Pythium, which causes the damping off of seedlings, reducing them to a putrid mass in a few hours, and Phytophthora, the agent of the potato disease.
Phytophthora in potatoes., If, on the other hand, the irritating agent is local in its action, causing only a few cells to react, we have the various pimples, excrescences, outgrowths, &c., exhibited in such cases as Ustilago Maydis on the maize, various galls, witchesbrooms, &c.
Brown spots are characteristic of Phytophthora, Puccinia, &c., and black ones of Fusicladium, Ustilago, Rhytisma, &c. Both are common as advanced symptoms of destruction by Fungi and insects.
Peronosporaceae are a group of endophytic parasites - about ioo species - of great importance as comprising the agents of "damping off" disease (Pythium), vine-mildew (Plasmopara), potato disease (Phytophthora), onion-mildew (Peronospora).
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.
- Phytophthora infestans.
Similar gradations are observed in the direct effect of the parasite on the host, which may be local (Hemileia) when the mycelium never extends far from the point of infection, or general (Phytophthora) when it runs throughout the plant.
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.
Pythium, Phytophthora omnivores.
The rotting of tubers after lifting may be due to various causes, but the infection of the tubers by the Phytophthora already mentioned is a frequent source of this trouble, while "Winter Rot" is due to the fungus Nectria Solani.