1. Introduction

1. INTRODUCTION Biodiversity refers to the variability of life all the living species of animals, plants and microorganisms on earth. According to Hawksworth (2002), fungi are a major component of biodiversities, essential for the survival of other organisms and are crucial in global ecological processes. Fungi being ubiquitous organisms, occur in all types of habitats and are the most adaptable organisms. The soil is one of the most important habitats for microorganisms like bacteria, fungi, yeasts, nematodes, etc. The filamentous fungi are the major contributors to soil biomass (Alexander, 1977). They form the major group of organotrophic organisms responsible for the decomposition of organic compounds. Their activity participates in the biodeterioration and biodegradation of toxic substances in the soil (Rangaswami and Bagyaraj, 1998). It has been found that more number of genera and species of fungi exist in soil than in any other environment (Nagmani et al., 2005). Contributing to the nutrient cycle and the maintenance of ecosystem fungi play an important role in soil formation, soil fertility, soil structure and soil improvement (Pan et al., 2008). Fungi take a very important position in structure and function of ecosystem. They decompose organic matter from humus, nutrients, assimilate soil carbon and fix organic nutrients. An intense study of the abundance and diversity of soil microorganisms can divulge their role in nutrient recycling in ecosystem. Studies on the soil mycoflora in chilli field of Thiruvarur district and biological control of 1 Pythium debaryanum Hesse 1.1 Biological control of Pythium debaryanum in Chilli pepper (Capsicum annuum L.) Systematic Position Class - Dicotyledons Order - Solanales Family - Solanaceae Genus - Capsicum Species - annuum Capsicum (Capsicum annuum L.) is an important spices crop extensively cultivated throughout the tropics and Southern countries such as Bangladesh, India, Pakistan and Sri Lanka. The genus Capsicum belongs to the family Solanaceae (Night shade). The members of the Solanaceae family are mostly herbs or undershurbs while some others are climbers. The family contains about 90 genera and nearly 3000 species (Vidhyarttie and Tripathi, 2002; Sterm, 2000). Capsicum is a crop that is widely cultivated because of its spicy nature and nutritional value. The crop accounts for a large portion of Vitamins A and C in many Nigerian diets. Heiser and Smith (1953) distinguished two Capsicum species cultivated as vegetables while varieties are all forms of either Capsicum annuum or C. frutescens. Capsicum annuum is not known in a wild state and species commonly cultivated are Capsicum annuum known as sweet pepper, bell pepper, cherry pepper and green pepper (Messraen, 1992). Studies on the soil mycoflora in chilli field of Thiruvarur district and biological control of 2 Pythium debaryanum Hesse Chilli pepper is better adopted to hot weather than sweet pepper, but it does not set fruit well when night temperatures are greater than 24°C. Optimum day temperatures for chilli pepper growth range from 20 to 30°C. When the temperatures falls below 15°C or exceeds 32°C for extended periods, growth and yield are usually reduced. Chilli pepper grows best in a loam or silt loam soil with good water-holding capacity, but can grow on many soil types, as long as the soil is well drained. Soil pH should be between 5.5 and 6.8. Small chillies are much hotter because, proportionally, they contain more seeds and veins than large specimens. Those seeds and membranes can contain upto 80 percent of the chillies capsaicin, the potent compound that gives chillies their fiery nature. 1.1.1. Common varieties Aleppo Dundicut Niora Super chilli Anaheim Fresno New Mexico Tepin Ancho Guajillo Pasilla TienTsin Bell pepper Hungarian wax Pepperoncini Cascabel Italian sweet pepper Piquin Cayenne Jalapeno Pimento Chilaca Japanese Poblano Chiltepin Mirasol Puya Cubanelle Macho chili Sanaam Dearbol Mulato pepper Serrano Studies on the soil mycoflora in chilli field of Thiruvarur district and biological control of 3 Pythium debaryanum Hesse Red chillies contain high amounts of Vitamin C and Carotene (Pro vitamin A). Yellow and especially green chillies (which are essentially unripe fruit) contain considerably a lower amount of both the substances. In addition, peppers are a good source of Vitamin B and Vitamin B6 in particular. They are very high in potassium, magnesium and iron. Their high Vitamin C content can also substantially increase the uptake of non-heme iron from other ingredients in a meal, such as beans and grains. India is the leading country in the world in chilli production with an area of 9,08,400 ha and the production of 9,70,800 tonnes of dry chillies. Recently, chilli is gaining greater importance in the global market because of its value – added products and diverse uses. This important chillies suffer from many diseases caused by fungi, bacteria, viruses, nematodes and also abiotic stresses. Pythium species are essentially soil-borne pathogenic fungi, that cause seed rot and damping off of many crops including chilli and tomato (Shah-Smith and Burns, 1996). 