Damping-off Diseases

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Damping – off and Fungal Root Rot Diseases : 

Damping – off and Fungal Root Rot Diseases

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Contents Introduction A- Damping – off Diseases Occurrence and importance Symptoms Pythium damping – off Diseaes Diseases cycle Phytophthora damping – off Diseas4es B- Fungal root rot Diseases Rhizoctonia root rot Diseas4es Fusarium root rot Diseas4es C- Management . Field history Cultural practices Soil solarisation Seed treatment Biological control Chemical control

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Root rots, damping-off before and after seedling emergence, and seed rots are destructive diseases of several hosts . These diseases are caused primarily by soilborne fungi and significant losses may occur to susceptible varieties, especially if cool, wet weather conditions prevail for the first few weeks after seeding and then are followed by hot, dry weather. Disease incidence and severity often vary greatly, even in areas with a history of root ro and damping – off diseases. In the same growing season, it is not uncommon to lose a crop completely and then re-seed and experience no problems. This situation results from changes in biological, environmental, and soil conditions.

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Damping – off Diseases

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Damping-off generally refers to sudden plant death in the seedling stage due to the attack of fungi . These fungi are soilborne and are stimulated to grow and infect the seed or seedling by nutrients released from a germinating seed. The effects of the seedling disease may appear as a seed rot (pre-emergence damping off), seedling decay before the seedling emerges (also a pre-emergence damping off), seedling decay after the seedling emerges (post-emergence damping off) .

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Occurrence and Importance Damping-off disease of seedlings is widely distributed and is a problem on a worldwide basis. It occurs in most soils, temperate and tropical climates , and in greenhouses. The disease affects seeds and seedlings of various crops. The amount of damage the disease causes to seedlings depends on the fungus, soil moisture, and temperature. Normally, however, cool wet soils favor development of the disease, depends on the fungi species . However, portions of the roots and stems still can be attacked, resulting in poor growth and reduced yields.

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Symptom s - Seeds : infected seeds becomes soft and mushy turning a brown to black color, and it eventually disintegrates. - Seeds that have germinated and become infected develop water-soaked spots that enlarge and turn brown. The infected tissue collapses, resulting in death of the seedling, this stage called pre- emergence damping –off . - emerged Seedlings are usually attacked at or below the soil line where the hydrolysis occur and followed by necrosis of the juvenile stem then loss of the vigor. The infected stem portion becomes discolored and begins to shrink, the supportive strength of the stem's invaded portion is lost, and the seedling topples over .

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The fungi continue to invade the remaining portion of the seedling, resulting in death. This phase of the disease is termed post - emergence damping-off. ( figure 1). Figure 1: damping – off - The disease often occurs in a roughly circular pattern. This is because of the tendency of fungi to grow radically from the point of origin, which is one of the field marker to distinguish the diseases between another factors that cause the same symptoms .

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Pythium Damping – off Disease

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Pythium spp is a member of class Oomycetes family Pythiaceae, live in soil saprobically on dead organic matter or parasitically on the young seedling of a of seed plants. The species most often encountered are Figure 2: sporangia and oospores of Pythium spp Pythium deparyanum, P. ultimum, P. aphanidermatum, and P. graminicola .

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Disease Cycle of Pythium Damping – off Disease

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Pythium spp causes damping-off most frequently during the first week of emergence of plant ( sugerbeet ) , while Aphanomyces cochlioides , another causal agent of damping – off does not begin to cause damping-off until after the first week of emergence (Figure 3 ). Also, A. cochlioides causes more extensive stand losses than do Pythium species. Figure 3: occurrence of damping – off caused by Pythium spp and Aphanomyces cochlioide

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Phytophthora Damping – off Disease

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Phytophthora species belong to class Oomycetes, family Pythiaceae. It reproduce sexually by producing oospores and asexually by production of sporangia which contain zoospores (Figure 5) . P. cactorum, P. fragaria, P. palmivora, and P. syringae cause primarily low stem rot , damping – off of vegetables , forest trees, and ornamentals . Figure 5: Phytophthora spp Unlike Pythium, Phytophthora is aggressive in warmer soil temperatures (15-23° C) , but still cool condition . Flooding, along with warm temperature within one week after planting, are favorable for development of Phytophthora damping-off.

