PENGENDALIAN HAYATI DAN MEKANISMENYA

I. PENDAHULUAN

Pengendalian hayati dilihat dari aspek ekologi adalah suatu fase dari pengendalian alami. Definisi pengendalian hayati adalah perbuatan parasitoid, predator dan patogen dalam memelihara kepadatan populasi organisme pada tingkat rata-rata yang lebih rendah dari pada apabila perbuatan itu tidak ada (DE BACH, 1964).

Menurut Istikorini (2002), mekanisme pengendalian hayati bisa terjadi melalui berbagai mekanisme, diantaranya :

1. Antagonisme

Mikroorganisme antagonis adalah mikroorganisme yang mempunyai pengaruh merugikan terhadap mikroorganisme lain yang tumbuh dan berasosiasi dengannya. Hal ini biasanya terjadi ketika terjadi persaingan antar mikroorganisme dalam hal ruang hidup, nutrisi dan cekaman faktor lingkungan.

2. ISR (Induced Systemic Resistance) atau Ketahanan terimbas

Ketahanan terimbas adalah ketahanan yang berkembang setelah tanaman diinokulasi lebih awal dengan elisitor biotik (mikroorganisme avirulen, non patogenik, saprofit) dan elisitor abiotik (asam salisilat, asam 2-kloroetil fosfonat).

3. Proteksi silang

Tanaman yang diinokulasi dengan strain virus yang lemah hanya sedikit menderita kerusakan, tetapi akan terlindung dari infeksi strain yang kuat. Strain yang dilemahkan antara lain dapat dibuat dengan pemanasan in vivo, pendinginan in vivo dan dengan asam nitrit.

2. AGENS HAYATI dan HABITATNYA

Pengertian agens hayati menurut FAO (1988) adalah mikroorganisme, baik yang terjadi secara alami seperti bakteri, cendawan, virus dan protozoa, maupun hasil rekayasa genetik (genetically modified microorganisms) yang digunakan untuk mengendalikan organisme pengganggu tumbuhan (OPT). Pengertian ini hanya mencakup mikroorganisme, padahal agens hayati tidak hanya meliputi mikroorganisme, tetapi juga organisme yang ukurannya lebih besar dan dapat dilihat secara kasat mata seperti predator atau parasitoid untuk membunuh serangga. Dengan demikian, pengertian agens hayati perlu dilengkapi dengan kriteria menurut FAO (1997), yaituorganisme yang dapat berkembang biak sendiri seperti parasitoid, predator, parasit, artropoda pemakan tumbuhan, dan patogen.

Lebih jauh, jika diperhatikan Peraturan Menteri Pertanian Nomor 411 tahun 1995 tentang pengertian agens hayati maka maknanya menjadi lebih sempurna lagi, yaitu setiap organisme yang meliputi spesies, subspesies, varietas, semua jenis serangga, nematoda, protozoa, cendawan (fungi), bakteri, virus, mikoplasma, serta organisme lainnya dalam semua tahap perkembangannya yang dapat dipergunakan untuk keperluan pengendalian hama dan penyakit atau organisme pengganggu, proses produksi, pengolahan hasil pertanian, dan berbagai keperluan lainnya (Menteri Pertanian RI 1995). Definisi terakhir mempunyai pengertian bahwa agens hayati tidak hanya digunakan untuk mengendalikan OPT, tetapi juga mencakup pengertian penggunaannya untuk mengendalikan jasad pengganggu pada proses produksi dan pengolahan hasil pertanian.

Secara keseluruhan habitat hidup mikroorganisme yang banyak berperan di dalam pengendalian hayati adalah di dalam tanah disekitar akar tumbuhan (rizosfir) atau di atas daun, balang, bunge, dan buah (fillosfir). Mikroorganisme yang bisa hidup pada daerah rizosfir sangat sesuai digunakan sebagai agen pengendalian hayati ini mengingat bahwa rizosfir adalah daerah yang utama dimana akar tumbuhan terbuka terhadap serangan patogen. Jika terdapat mikroorganisme antagonis padd deerah ini patogen akan berhadapan selama menyebar dan menginfeksi akar. Keadaan ini disebut hambatan alamiah mikroba dan jarang dijumpai, rnikroba antagonis ini sangat potensial dikembangkan sebagai agen pengendalian hayati (Weller 1988).

