Internal mycelium of Pseudoperonospora cubensis, the causal agent of cucurbit downy mildew

The internal mycelium of Pseudoperonospora cubensis has been observed in transmission and scanning electron microscopic preparations. The internal mycelium may be inter- or intracellular. Haustoria of short swollen bundles of hyphae have been observed. Actively growing hyphae contain numerous mitochondria, nuclei, active dictyosomes, low amounts of storage materials (lipid) and microbody-like structures with a laminate inclusion. Thick walled hyphae with a diameter which is smaller than the actively growing hyphae have been observed. These thick wallcd hyphae contain large amounts of reserve material (lipid) and it is suggested that they may function as resting propagules.