Causal Organism: Pseudoperenospora cubensis
Downey mildew symptoms are variable. Sometimes, the first symptoms on leaves remind one of mosaic; pale green areas are separated by islands of darker green. Soon the pale green areas change yellow angular spots bounded by leaf veins. During moist weather, the corresponding lower surface is covered with a faint purplish fungus-fruiting layer.
Occasionally, the purplish hue lacking and the colour ranges from white to almost black. The entire leaf dries quickly. Usually leaves near the centre of the hill are affected first. Then the symptoms appear progressively on younger leaves until most leaves are killed.
Etiology: Spore-bearing tips are subacute. Sporangia are greyish to olivaceous purple, ovoid to ellipsoidal, thin-walled with a distal Perpetuation: Epidemiology: Management:
It is an obligate parasite. The mycelium is coenocytic and intercellular with small, ovate or finger-like haustoria. One to five sporangiophores arise through the stomata. The upper third of the sporangiophore is branched either dichotomously or intermediately between dichotomous and monochotomous branching.
papilla and 21 to 39 x 14 to 23 um. Zoospores are 10 13 um in diameter. Oospores are not common.
The pathogen survives on the diseased plant debris. In warm and humid climates from old to younger crops takes place all the year round. Where warm and dry summer alternate with cooler and wet winters, year-round survival is possible on summer crops. They may overwinter as thick walled oospores. Sporangia are disseminated by wind.
The fungus can infect plants at temperatures from 10 to 27°C with an optimum of 16.1 to 22.2°C. They survive several days when temperatures are over 37°C. lesion development occurs at cycles of about 25°C day time, 15°C night temperature and a photoperiod of 6 h darkness.
Spore-bearing tips are subacute. Sporangia are greyish to olivaceous purple, ovoid to ellipsoidal, thin-walled with a distal