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

Abstract

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.

Afrikaans: Mielies (Zea mays L.) is een van Suid Afrika se belangrikste graangewasse omdat baie mense daarvan afhanklik is vir stapelvoedsel. Mielies is voorts 'n belangrike komponent in dierevoeding en word ook as grondstof vir die vervaardiging van industriële produkte gebruik. Waarnemings die afgelope paar seisoene het duidelik getoon dat gewone roes, veroorsaak deur die swam Puccinia sorghi Schwein., meer algemeen voorkom. Hierdie voorkoms het vrae vanaf mielieprodusente en -navorsers laat ontstaan oor die epidemiologie van die siekte, die ekonomiese impak en beheermaatreëls. In hierdie studie is die spoorstadia van ie a rosi liese s am P. sorghi op beide die primêre (mielies) en alternatiewe gashere (Oxalis corniculata, ranksuring) beskryf. Gedetailleerde skematiese illustrasies sowel as mikroskopiese rekords van die vyfspoorstadia is voorberei. 'n Opname wat gedoen is om die voorkoms van die geslagtelike fase en ranksuring in die belangrikste mielieproduserende gebiede in Suid Afrika te bepaal, het gewys dat KwaZulu-Natal waarskynlik die grootste bron is van nuwe, rekombinante patotipes. Op versoek van die PANNAR Saadmaatskappy Edms. (Bpk) is alle lewenskragtige mielie-aanwinste in hulle kiemplasmaversameling vir reaksie tot roesinfeksie getoets. Uit 'n totaal van 2594 lyne was 185 bestand in beide die glashuis- en veldevaluasies, terwyl 'n verdere 426 weerstand in die glashuis, maar nie in die veld, getoon het nie. Hierdie inligting stel PANNAR instaat om sekere aannames oor roesweerstand of -vatbaarheid in hulle teelmateriaal te maak. In spuitproewe met verskillende mieliekultivars is aangetoon dat positiewe opbrengsverhogings verkry kan word met fungisiedtoedienings. Eria® + Eria® en Amistar® + Eria® behandelings het die beste resultate getoon. Opbrengste van kultivars is met tot 2.97 tlha (42.7%) verhoog. Die effek wat fungisiedes op kultivars het is egter afhanklik van die vlak van volwasseplantweerstand, omgewing en ook die voorkoms van siektes bo en behalwe gewone roes. Alhoewel mielietelers besef dat enkelgeenweerstand nie noodwendig langdurige beskerming teen roes sal verleen nie, maak hedendaagse tegnologie dit moontlik om sulke gene met nie-spesifieke, volwasseplantweerstand te kombineer. Ten einde inligting aan telers te voorsien oor die gemak waarmee enkelgene in segregerende populasies gevolg kan word, is die oorwerwing van Rp 1-A, Rp 1-C, Rp 1-G, Rp 1-L, Rp3- D, Rp3-E en Rp3-F in F2 nageslagte bestudeer. AI die gene is as dominante enkelgene oorgeëri, alhoewel F2 segregasieverhoudings van die kruisings lyn 1 X Rp 1-L en lyn 1 X Rp 3-F betekenisvol vanaf die verwagte 3 bestand: 1 vatbaar afgewyk het. Chi-kwadraattoetse vir homogenisiteit het gewys dat, soos verwag, geelof wit saadkleur nie die oorwerwing van Rp1-A, Rp1-C, Rp1-G, Rp1-L, Rp3-D en Rp3-E beïnvloed het nie. Op grond van die duidelike uitdrukking van infeksietipes op roesbestande plante behoort telers hierdie gene maklik in kultivarontwikkeling te kan volg.