LESIONS. The most characteristic symptoms caused by M. laxa are the blight of blossoms and green tips of twigs due to the penetration of the pathogen into opened blossoms through the stigma of pistils or anthers. This usually results in wilting of the whole part of a one-year-old twig. Its leaves first hang down, later become brown and rigid, but usually do not fall and remain on trees until the spring of the next year. At the base of the affected inflorescence and between the drying up and the living parts of the affected twig, sporodochia and occasionally concentrated droplets of gum appear in humid weather.
The infected fruits gradually cover with enlarging putrefactive spots, from which warty sporodochia with conidia of the "summer" form appear. Additionally, in late autumn and winter, the fungus produces sporodochia of the "winter" form on infected twigs. With time, severely affected fruits become mummified. The mycelium growing in such mummies gradually aggregates into sclerotia. Such fruits remain on the tree during winter.
SPORODOCHIA OF THE "SUMMER" FORM (s) pale grey, 1.5-2 mm diam, consist of loose conglomerations of straight or branched conidiophores with conidia arranged in long, branched chains.
Conidia (c) hyaline, lemon-shaped, 19 x 13.5 µm.
SPORODOCHIA OF THE "WINTER" FORM scutate-warty, with chains of conidia, 11.5 x 8 µm.
PLANT HOST AND DISTRIBUTION. The plant hosts of M. laxa are edible and ornamental species of the genus Prunus and other genera of the family Rosaceae (Agrios 1988; Brooks 1953; Kochman 1973; Farr et al. 1989).
According to Farr et al. (1989), M. laxa occurs in temperate northern hemisphere.
NOTES. The teleomorph of M. laxa is Monilinia laxa (Aderth. & Ruhl.) Honey ex Dennis (Holliday 1989). However, it infrequently appears only after a two-time overwintering of rotted fruits (Kochman 1973; Smith et al. 1988). Monilinia laxa has not been recorded in Poland to date (Kochman 1973).
The primarily sources of inoculum of the pathogen in the spring are conidia of the "winter" form. A low part of conidia of the "summer" form may also retain their vitality and take place in infection of trees.
Conidia may be dispersed by air or water, or carried on insects attracted to infected or injured fruit (Smith et al. 1988).
The germ tubes of conidia of M. laxa mainly penetrate their plant hosts through wounds caused by, e. g., aphids. During a wet summer, the fruit may be also infected through stomata and directly through the cuticule.
The conditions favourable to the pathogen are a cold, wet spell in the spring following warm weather that has stimulated the blossoms to open (Brooks 1953). At 20-25oC, infected fruits completely rot within 1-2 days (Kochman 1973).
Agrios G. N. 1988. Plant pathology, 3rd edition, Academic Press, INC. San Diego, New York, Berkeley, Boston, London, Sydney, Tokyo, Toronto.
Brooks F. T. 1953. Plant diseases. Geoffrey Cumberlege. Oxford University Press. London, New York, Toronto.
Farr D. F., Bills G. F., Chamuris G. P., Rossman A. Y. 1989. Fungi on plants and plant products in the United States. APS Press. The American Phytopathological Society. St. Paul, Minnesota. USA.
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.