1.2 Causal organism - Pythium debaryanum Systematic Position Class : Phycomycetes Order : Pythiales Family : Pythiaceae Genus : Pythium Species : debaryanum Pythium species are fungal like organisms (Oomycetes), commonly referred to as water moulds, which naturally exist in soil and water as saprophytes, feeding on organic matter. Some Pythium species can cause serious diseases on greenhouse Studies on the soil mycoflora in chilli field of Thiruvarur district and biological control of 4 Pythium debaryanum Hesse vegetable crops resulting in significant crop losses. Pythium infection leads to damping off in seedlings and crown and root rot in older plants. The genus Pythium is a complex genus containing over 200 described species that occupy a variety of terrestrial and aquatic ecological habitats (Dick, 2001). Perhaps the most economically important members of this genus are plant pathogens (Hendrix and Campbell, 1973), many of which have a broad host range and cause losses by both pre and post emergence damping off (Erwin and Ribeiro, 1996), as well as by reduction in plant growth and yield, due to root rot (Vander Plaats-Niterink, 1981). Pythium damping off is a very common problem in fields and green houses, where the organisms kill newly emerged seedlings. (Jarvis, 1992). Pythium debaryanum parasitizes seedlings of many plants cause a destructive disease known as damping off. The fungus live saprophytically in the moist, humus soil and infect the hypocotyl of seedlings. Thereafter they live as parasites. 1.2.1. Damping off The single term used to describe underground, soil line or crown rots of seedlings due to unknown causes is damping off. The term actually covers several soil borne diseases of plants and seed borne fungi. Pythium root rot (Pythium sp.) is similar to Rhizoctonia in that it causes damping off of seedlings and foot rot of cuttings. However, infection occurs in cool, wet, poorly drained soils, and by over watering. Infection results in wet odourless rots. When severe, the lower portion of the stem can become slimy and black. Usually, the soft to slimy rotted outer portion of the root can be easily separated from the inner core. Species of Pythium can survive for several years in soil and plant refuse. Studies on the soil mycoflora in chilli field of Thiruvarur district and biological control of 5 Pythium debaryanum Hesse 1.2.2. Symptoms of Damping-off Seeds were infected as soon as moisture penetrates the seed coat or a bit later as the radicle begins to extend, all of which rot immediately under the soil surface (pre- emergence damping-off). This condition, results in a poor, uneven stand of seedlings, often confused with low seed viability cotyledons may break the soil surface and only looking seedlings may suddenly fall over (post-emergence damping-off). Infection results in lesions at or below the soil line. The seedlings will discolour or wilt suddenly, or simply collapse and die. Weak seedlings are especially susceptible to attack by one or more fungi when growing conditions are only slightly unfavourable. Damping-off is easily confused with plant injury caused by insect feeding, excessive fertilization, high levels of soluble salts, excessive heat or cold, excess or insufficient soil moisture, or chemical toxicity in soil. 1.2.3. Disease cycle Pythium spreads by forming sporangia, sack-like structures, each releasing hundreds of swimming zoospores. Zoospores that reach the plant root surface encyst, germinate and colonize the root tissues by producing fine thread-like structures of hyphae forming masses of mycelium. These hyphae release hydrolytic enzymes destroy the root tissue and absorb nutrients as a food source. Pythium forms Oospores and Chlamydospores on decaying plant roots which can survive prolonged adverse conditions in soil, greenhouse growing media and water, leading to subsequent infections. Studies on the soil mycoflora in chilli field of Thiruvarur district and biological control of 6 Pythium debaryanum Hesse 1.2.4. Control of plant disease The edaphic environments of plants are inhabited by a wide range of microorganisms including various fungi, bacteria, mycorrhizae etc., which may be pathogenic or non pathogenic. Certain microbes produce compounds which affect the growth of other microbes and may sometime beneficial to the plants. Such microbes act naturally as biocontrol agents by producing certain compounds which act as growth promoting substances as well as plant protectants. Thus they form a very good alternative over chemical fertilizers. 1.2.5. Biological methods These methods aim at direct protection of plant from pathogens or at eradication or reduction of inoculum by using antagonistic microorganisms. Biological methods include reduction of pathogen inoculums by antagonist, which is achieved by the use of suppressive soil that contains many kinds of antagonistic microorganisms like Trichoderma, Aspergillus, etc., Studies on the soil mycoflora in chilli field of Thiruvarur district and biological control of 7 Pythium debaryanum Hesse 1.2.6.

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