Peronospora
Peronospora:-
1. Classification:-
Kingdom:- Myceteae
Division:- Mastigomycota
Sub division:- Diplomastigomycotina
Class:- Oomycetes
Order:- Peronosporales
Family:- Peronosporaceae
Genus:- Peronospora
2. Habit and Habitat:-
> Most of the Peronospora species are highly specific to their hosts and can generally be found anywhere the host plant grows, or is being cultivated.
> A large portion of their life cycle is spent inside their host plant.
> Many species of Peronospora are seedborne pathogens, so the worldwide spread of Peronospora crop-plant pathogens is likely to be a result of unknowingly trading infected seeds to new areas.
> There are also many Peronospora species that are spread by wind currents, which allows them to disperse over large distances.
> Peronospora species prefer humid air and cool temperatures.
> It causes downy mildew disease in crops.
i. P. brassicae:- Downy mildew of Mustard.
ii. P. destructor:- Downy mildew of onion.
3. Thallus structure:-
> Well developed and profusely branched mycelium.
Hyphae are aseptate, coenocytic and branched.
4. Reproduction:-
> The first stage in the Peronospora life history is the sporangia.
> The sporangia are small spore-like structures about 65 um long that germinate a germ-tube when they are near a leaf stoma.
> A germ tube will come from the sporangium and penetrate the leaf cell where it will form a haustorium.
> The haustorium absorbs nutrients from the leaf, while hyphae invade the intercellular space, and the leaf will eventually develop a lesion.
> These lesions often start out yellow and then turn brown as the leaf starts to undergo necrosis.
> From here, Peronospora can undergo either asexual reproduction or sexual reproduction.
a. Asexual reproduction:-
- Asexual reproduction occurs when the air outside is moist making for favourable conditions.
- During asexual reproduction, hyphae on the host plant will form sporangiophores, which will produce conidia.
- The conidia will be dispersed by the wind is able to infect other plants.
- The asexual cycle only takes five to 7 days to complete.
b. Sexual reproduction:-
- Sexual reproduction occurs when the conditions are unfavourable and it needs to withstand harsh environmental conditions.
- During sexual reproduction, the hyphae will undergo meiosis forming antheridia and oogonia, the only haploid structures in the Peronospora life history.
- The antheridia will fuse to the oogonia, initiating plasmogamy and then karyogamy, and will result in the production of many oospores.
- The oospores can then be dispersed by the wind to infect more plants.
- Peronospora is characterized by their ability to produce melanized sporangia, but do not produce zoospores.
