in the main maize producing areas in South Africa. Most aecial infections were observed in KwaZulu-Natal, implying this region to be a source of new recombinant pathotypes. On request of the PAN NAR Seed Company (Pty) Ltd., all viable maize accessions in their germ plasm collection were tested for reaction to common rust infection. Out of 2594 lines only 185 were resistant in the glasshouse as well as in the field, while 426 were resistant in the glasshouse but not in the field. This information should help PANNAR to make certain decisions about rust resistance or susceptibility in their breeding material. Spray trials with different fungicides and maize cultivars showed that positive yield increases can be obtained. Eria®+Eria® and Amistar®+Eria® showed the best results. The effects of fungicides were, however, dependent on the level of adult plant resistance, locality, as well as the occurrence of other diseases. Even though maize breeders realize that single gene resistance does not ensure long-term protection against rust, breeding approaches allow combination of such genes with non-specific adult plant resistance. The inheritance of Rp1-A, Rp1-C, Rp1- G, Rp1-L, Rp3-D, Rp3-E and Rp3-F in F2 progenies were studied to provide breeders with information on the ease of following single genes in segregating populations. Although all the genes indicated monogenic inheritance, F2 segregation ratios from crosses inbred 1 X Rp 1-L and inbred 1 X Rp 3-F differed significantly from the expected 3 resistant: 1 susceptible ratio. Chi-square tests for homogeneity showed that white and yellow seed colour did not, as expected, influence the inheritance of Rp1-A, Rp1-C, Rp1-G, Rp1-L, Rp3-0 and Rp3-E. Based on the distinct expression of infection types, breeders should be able to efficiently select for these genes in cultivar development.