Phylum Oomycota.

1. They are facultative parasites growing within plant tissues i.e. within the host; the mycelium is intercellular with the haustoria.
2. The body is mycelium i.e. thread like structures with no septor (non septate).
3. Have cell wall made of cellulose with deposits of fat substance, peptic materials and impregnated with chitin.
4. They reproduce sexually and asexually and under unfavorable conditions conidia forms which develops into mycelium.

1. Have extensively branching haustoria which penetrates into mesophyll cells or other tissues.
2. They produce enzymes i.e. pectinases and cellulases which destroy the host cells.
3. They are non septate with thin walls thus easy absorption of food materials into their bodies. (The walls are permeable).
4. The sporangium hanging on the long sporangiophore makes it easy for the spores to be spread by water current/wind for other plants. By being on the lower part of the leaf they are easily protected from dehydration.
5. They have high reproductive rate i.e. produce many spores asexually and rapidly also when conditions are not favorable they can still undergo asexual reproduction by formation of conidia.
6. The spores can remain dormant for a long period (more than one year) thus increases it chances of survivals.
7. The zoospores can penetrate through different parts of the plant like stomata in the leaves, lenticels in stem, epidermis or wounds. Phytophthora infestans growing in a diseased potato leaf with sporangiophores emerging from the underside of the leaf.

The vegetative body of Phytophthora consists of mycelium which is abundantly branched where septa may develop in the older parts and at the base of sex organs.

The mycelium branches arise at right angles. The mycelium grows both intra- and intercellularly. During intercellular growth, it develops finger like haustoria inside the neighbouring host cells, which are normally the mesophyll cells of the plant leaves.

The haustoria absorb nutrients from the leaf cells. The hyphae possess sporangium which produces spores

The Phytophthora have the following adaptations to their mode of life:

1. Formation of cysts during unfavourable conditions ensures its survival.
2. Ability to reproduce both sexually and asexually, with a high reproductive output, increases its chances of survival.
3. It has haustoria with a large surface area for penetration into the plant cells and absorption of nutrients from them.
4. The haustoria secrete enzymes which help the parasite to penetrate its host cell.
5. It has spores which withstand the adverse conditions, such that, under favourable conditions, they germinate to Phytophthora infestans.