Many fungi of native origin are pathogenic to specific species of weeds. Such fungi are used to kill the weeds selectively and eliminate them from the area. These weeds-killing fungi are called mycoherbicides. eg. Phytopthora palmivora is used to control the weed Morrenia odorata.

Production of Mycoherbicides:

The phytopathogenic fungus that attacks the target weed in a large proportion is isolated from the infected weed species. It is cultured in a suitable nutrient medium. Fungal spores are isolated in a suitable nutrient medium. Fungal spores are isolated and sprayed on a wide variety of plant species including weeds and main crops to assess the host range of the fungus. If the fungus attacks the target weed species alone, it is elected as my mycoherbicide for that weed.

The successful mycoherbicide must produce abundant spores in culture medium. And it must be efficient in a broad range of climatic conditions.

Methods of Applications: Some mycoherbicides can multiply on the target weeds and spread rapidly. In addition, they persist in the killed weeds for a long time in the form of spores till new weeds appear. These mycoherbicides are introduced are introduced here and there in the field for weed control. This method is called inoculation or introduction. e.g. Puccinia chondrillina and Cercospora rodmanii.

Some mycoherbicides kill weeds and get destroyed. So a known volume of a mycoherbicide is diluted and sprayed directly over the plants. This method is called inundative method. The fungus cannot persist for the forth coming seasons. e.g. Phytopthora palmivora.

Important Mycoherbicides:

Puccinia chondrillina causes rust in Chondrilla juncea and kills the weed. It is applied by inoculation method.

Cercospora rodmanii is pathogenic to water hyacinth, Eichhornia crassipes. Therefore, it is inoculated in ponds and lakes to control overgrowth of water hyacinth. In the USA, it is available in the name ABG5003.

The spores suspension of Phytopthora palmivora is sprayed on milk weed vine (Morrenia odorata) in citrus orchards to control the weed. It is available in the name De VINE.
Spores of Alternaria cassia and sprayed on sickle weed Cassis obtusifolia among soyabean and groundnut to kill the weed. It is available in the name CASST.

The preparation containing spores of Colletotrichum coccoides is used to control Abutilon theophassti. It is available in the trade name VELGO.

COLLEGO is the trade name of fungal preparation containing spore of Colletotrichum gloeosporioides f.sp. aeschynomene. It is used to control Aeschynomene virginica, a commom weed in paddy fields.

LUBOA2 is the trade name of spores of Colletotrichum gloeosporioides f.sp. cussutae. It is used to control various species in Cuscuta.

BioMal is the trade name of spores of Colletotrichum gloeosporioides f.sp. malvae. It is used to control the weed Malva pusilla.

Some insects cause serious illness to weeds. They are actually pests for the weeds. They are known as weed-killing insects.

When weed-killing insects are released into an area, they attack the weed and multiply till the weed is destroyed. Then they disappear due to shortage of food. So there is no risk in releasing the insects in nature.

A few examples of weed control using insects are discussed here under:

Parthenium: Parthenium hysteroporus has been controlled by releasing Mexican beetle (Zygogramma bicolorata) into the area. This beetle feeds on all parts of Parthenium and breeds very rapidly within 30-35 days. As a result the growth of Parthenium is suppressed.
Lantana: Lantana camara is a serious weed occurring in all parts of the world.

The growth of Lantana is suppressed by using the following insects-

• Caterpillers of Plusia verticillata.
• Lantana caterpillar (Lannophaga pusillidactyla).
• Larvae of seedfly (Agromyza lantanae).
• Lantana gall fly (Aspondylla lantanae).
• Lace bug (Teleonemia scrupulosa).
• Lantana bug (Ortheza insignis).

Water hyacinth (Eichhornia crassipes) is a free floating weed plant found in water bodies. The weevils Neochentia olchhorniae and N. brunchi in the USA are pests for water hyacinth.

Salvia molesta (a water fern) is controlled by a weevil Cyrtobagous salviniae.
Opuntia: The prickly desert plant Opuntia cannot be destroyed easily by man, because of the presence of thorns. Two insects namely Dactylopius coccus and D. opuntiae are effective biocontrol agents against Opuntia.

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