BUILDING SOIL FOR BETTER CROPS

Part 2

When soil organisms and roots go about their normal functions of getting energy for growth from organic molecules they “respire” — using oxygen and releasing carbon dioxide to the atmosphere. (Of course, as we take our essential breaths of air, we do the same.) An entire field can be viewed as breathing as if it is one large organism. The soil is like an organism in another way too — a field also may get “sick” in the sense that it becomes incapable of supporting healthy plants.

The organisms living in the soil, both large and small, play a significant role in maintaining a healthy soil system and healthy plants. One of the main reasons we are interested in these organisms

is because of their role in breaking down organic residues and incorporating them into the soil. Soil organisms influence every aspect of decomposition and nutrient availability. As organic materials are decomposed, nutrients become available to plants, humus is produced, soil aggregates are formed, channels are created for water infiltration and better aeration, and those residues originally on the surface are brought deeper into the soil.

We classify soil organisms in several different ways. Each organism can be discussed separately or all organisms that do the same types of things can be discussed as a group. We also

can look at soil organisms according to their role in the decomposition of organic materials. For example, organisms that use fresh residues as their source of food are called primary (1°), or first-level, consumers of organic materials. Many of these primary consumers break down large pieces of residues into smaller fragments. Secondary (2°) consumers are organisms that feed on the primary consumers them14 selves or their waste products. Tertiary (3°) consumer then feed on the secondary consumers.

Another way to treat organisms is by general size, such as very small, small, medium, large,

and very large. This is how we will discuss soil organisms in this chapter. There is constant interaction among the organisms living in the soil. Some organisms help other organisms, as when bacteria that live inside the earthworm’s digestive system help decompose organic matter. Although there are

many examples of such mutually beneficial symbiotic relationships, an intense competition occurs among most of the diverse organisms in healthy soils. Organisms may directly compete

with each other for the same food. Some organisms naturally feed on others — nematodes may

feed on fungi, bacteria, or other nematodes, and some fungi trap and kill nematodes.

Some soil organisms can harm plants either by causing disease or by being parasites. In other

words, there are “good” as well as “bad” bacteria, fungi, nematodes, and insects. One of the goals of agricultural production systems should be to create conditions that enhance the growth of beneficial organisms, which are the vast majority, while decreasing populations of those few

that are potentially harmful.

SOIL MICROORGANISMS

Microorganisms are very small forms of life that can sometimes live as single cells, although many also form colonies of cells. A microscope is usually needed to see individual cells of these organisms. Many more microorganisms exist in topsoil, where food sources are plentiful, than in subsoil. They are especially abundant immediately next to plant roots, where sloughed off cells and chemicals released by roots provide ready food sources. These organisms are important primary decomposers of organic matter, but they do other things, such as providing nitrogen through fixation to help growing plants. Soil microorganisms have had another direct importance for humans — they are the origin of most of the antibiotic medicines we use to fight various diseases.

Bacteria

Bacteria live in almost any habitat. They are found inside the digestive system of animals, in the ocean and fresh water, in compost piles (even at temperatures over 130°F), and in soils. They are very plentiful in soils; a single teaspoon of topsoil may contain more than 50 million bacteria.

Although some kinds of bacteria live in flooded soils without oxygen, most require wellaerated soils. In general, bacteria tend to do better in neutral soils than in acid soils. In addition to being among the first organisms to begin decomposing residues in the soil,

bacteria benefit plants by increasing nutrient availability. For example, many bacteria dissolve phosphorus, making it more available for plants to use. Bacteria are also very helpful in providing nitrogen to plants. Although nitrogen is needed in large amounts by plants, it is often deficient in agricultural soils. You may wonder how soils can be deficient in nitrogen when we are surrounded

by it — 78 percent of the air we breathe is composed of nitrogen gas. Yet plants as well as animals face the dilemma of the Ancient Mariner, who was adrift at sea without fresh water: “Water, water, everywhere nor any drop to drink.” Unfortunately, neither animals nor plants can use nitrogen gas (N2) for their nutrition. However, some types of bacteria are able to take nitrogen gas from the atmosphere and convert it into a form that plants can use to make amino acids and proteins. This conversion process is known as nitrogen fixation.