The Fungi belonging to Kingdom Chromista: a. Oomycota:
Dr. Basudha Sharma Chromista
► Wittaker divided living beings into 5-Kingdom-Monera (Prokaryotic), Protista (Eukaryotic), Fungi, Plantae and Animalia. Molecular phylogeny, however suggested a new division: Chromista
► The chromists represent an independent evolutionary lineage that appears to have diverged from the same common ancestor as plants, animals and fungi.
► Chromista means ‘coloured’ and includes some colourless chromists like oomycota and certain algae.
► they all possess flagellated cells at some stage of their life cycles, and the flagella are typically of two types-hetrokont (whiplash and tinsel)
► the chloroplast is bounded by a double membrane, but has an extra layer of ER (also two-layered) that is often continuous with the nuclear envelope Chromista Oomycota
► commonly known as water moulds
► some are unicellular, however majority are multicellular and mycelial (branched filamentous coenocyte)
► They are classified as chromists because their free-swimming zoospores possess the heterokont-type flagella also, reserve food is stored in the form of mycolaminarin, an energy storage molecule similar to that found in diatoms and brown algae.
► oomycetes cell walls are composed of cellulose, free-living stage of the oomycetes has a diploid chromosome complement while that of the fungi is haploid.
► Asexual reproduction is by biflagellate zoospores
► Sexual reproduction is oogamus and involves the formation of oogonia and antheridia Saprolegnia
► These fungi are saprophytic organisms that are widely distributed in the aquatic environment and can derive nutrients from any organic source in water
► They become pathogenic to fish only when fish are stressed or eg. diseased. They attach to surfaces like gills and fins of fishes
► Reproduction in Saprolegnia may be sexual or asexual. Asexual by means of zoospores and sexual is by fusion of two gametes to form a thick-walled oospore
► Zoospores escape rom the zoosporangia. The zoospores undergo primary and secondary stages and the attach to dead/ injured tissue to form hypha. Mating (with help of sex hormones)
► Seen in Oomycota Achlya ambisexuals Pythium (Damping off/ Root rot disease)
► Pythium is a paracytic oomycete. The fungus lives saprophytically in the moist, humus soil and attacks the seedlings at the soil level. Thereafter it lives as a parasite.
► Phythium affecting the seeds or seedlings before or after germination (damping-off) Pythium spp. can inhibit seed germination or disrupt the development of young seedlings soon after colonizing the roots and/ or stem base
► The mycelium is coenocytic, haustoria absent. Asexual reproduction also takes place by the formation of gemmae and chlamydospores. Sexual reproduction is oogamous and the fungus is homothallic Phythium Phytophtora infestans (Late blight of Potato)
► Terrestrial fungi which develop their entire life cycle on living hosts and cannot be grown in laboratory culture
► This fungi caused the Irish potato blight of the 1840's which initiated in Mexico and later spread throughout the world
► Phytophthora infestans, which thrives in cool, moist conditions and is spread by contact or wind-blown spores. The disease, which can destroy a field in days, causes mottled, dark lesions on leaves and stems from which develop a white, velvety growth that kills the plant. Blighted potatoes develop a dark, corky rot and appear dehydrated Late blight of potato
Symptoms Lesions appear on leaf which appear water soaked and necrotic Potato tubers can become infected due to sporangia which arise from lesions of infected leaves and then cause rotting of tubers. Albugo/White rust of Crucifers
► Albugo is a member of oomycote which attacks mostly crucifers like turnip, mustard, radish, cabbage, cauliflower, causing white rust which causes the appearance of shiny, white, smooth irregular patches (pustules) or blisters on the leaves, stems and other aerial parts of the plant. It is an obligate para site ► Along with the white postules there is also an increase in the size of the cells (hypertrophy) and organs takes place. It results in the formation of large galls on the various parts of the host ► It also causes proliferation of the lateral buds, discoloration of flowers, malformation of floral parts and sterile gynoecium. ► Reproduction occurs by sexual and asexual means. Asexual reproduction occurs by means of sporangiophore. In the presence of moisture and low temperature (10°C) the sporangium like zoosporangium and produces zoospores. It absorbs water, swells up, and its contents. Zoospores are uninucleate, slightly concavo- convex and biflagellate. The flagella are attached laterally near the vacuole. Of the two flagella one is of whiplash type and the other tinsel type White rust of crucifers
► Sexual reproduction occurs when the growing season comes to an end.Externally sexual reproduction can be visualized by hypertrophy
► The antheridium and oogonium develops deeper in the host tissue in close association within the intercellular spaces.
