Which fungus is causing the disease grey leaf spot on maize in southern Africa?

Barbara Meisel1,   Jeanne Korsman1,   Frederik J Kloppers2,   and Dave K Berger1
1Department of Plant Science,   FABI,   University of Pretoria,   Pretoria,   South Africa.

2 PANNAR Research Services (Pty) Ltd, PO Box 19, Greytown, South Africa

When you grow maize plants in warm and humid climates you will often find long rectangular grey to tan lesions on the leaves (Figure 1A). This disease is known as grey leaf spot. The pathogen not only uses nutrients from the plant it also reduces the plants yield.  This results in huge financial losses for maize farmers. Depending on the geographic region, the disease can be caused by two different fungi: Cercospora zeae-maydis or Cercospora zeina. In our study, we wanted to find out which of the two pathogens was causing the disease in South Africa, Zimbabwe and Zambia. We collected maize leaves with grey leaf spot symptoms from fields in these countries. When you have a close look at the lesions you can see that spore-bearing structures called conidiophores from the fungus are sticking out of the surface of the leaves (Figure 1B & 1C). We recovered spores (Figure 1D) from the fungus growing in the maize leaves and transferred them one by one onto petri-dishes with a special medium (Figure 1E). The spores absorbed nutrients from the medium and as they grew they formed separate pure fungal cultures. Because the two fungi Cercospora zeae-maydis and Cercospora zeina are very similar it was not enough to study them just under the microscope and find morphological differences. We also had to study differences in their genetic information. We therefore isolated DNA from 71 cultures and performed different molecular tests to see if the nucleotide sequences of different genes from our fungal cultures matched the sequences known from Cercospora zeae-maydis or Cercospora zeina. It turned out that all grey leaf spot lesions we investigated had been caused by Cercospora zeina.

Can you get quicker results?
Because it took several months for the fungus to grow on the medium before we could isolate DNA from it, we developed a new protocol to actually skip this step and to isolate DNA directly from a grey leaf spot lesion (Figure 1A). We now had a mixture of DNA from the fungus and the plant, but if we applied a stringent molecular test we could retrieve results from the fungal DNA only. Our new idea worked well and can help plant pathologists to save time when testing for this fungus.
Related article:
Meisel, B., Korsman, J., Kloppers, F.J. and Berger, D.K. (2009) Cercospora zeina is the causal agent of grey leaf spot disease of maize in southern Africa. European Journal of Plant Pathology, 124 (4) 577-583.
 


Figure 1 Grey leaf spot on maize caused by Cercospora zeina. A) close-up of grey leaf spot lesions on maize leaf, B) conidiophores on leaf surface, C) spore-bearing structures of the fungus called conidiophores, D) spores of the fungus called conidia, E) Cercospora zeina in culture, and F) artificially inoculated leaf.