Abstract:
Conidia of Cercospora arachidicola are straw-coloured or olivaceous, subfusiform,
multicellular, 28.3-68.1(jm in lengths. They contain 3-7 cells. They
germinated in distilled water and at 85-100% R.H. The optimum pH for
germination was 5.0-6.0; any cell of the conidium germinating produced only one
germ tube. The germ tube commonly comes from the end cells with occasional
germ tube emerging from one or two median cells. Pollen of Zea mays, glucose
(1.0-8.0%) and peptone (0.1-2.0%) failed to stimulate more median cells to
germinate. Maize pollen, however, stimulated germination of the conidia at pH 3
and 10.
During germination tests using conidia in water drops on sterile glass slides, the
conidia germinated better and produced slightly longer germ tubes in the
absence of pollen grains of Zea mays than in the presence of both non-sterile
and autoclaved pollen grains. Particularly with the sterile pollen grains,
percentage germination decreased with increasing pollen density. Maize pollen
when added to solutions of glucose (1.0-8.0%) and peptone (0.1-2.0%) did not
improve germination of the conidia. Conidia in pollen-free aqueous suspension
drops on leaflets of groundnuts also germinated well than in suspension drops
containing the pollen. Percentage germination in the presence of the pollen on
leaflets with reduced fungal flora (a third of the original) and bacterial flora (a
sixth of the original) was still lower than in the absence of pollen Maize pollen at
moderate densities did not alter the infection rate of groundnut by C. arachidicola
but high density of pollen on non-sterile groundnut leaves encouraged excessive
phylloplane microfloral population which suppressed germ tube growth.
Conidia stored at 0, 20, 40, 60, and 80% R.H. survived better during the initial 30
days at 20, 40 and 60% R.H. than at 0 and 80% R.H. The pattern of survival
changed thereafter and conidia stored at 0% R.H. showed the highest survival on
the 50th day; 35.1 percentage viability at 0% R.H. decreased with increasing
relative humidity to 29.2% at 80% R.H. Maize pollen could not rejuvenate the
aged conidia.
It was concluded that since maize pollen reduced percentage germination o f C.
arachidicola conidia, and high pollen density suppressed germ tube growth on
non-sterile groundnut leaves, a closer spacing of maize plants in mixed farms
could be recommended to encourage greater pollen deposit on the groundnut
plants. This would be an environmentally friendly control measure provided the
conidia of the other Cercospora leaf spot fungus, Cercospora personata, are
similarly affected by maize pollen.
