Damping off of seedling

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1. Damping off of seedling By : Dr Alok

2. SUBJECT : BOTANY CLASS : B.Sc (BIO) YEAR : I YEAR (Semester 1 ) PAPER : I TOPIC : MICROBIOLOGY AND PLANT-PATHOLOGY SUB TOPIC : PLANT-PATHOLOGY KEYWORDS : DAMPING-OFF OF SEEDLINGS PREPARED BY : DR.ALOK DESIGNATION : ASSISTANT PROFESSOR DEPARTMENT : BOTANY E MAIL : [email protected] FAULTY: SCIENCE COLLEGE NAME: S.K.B.B. GOVT.P.G.COLLGE, HARAKH, BARABANKI, U.P AFFILIATION: DR.R.M.L.AWADH UNIVERSITY, AYODHYA SELF DECLARATION The information given in the E-content/material/manuscript is authentic to the best of my knowledge .The use of above said content is purely for academic/educational purpose to promote teaching and learning. DR.ALOK

3. Damping off disease of seeding is widely distributed all over the world. This disease occurs in vegetables, flower, cereals and many fruits and forest trees. This condition damages or weakens the seedlings during the process of germination either before or after emergence. Poor seed germination of seedling is due to dumping off infections in pre-emergence stage. Older plants are not killed but develop stem lesions or root rot, thus retards their growth. It is caused by different pathogens and is mostly found in wet and cool conditions. Damping-off is caused by several seed-borne and soil-borne fungi including Rhizoctonia solani and Aphanomyces cochlioides . It is also caused by oomycetes and different fungal species which include Pythium, Fusarium , Phytophthora , Botrytis, Cylindrocladium , Diplodia , Phoma , and Alternaria . Damping off caused by these pathogens has a severe impact on the plant species and can cause great loss to economically important food crops .

4. Damping Off Symptoms Seeds in infested soils fail to germinate, they become soft and mushy, turn brown, shrink, and finally disintegrates. In initial infections appears as a slightly darkened, water soaked spot.Infected area enlarges rapidly, the invaded cells collapseband the seedling is overrun by the fungus and dies shortly. Rotting of seeds before the process of germination causing the seedlings to die before emerging is the first symptom of damping off. This process is also regarded as Pre-emergence damping-off . Rotting of young seedlings around the crown region, resulting in softening of tissue, wilting, and falling over of plants is another common symptom known as Post-emergence damping-off . In most cases, the seedlings turn to form a dull green colour along with the drooping of cotyledons. This is mainly caused by Pythium spp. which also causes the development of water-soaked lesions on the hypocotyls at the soil line resulting in seedlings wilt and collapse.  Seedlings that have already emerged are usually attacked at the root below the soil line. The succulent tissues are invaded and killed rapidly, infected areas become water soaked and cells soon collapses. The basal part of seed is much thinner and softer than upper part. Due to infection basal part can not support upper part and the seedling falls over on the soil.

5. Causal organism Several fungi have been reported to be responsible for damping off of seedling. These are species of Rhizoctonia , Fusarium , Phytopthora and Pythium . Pythium appears to be the most important . Several species of this genus are involved P. debaryanum , P. aphanidermatum , P. ultimatum but the effect of each of them on its hosts is usually similar to that of others. Here we discuss about Pythium species Kingdom-Fungi Division- Eumycota Class-Oomycetes Order- Perenosporales Family- Pythiaceae Genus- Pythium Species- debaryanum •Species of Pythium grow in water and soil as saprotrophs. •Intolerant of highly acidic soils. •Grow commonly where seedlings are grown crowded together in poorly drained soil. •Produce branched colonies of tip-growing hyphae. •Reproduction can be asexual and sexual. •Pythium are Necrotrophic pathogens i.e. they kill the cells which they attack.

6. Most species are plant parasites . Pythium penetrates plant directly by physical force and by using enzymes like pectinase or cellulase . Pectinases break down pectins in the middle lamella causing cells to part and tissues to break up . Cellulases result in plant cell wall disintegration . Morphology: The mycelium of Pythium is white, slender, profusely branched, with coenocytic hyphae. They have terminal and intercalary sporangia, which may be spherical or filamentous. Sporangia produces zoospores, oogonia , antheridia, and thick-walled oospores. Sporangia germinate and form vesicles(zoosporangium) in which zoospores are formed. Zoospores ovoid and biflagellate. Zoospores released in favorable condition and infect host by germ tube. Reproduction: Pythium reproduces both asexually and sexually. Asexual mode is most common while sexual reproduction occurs mostly towards the end of the growing season. Asexual reproduction is favoured by humid conditions. The reproductive phase sets on before the death of the host in case of parasitic species.

7. Asexual reproduction: Pythium spp . produce sack-like structures called sporangia. Sporangium germinate to produce a hypha or give rise to zoospores. Zoospores that are formed in a vesicle and get released when the vesicle wall ruptures. After a period, the zoospores slow their movements and become quiescent, retract their flagella, secrete a wall around themselves, and become encysted to form cystospores . C ystospores on the arrival of favourable conditions, either directly germinate to establish a new mycelium, or give rise to secondary zoospores which then germinate to establish the mycelium. Sexual reproduction: Sexual reproductive structures include an oogonium and a club-shaped antheridium . The two structures form from a common strand of hyphae, or form two different hyphal strands . Two structures come in contact, a fertilization tube enters the oogonium from the antheridium, and the antheridial nucleus is transferred to the egg within the oogonium . Then antheridial and oogonial nuclei unite and form a zygote. The wall of the egg cell then thickens creating an oospore which contains the zygote. The oospore can survive periods of drying and remain viable for many years.

8. The infected area enlarges rapidly, the invaded cells collapse . The basal part of the stem of the seedlings is much thinner and softer than the above. The invaded area become water-soaked and discoloured, and the cells soon collapse. Cotyledon and leaf chlorosis occurs and the plant soon shows signs of infection in the form of wilting, result in its death. Control: Soil disinfection with chemicals like Formalin, Captain, Thiram , Blitox-50. Soil sterilization by steam or dry heat. Seed protectants,which includes several type of chemicals as phygon,agrosan GN,ceresin . These are applied to seed in dry or wet form. Seed treatment followed by spraying of seedings with ziram , chloranil , captan , soluble copper etc. Cultural practices that includes chiefly good drainage, improvement of soil aeration, check on excessive use of nitrate forms of fertilisers etc.