In this article we will discuss about:- 1. Introduction to Damping-Off of Seedlings 2. Symptoms of Damping-Off of Seedlings 3. Causal Organism 4. Disease Cycle 5. Control.
Contents:
- Introduction to Damping-Off of Seedlings
- Symptoms of Damping-Off of Seedlings
- Causal Organism of Damping-Off of Seedlings
- Disease Cycle of Damping-Off of Seedlings
- Control of Damping-Off of Seedlings
1. Introduction to Damping-Off of Seedlings:
Damping-off is a term applied to a very common trouble in which seedlings are attacked by soil-inhabiting fungi usually just at the soil level on to hypocotyl or upper taproot causing partial or complete rot and the seedlings suddenly topple over in a characteristic manner.
Of the various species of soil-inhabiting fungi responsible for damping-off of seedlings, Pythium debaryanum ranks first in importance, in relation to this disease as a damping-off parasite.
The damping-off of seedling disease has been known for a long while both in Europe and America. It was first studied in Germany by Hesse in 1874 and De Bary in 1881. In the United States this type of disease attracted attention by the end of the nineteenth century and the beginning of the twentieth century.
As early as 1901 to 1905, however, the damping-off disease began to receive attention as a trouble in forest nursery seed beds.
This disease is encountered in seed beds where plants are propagated to be transplanted later to the field. But green houses and nursery seed beds are ideal places for outbreaks of damping-off if moisture and temperature conditions are favourable.
Almost any kind of plant may be attacked by damping-off fungi while in young, tender, succulent stage of development. But the disease incidence is dependent more upon the conditions under which the seedlings are grown than upon the particular species of plant concerned.
2. Symptoms of Damping-Off of Seedlings:
The most striking symptom is the sudden toppling over of the seedlings. The fungus attacks the seedlings at or near the surface of the ground, according to the moisture level and depth of planting, on the hypocotyl or taproot (Fig. 353 A & B). The cell wall of the rapidly growing seedlings is generally thin, and as such the tissue is particularly vulnerable.
In a very short time the pathogen causes upon invasion, rapid killing and rot of cells (Fig. 353C), and collapse of the tissue to such an extent that the seedlings suddenly fall over (Fig. 353B).
This occurs so quickly that the seedlings are found in a prostrate condition even before the top has had time to wilt. Often the damage may be complete before emergence of the seedlings above ground. After the seedlings have fallen over they may continue to decay, if the ground is wet and the air humid, until entirely rotted away, or if drier conditions prevail the top may wilt and dry up after toppling over.
Damping-off of seedlings actually occurs in two stages:
(i) Pre-emergence stage
(ii) Post-emergence stage.
(i) Pre-Emergence Stage:
This is the stage when the young seedlings are killed before they reach the surface of the soil when the hypocotyl has just emerged. The radicle and the plumule are destroyed as soon as they come out of the seed coat. Since this happens below the surface of the soil, it remains unnoticed and as such the cause of failure of seedling emergence is given to the poor quality of the seed.
(ii) Post-Emergence Stage:
During this stage the seedlings are out on the surface of the soil and on being infected by the pathogen they topple over and lie on the surface of the soil. This toppling is due to the killing of host tissue by the pathogen at or below the ground level. Initially the infected areas appear soft and water-soaked.
With the further growth of the pathogen the stem or the hypocotyl, as the case may be, becomes constricted and the seedlings collapse. The seedlings remaining erect on the surface of the soil may thus collapse any time.
The disease usually radiates from initial infection points causing areas of dead seedlings that have been toppled. The disease is most severe when the moisture content of the soil is medium to high. Under favourable conditions, the rapidity of onset of the disease may be to such an extent that within a very short time the seedlings of the entire seed bed may collapse.
The losses due to damping-off are confined largely to seed beds, wherever they may be.
3. Causal Organism of Damping-Off of Seedlings:
Pythium debaryanum Hesse, is the common cause of damping-off. It is eminently a soil-borne organism and is of common occurrence in field and garden soils, in which it survives in the form of oospores. It lives in soil saprophytically on dead organic matter or parasitically on the young seedlings of a great many susceptible hosts.
The non-septate somatic hyphae of the organism may survive both intra- and intercellularly without any haustoria.
The organism reproduces asexually by the development of sporangia which are either terminal or intercalary on the somatic hyphae. Depending on the environmental conditions, the sporangia produce either biflagellate secondary zoospores or germ tubes.
The contents of the sporangium are discharged into a protruding vesicle within which a large number of zoospores are developed. After release, the zoospores swarm for some time, come to rest, round off, and germinate to produce somatic hyphae.
The sex organs, oogonia and antheridia, usually appear at a later stage than sporangia and may arise within or outside the substratum.
Paragynous antheridia, of which one or more may invest an oogonium, may arise either from the same, hyphae bearing the oogonium or from a neighbouring branch. Following fertilization by a fertilization tube, each oogonium produces a single spherical oospore which becomes thick-walled.
