PERONOSPORALES
Order Peronosporales is more advanced than Saprolegniales . Some of the families of order Peronosporales are -
1. Pythiaceae - Phytophthora , Pythium
2. Albuginaceae - Albugo
Phytophthora
Class - Oomycetes
Order - Peronosporales
Family - Pythiaceae
Genus - Phytophthora
Phytophthora means plant that causes destruction . Many of the species of Phytophthora are pathogenic . P. infestans causes late blight of potato . Irish potato famine occurred in north europe during 1845 was due to late blight of potato . Symptoms of Phytophthora appear in the form of dark coloured irregular water soaked areas on tips and margins of old leaves . Tuber shows wet rot ( soft tuber ) in humid conditions and dry rot in dry conditions . Low temp. and humid conditions favour the rapid spread of Phytophthora . Mycelium is intercellular in most of the species . They have club shaped haustoria and branched conidiophores.
Pythium
They have similar classification like Phytophthora but mycelium is intracellular in most of the species . P. debaryanum causes dampimg off seedlings in tobaco , tomato , mustard and chillies . In pre-emergence phase of disease the young seedlings are killed before reaching the soil surface . In post-emergence phase stem becomes constricted at the base and finally the plant collapses .
Albugo
Class - Oomycetes
Order - Peronosporales
Family -Albuginaceae
Genus - Albugo
Albugo is also called Cystopus . They usually infect families like Cruciferae , Convolvulaceae , Compositae , Amaranthaceae etc . The most common species is Albugo candida which causes white rust of crucifers . Floral parts become fleshy and hypertrophied ( abnormal growth of infected part ) and the growth of entire plant is stunted . Bordeaux mixture is an inorganic copper fungicide which is effective in controlling white rust of crucifers . Mycelium is aseptate , coenocytic , intercellular and has knob like haustoria . Multinucleate Conidia are formed in Albugo . They develop in basipetal succession and intercalary mucilaginous disc called disjunctor joins successive conidia . Antheridia and oogonia develop side by side , this condition is called paragynous .