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- Soil type is another factor that influences Phytophthora disease. Poorly drained or compacted soils are favorable to Phytophthora . Phytophthora risk is higher for no-till fields than tilled fields. Symptoms Initially affected tissue develops a soft, watery brown rot. Within several days, the affected plant parts may dry out and shrivel up becoming dark, dry and brittle. This early stage Phytophthora is difficult to distinguish from Pythium damping-off

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Fungal Root Rot Diseases

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There are many individual diseases within this group but most of any economic important are caused by fungi. In each there is a progressive rotting and root decay of the root system which often involves the basal portion of the stem ( foot rot ) and, as a result , the plant cannot obtain the water and nutrients it needs. This gives rise to a number of symptoms of the shoot : growth is checked and the plant becomes stunted , the leaves yellow and wilt , some drop and eventually the plant collapse and dies ( Figure 6) Root rot pathogen attack only the cortical tissues provide the appropriate milien for their activities Figure 6: fungal root rot

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Rhizoctonia Root Rot Disease

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Rhizoctonia solani belong to class Deuteromycets , order Ceratobasidiales, it exist primarily as a sterile mycelium and , sometimes as small sclerotia that show no internal tissue differentiation . In older seedling the invasion of the fungus is limited to the outer cortical tissue which develop elongate tan to reddish – brown lesion . The region may increase in length and width until they finally girdle the stem and the plant may die . Rhizoctonia solani

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Root rot Disease Cycle

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Fusarium root rot Disease

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Fusarium species belong to class Deuteromycets, family Tuberculariaceae . The fungus produces only asexual spores , although under certain condition it produces its perithecial stage , Netria haematococca . The asexual spores are microconidia , macroconidia , and thick-walled chlamidospores ( Figure 9 ) Figure 9 : Fusarium solani

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Tap roots of young plant show a reddish discoloration that later becomes darker and larger . The discoloration may cover the tap root and the stem below the soil line without a definite margin , or it may appear as streaks extending up to the soil line . Longitudinal cracks appear along the main root , whereas small lateral root are killed . Symptoms

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Management

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Field history Prior to planting it is important to be familiar with the history of a field which can provide clues important to understanding the cause of stand establishment problems. Consider the past and present climate and production history of the field. A number of factors cause symptoms similar to seedling diseases, including insect damage, wind injury, heat, drought, frost, insecticides, soil fertility, and misapplication, drift, or carryover of herbicides.

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Prevention Prevention is most important: -         avoid contamination of the growing medium and purchase quality seed -         Using a soil-less medium -         Keep flats off of dirt floors. Hang up the hose ends -         Avoid use of low, poorly drained areas nursery production -         Restrict use of machinery in infested areas, particularly when soil is wet -         Prevent movement of soil from infested to non infested areas of nursery -         Avoid over watering to puddle or run-off point - Avoid movement of infected trees within and between nurseries

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Cultural practices - providing good soil drainage and good air circulation among plants -         planting when temperatures are favorable for fast plant growth -         avoiding application of excessive amounts of nitrate   - -     - crop rotation . -         Delay planting until the soil is warm (above 65°F) and seed shallow   -Avoid planting seeds too close together         -Using of resistant plant cultivars

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Soil Solarization Solarization of soil by covering it with transparent polyethylene traps is effective in controlling soil borne pathogen higher soil temperature and a more rapid decline in pathogen population density occurred in soils irrigated after replacement of the than in those preirrigated and then traped Seed Treatment Since the possibility exists for recontamination of pasteurized soil and thus for damping-off losses to occur, a fungicide seed treatment should be considered to minimize losses. Examples of fungicides used as seed treatments include Thiram and Captan.

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Biological Control Tow types of disease suppression: specific and general . A good example of specific suppression is provided by a strategy used to control one of the organisms that cause damping - off Rhizoctonia solani. by the beneficial fungi, Trichoderma, locate then attack Rhizoctonia through a chemical released by the pathogen. Beneficial fungal strands (hyphae) entangle the pathogen and release enzymes that dehydrate Rhizoctonia cells, eventually killing them (Figure 10) Figure 10: the beneficial fungus Trichoderma wrap around the pathogenic fungus Rhiozoctonia

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Protection from the pathogen Fusarium oxysporum using Mycorrhizal fungi The mycorrhizal fungi protect plant roots from diseases in several way - By providing antagonistic chemicals. Mycorrhizal fungi can produce a variety of antibiotics and other toxins that act against pathogenic organisms. - By competing with the pathogen -By increasing the nutrient-uptake ability of plant roots. For example, improved phosphorus uptake in the host plant has commonly been associated with Mychorrhizal fungi. When plants are not stressed for nutrients they are better able to tolerate or resist disease-causing organisms. -By changing the amount and type of plant root exudates. Pathogens dependent on certain exudates will be disadvantaged as the exudates

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chemical Control -         Use of fungicide-treated seeds will minimize problems with damping off and root rots., most effective and recommended fungicide are Subdue, Truban, Banol, Terraclor, Domain, Banrot, and Captan, Benlat and Tachigaren -  Fumigate nursery soils with methyl bromide, chloropcrin; maintain good soil drainage in nurseries.

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Thank you

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