SA Graan April 2016

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April 2016
known to occur on barley in South Africa, until barley samples
with small, brown lesions, as well as circular to oval-shaped, inter-
mediate, brown lesions with yellow halos (Photo 6), were submitted
to the Disease Clinic.
After incubating the isolates for two weeks, a very slow-growing
fungus was isolated. This isolate was molecularly identified as
Ramularia collo-cygni, using a DNA sequence technique. Species-
specific primers, for the positive identification of this species, was
obtained and positive Ramularia collo-cygni DNA have been re-
quested from the groups of Proff Andreas von Tiedemann and
Petr Karlovsky at the University of Göttingen, Germany.
The probable South African Ramularia collo-cygni culture, together
with similar isolates from other samples, will be compared to the
published German Ramularia collo-cygni DNA. The Disease Clinic
is currently in the process of conducting pathogenicity tests as
verification to the above-mentioned findings. Should the South
African isolates induce the same symptoms in the artificially inocu-
lated barley, this would be the first confirmed report of Ramularia leaf
spot on barley in South Africa.
Foliar diseases may also be caused by bacterial infections. Bacte-
rial Chaff, caused by Xanthomonas translucens pv. undulosa, is rec-
ognised by dark, linear, water-soaked streaks on the neck and head
of the plant. As the disease develops, the lesions merge, darkening
the glumes, lemmas and peduncles. Pseudomonas spp. may cause
bacterial leaf blight on wheat.
Initial water-soaked spots expand, become necrotic and turn from
gray-green to tan-white. The spots may coalesce into irregular
streaks or blotches within two to three days. Various Pseudomonas
spp. occur naturally as epiphytes in the phyllosphere of wheat
(sometimes non-pathogenic). When isolating for bacteria, leaf le-
sions are excised, soaked in PBS (a buffer), macerated and streaked
out onto a general bacterial growth medium (Photo 7).
Non-pathogenic foliar symptoms, caused by environmental con-
ditions such as hail, waterlogging, frost and drought, can be mis-
takenly identified as a cereal disease. Nutrient and trace element
deficiencies (i.e. chloride and zinc) or toxicity must also be taken
into account. Chemical injury can, in some cases, also cause ‘dis-
ease-like’ symptoms.
An effective plant disease management strategy is required to re-
duce the economic and aesthetic damage caused by plant diseases.
Proactive plant disease management practices rely on anticipat-
ing the occurrence of disease, and attacking the vulnerable points
in the disease cycle.
Once a disease has established in a field, a reactive approach to
control the outbreak has to be followed. Therefore, the correct
identification of the causal agent of plant disorders is essential when
deciding on a management strategy and control measures.
Contact the authors at or
or at 021 808 4798/4223.
6: Barley leaves with small, brown lesions as well as circular to oval-shaped, intermediate, brown lesions with yellow halos, preliminary identified as
Ramularia leaf spot caused by Ramularia collo-cygni.
7: Bacterial growth on the general growth media for bacteria, Nutrient Agar (NA).
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