The development of the "conidia" as true conidial spores or as zoosporangia may occur in one and the same species (Cystopus candidus, Phytophthora infestans) as in Pythium described above; in other cases the direct conidial germination is characteristic of genera - e.g.
Tophthora infestans passing K, Germination of the zoospores through the stomata D, on formed in the sporangia.
Some parasites attack many hosts and almost any tissue or organ (Botrytis cinerea), others are restricted to one family (Cystopus candidus) or genus (Phytophthora infestans) or even species (Pucciniastrum Padi), and it is customary to speak of rootparasites, leaf-parasites, &c., in expression of the fact that a given parasite occurs only on such organs - e.g.
The significance of these facts from a cultural point of view is twofold: for, while a late variety is desirable for culture in Great Britain, as ensuring more or less immunity from spring frost, it is, on the other hand, undesirable, because late varieties are more liable to be attacked by the potato disease (Phytophihora infestans) which as a rule appears about the time when the earliest varieties are ready for lifting, but before the late varieties are matured.
The best-known disease of potatoes is caused by the growth of a fungus named Phytophihora infestans, within the tissues of the host plant, and this fungus has the peculiar property of piercing and breaking up the cellular tissues and setting up putrescence in the course of its growth.