SPOTS at first circular to subcircular, olive, up to 1 cm diam, later connecting with one another, and darkening to blackish brown, present on leaves, green flower parts, and fruits. Spots on fruits gradually become scabby, dark, hard, and finally crack.
CONIDIOPHORES (cp) arising from localized stromatic thickenings of subcuticular to subepidermal mycelium (conidiomata), cylindrical, usually slightly tapering toward their tops, erect septate, wavy, pale brown to brown, terminating in a conidiogenous cell, 50-60 x 4-6 µm. In young leaf lesions, the mycelium develops radially in branched ribbons of hyphae.
CONIDIA (c) terminal, obclavate, 1-septate, olive to brownish, 30 x 7-9 µm.
PLANT HOST AND DISTRIBUTION. The plant hosts of S. pomi are different species of the genus Malus.
Spilocea pomi occurs around the world.
NOTES. The teleomorph of S. pomi is Venturia inaequalis (Cooke) Winter.
Venturia inaequalis and its conidial stage S. pomi cause apple scab, the most important disease of apples (Agrios 1988).
Venturia inaequalis is a typical ascomycete fungus. It forms pseudothecia with ascospores in the spring. The ascospores are divided into two cells of unequal size, and hence the name of the species. Ascospores in pseudothecia are formed in overwintered leaves lying on the ground. Sometimes, pseudothecia also develop on overwintered fruits.
Pseudothecia initially develop in the fall and early winter. In the spring, when dead leaves with pseudothecia become thoroughly soaked, the asci elongate, push through the ostiole, and forcibly discharge the ascospores into the air and then to susceptible green plant host tissues. Spore trapping showed that the first spores were released within 30 min of wetting, and a maximum was reached after 1-2 h (Smith et al. 1988). The time needed for germination of ascospores, provided the surface remains wet, is 25 h at 6oC or 9 h at 16-24oC (according to Mills). Conidia have the same germination requirements as ascospores, but need longer periods of wetness.
Upon germination on a host leaf or fruit, the ascospores produce an appressorium from which a slender hyphal tube pierces the cuticule. A mass of closely packed hyphal cells (conidiomata) forms between the cuticule and the epidermis. Here, the fungus obtains its nutrients. Several days after infection, the fungus produces conidiophores on the mass of hyphal cells. Then, the conidiophores produce conidia. The conidia are carried by wind and splashing rain. They germinate, form appressoria, and infect the plant host in a manner similar to the ascospores. These are secondary infections, because they result from inoculum produced by primary infections during the current season. The secondary infections soon produce conidia and the cycle of conidia-infection-conidia-infection may repeat many times, when weather conditions, mainly temperature and humidity, are favourable.
Agrios G. N. 1988. Plant pathology, 3rd edition, Academic Press, INC. San Diego, New York, Berkeley, Boston, London, Sydney, Tokyo, Toronto.
Smith I. M., Dunez J., Lelliott R. A., Phillips D. H., Archer S. A. 1988. European handbook of plant diseases. Blackwell Scientific Publications.