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.