Article by Lindy J. Rose,
Department of Plant Pathology, Stellenbosch University,
and Prof. B.C. Flett
ARC-GCI (Potchefstroom)
D
iplodia ear rot (DER) is
an economically important disease of maize
caused by the fungi
Stenocarpella maydis and S. macrospora. DER caused by S. maydis can
occur throughout the entire maize
production area of South Africa,
whereas S. macrospora appears to
be predominant in areas of the
KwaZulu-Natal province. Maize kernels infected with these fungi result
in reduced grain yield and quality
due to reduced kernel size and lower test weight. Grain infected by the
fungus can also be contaminated
with secondary metabolites that
have detrimental health effects on
livestock (known as diplodiosis).
The fungal life cycle and disease
symptom
Stenocarpella maydis survives on
debris left in maize fields following
the previous season’s harvest. Fungal spores are produced in spring,
and are then distributed by wind or
rain to infect the ear leaf, base of
the ear and maize silks of the new
crop. Inoculum pressure is particularly high in fields where maize is
grown in monoculture with minimum and no-tillage. Fungal spores
that germinate on ear leaves infect
the ear from the base and then
grow into the maize cob and the
kernels during the grain-fill stage of
plant development.
Epidemics of DER are most severe
when drought experienced early in
the season is followed by lateseason rainfall. Symptoms become
visible as infected ear bracts on
green plants that turn yellow and
dry out. Infection of the ears itself
typically starts as a dense white to
greyish mold growing from the
base of the ear that eventually covers the entire ear (Fig. 1). Additionally, fruiting bodies (pycnidia),
which appear as raised black
bumps, may be scattered on the
husks, ears and sides of rotten kernels (Fig. 2). Dormant infection of S.
maydis shows no apparent symptoms, but when the ears are broken
in half and the kernels removed,
pycnidia can be found on the kernels with discoloured embryos (Fig.
3). This is also known as “hidden
Diplodia (skelm Diplodia)”. Stenocarpella macrospora ear rot symp-
toms are almost identical to those
produced by S. maydis, but occur
under different climatic and epidemiological conditions.
Fungal metabolites and mycotoxicoses
Stenocarpella maydis produces several mycotoxins, which include diplonine, diplodiatoxin, dipmatol,
chaetoglobosins (K, L, M and O),
stenocarpin and carpellin. The fungus is associated with diplodiosis,
the last major veterinary mycotoxicosis for which the causative mycotoxin is still unknown. Diplodiosis is
associated with nervous system defects and neonatal losses in livestock, as well as acute toxicity in
ducklings and chickens. Clinical
signs in cattle include standing with
legs wide apart, ataxia and high
stepping gait, falling and recum-
Figure 1. Diplodia ear rot of maize characterised by dense white/grey
mycelia commencing from the ear base to the tip of the maize ear
(Photo: I. Beukes).