LEAF SPOTS mainly on leaves and petioles, occasionally also on fruits. Leaf spots first single, circular to angular, dark brown, surrounded with a brown border, later confluent and covering a large part of the leaf area. The spots on petioles are dark brown and extend along them. On the under leaf side, directly below the spots present on the upper leaf side, small, 0.1-0.2 mm, slightly glistening and concentrically distributed elevations occur, under which acervuli of the conidial stage form. With time, the conidia produced push the cuticule that finally cracks and delivers them. Spots on fruits are darker-coloured (inky) and more irregular than those on leaves. They are slightly immersed and may fuse with each other.
ACERVULI (a) subcuticular, yellow brown to dark brown, consist of a layer formed by a pseudoparenchymatous fungal tissue spread on the surface of the leaf peel and a layer of filiform conidiophores, from which conidia arise.
CONIDIOPHORES (cp) erect, hyaline, branched irregularly, septate, smooth, arranged in a palisade.
CONIDIA (c) falcate, 2-celled, hyaline, 14-24 x 2-3 µm.
PLANT HOST AND DISTRIBUTION. Marssonina juglandis affects different plant species of the genus Juglans (Kochman 1973).
According to Kochman (1973), M. juglandis occurs in many countries of Asia, Europe, and America.
NOTES. The teleomorph of M. juglandis is Gnomonia leptostyla (Fr.) Ces. & de Not. (Holliday 1989).
Marssonina juglandis causes the anthracnose of walnut.
The fungus overwinters in the form of perithecia in fallen leaves. In the spring, the primarily sources of infection are ascospores. Although the ascospores begin to form in December of the preceding year, they mature at the beginning of April of the next year and disseminate from the first days of May to the second half of June. When they fell on wetted leaves or fruits, they germinate and the germ tubes produced penetrate the leaf of fruit tissue either through stomata or the cuticule.
Marssonina juglandis is favoured by wet and humid weather (Agrios 1988). The best conditions for infection are a temperature of ca. 21oC and a relative humidity of 96-100% (Smith et al. 1988). However, conidia can survive the absence of free water for at least two weeks. Both ascospores and conidia are disseminated only during rainy weather.
Agrios G. N. 1988. Plant pathology, 3rd edition, Academic Press, INC. San Diego, New York, Berkeley, Boston, London, Sydney, Tokyo, Toronto.
Holliday P. 1989. A dictionary of plant pathology. Cambridge University Press. Cambridge, New York, New Rochelle, Melbourne, Sydney.
Kochman J. 1973. Fitopatologia. PWRiL. Warszawa.
Smith I. M., Dunez J., Lelliott R. A., Phillips D. H., Archer S. A. 1988. European handbook of plant diseases. Blackwell Scientific Publications.