► On fertilization oospore is formed
► Each oospore produces about 40-60 zoospores which enter the stoma again White rust of crucifers
► The causal organism perennates as oospore left either in soil or in previous year’s plant debris. Thus the oospore is the source of primary inoculum and primary infections are caused by zoospores that are developed during the germi nation of the oospore with the return of favourable conditions.
► Disease may be controlled by
► Crop rotation
► Removal of infected plant
► Spray of Bordeaux mixture Disease cycle of white rust Downy mildew of grapes ( Plasmopara viticola)
► It is member of Pernosporales ► The term “mildew” refers to the cottony white growth that develops on infected tissue under moist conditions. It is a serious disease in the united states, Australia, and other parts of the world where grapes are grown. ► The fungus causes direct yield losses by rotting inflorescences, berries, clusters and shoots. Indirect losses can result from premature defoliation of vines due to foliar infections. ► Symptoms ► On leaves, small, greenish-yellow, translucent spots appear, On the underside of the leaf, the mycelium can be seen within the border of the lesion as a delicate, dense, white to grayish, cotton-like growth ► young berries turn light brown and soft, shatter easily, and under humid conditions are often covered with the downy- like growth of the fungus. Infected fruit never matures normally ► Causal organism: The fungus overwinters in infected leaves on the ground and possibly in diseased shoots. The overwintering spore (oospore) germinates in the spring and produces a different type of spore (sporangium). These sporangia are spread by wind and splashing rain. When plant parts are covered with a film of moisture, the sporangia release small swimming spores, called zoospores. ► Zoospores germinate by producing a germ tube that enters the leaf through stomates present on the lower surface of leaf. The mycelium grows intracellulary and then produces a branched, tree-like structures (sporangiophores) on the lower leaf surface of the leaves.. The small sporangiophores and sporangia make up the cottony, downy mildew growth. Sporangia cause secondary infections and are spread by rain. Disease cycle Disesease control
► Vines are exposed to all-day sun, with good air circulation and soil drainage to reduce the potential of infection
► Sanitation, i.e, to reduce the debris, dead leaves and berries from vines and the ground
► Cultivation practices
► fungicide spray program References
► Maneveldt, G.W. & Keats, D.W. (1997). The chromista. ENGINEERING IN LIFE SCIENCES
► https://www.apsnet.org/edcenter/disandpath/oomycete/pdlessons/Pages/ LateBlight.aspx assessed 8th April, 2020
► http://www.biologydiscussion.com/plants/plant-diseases/2-major-diseases- of-crucifers-plant-diseases/58643 assessed 10 April, 2020
► https://repository.uwc.ac.za/bitstream/handle/10566/1234/ ManeveldtChromista1997.pdf?sequence=1&isAllowed=y
► https://ohioline.osu.edu/factsheet/plpath-fru-33 Questions
► 1. Why are the oomycetes not considered as true fungi?
► 2. Write a note on hormones present in fungi.
► 3. Write a note on white rust of crucifers.
► 4. Write a note on Sparolegnia.
► 5. write a note on damping off of seedlings
► 6. Write a note on late blight of Potato
► 7. Differentiate white rust of crucifers and finger and toe of crucifers
► 8. Name the causal organism of Downy mildew of Grapes