On the decay of the host tissue, the oospores pass into the soil where they germinate after a period of rest. At high temperatures the oospore germinates directly by germ tube which develops into somatic hyphae. At low temperatures, however, a vesicle is developed at the tip of the germ tube in which zoospores develop. Meiosis occurs in the first division of the diploid oospore nucleus.
4. Disease Cycle of Damping-Off of Seedlings:
The survival of the fungus in the soil suggests that it lives largely as a saprophyte on plant debris or humus in the soil, and becomes parasitic when the proper host and proper growing conditions are present. The temperature and moisture conditions play a very large part in determining the severity of infection.
Abundant moisture and a fairly warm temperature are conducive to invasion of the seedlings. Soil temperatures of 20°C to 28°C. are ideal for the host infection by direct penetration. Abundance of moisture in the surface layer of the soil due to overwatering, thick sowing in the seed bed causing heavy stands, lack of aeration of the surface soil, and too much shade are conditions which favour damping-off.
Dissemination of inoculum takes place through oospores in the plant debris and mycelium remaining in the soil. A very common source of contamination in seed beds is in the soil or compost used in making up the seed beds. Sometimes these materials are obtained from an infested source and of course the organism is introduced along with the soil or compost.
Disease cycle of Damping-off is presented in Figure 354.
5. Control of Damping-Off of Seedlings:
Since the fungus responsible for causing damping-off trouble is soil- borne and since the temperature and moisture factors of the environment play important role in determining the severity of infection, control measures are largely based on:
(i) Proper seed bed soil preparation
(ii) Soil treatment
(iii) Seed treatment
(iv) The manipulation of watering system
(v) Application of improved cultural practices.
(i) Seed Bed Soil Preparation:
Every effort should be made to secure seed bed soil free from damping-off fungus. The seed bed soil should be light, having good proportion of sand. Pure fine sand or saw dust is also a suitable medium for growing seedlings. An alkaline reaction in the soil favours the growth of the fungus causing damping-off.
As such every effort should be made to reduce alkaline reaction of the soil. Manure used in the seed bed should be well decomposed. The seed bed should be at a raised level to maintain good water drainage.
(ii) Soil Treatment:
The most perfect control of damping-off is by soil treatment. The seed bed soil should be sterilized by suitable methods to make it free from the fungus inducing the disease. Some of the common methods of soil sterilization are by using steam, dry heat, and chemicals. Steaming of soil should be done continuously for thirty minutes.
Sterilization by dry heat is usually accomplished by burning wood upon sped bed soil. Seed bed soil can be sterilized by using formaldehyde diluted in the proportion of 1 part to 50 parts of water to be sprinkled over the loose soil in sufficient amount to soak it to a depth of at least 4 inches, which will mean one gallon for 2-3 square feet of soil.
Soil treatment with formaldehyde should be applied several days before sowing of seeds and the soil should be covered with paper, tarpaulin or other covering until ready for seed sowing.
In using sulphuric acid in seed beds, three-sixteenths fluid ounce dissolved in 1 to 2 pints of water is allowed for each square foot of seed bed if the soil is heavy; a little less is used if the soil is sandy. This acid treatment can be applied without danger immediately after the seed is sown.
The formaldehyde and stream treatment are most commonly used in greenhouses and in seed beds other than in forest nurseries; the sulphuric acid is in common use in forest nurseries. Drenching of seed beds with 0.1 per cent. Captan is also recommended for soil sterilization.
Two sprayings with Bordeaux mixture at the rate of one-half gallon per square foot, one before seed germination, often produce good results. Post-emergence damping-off can be controlled by drenching seedlings sprayed with 0.3 per cent. Blitox-50 or 0.2 per cent. Ziram. Spraying at frequent intervals gives satisfactory results.
The sprays should begin as soon as the first leaves are formed and should be repeated every week as long as damping-off is likely to affect the seedlings. Disinfection of the seed bed soil and control of post-emergence damping-off can also be achieved by drenching the seed bed soil with Bordeaux mixture, 0.2 per cent. Fytolan, 0.5 per cent. Perenox, and 0.2 per cent. Flit 406.
(iii) Seed Treatment:
One of the most effective control measures of the disease is the use of seed-protectants and is a very effective measure against pre-emergence damping-off. The chemicals used as seed-protectants are applied to the seed in dry or in wet form producing a protective layer around the seed coat to enable the seedlings to grow uninterrupted.
Some of the chemicals that have produced effective results are:
Arasan, Agrosan GN, N.I. Ceresan, Captan, Blitox-50, Copper sulphate, Mercuric chloride, and Semesan.
(iv) Manipulation of Watering System:
A system of watering should be maintained to ensure that the seedlings and the surface of the seed bed soil remain dry without permitting the seedlings to suffer from lack of water. A thin layer of sand should be sprinkled over the surface of the soil. This aids in keeping the surface of the seed bed soil dry. Light irrigation at frequent intervals should be given, water-logging should be avoided.
(v) Improved Cultural Practices:
Avoid heavy seeding so that there is no overcrowding of seedlings in the seed bed and thereby no shedding effect is produced. Seedlings are liable to severe infection in poorly aerated soil and growing in shed. To avoid water stagnation, seed bed soil should have proper drainage facilities.