About Sterol Biosynthesis Inhibitors
According to mode of action these sterol inhibiting fungicides are divided into 2 groups: (1). Inhibitors of sterol C-14 demethylation (DMIs). (2). Group which interfere with the C-14 reductase.
(1). Inhibitors of sterol C-14 demethylation (DMIs) which affect cytochrome P-450 enzymes. This group includes pyrimidines (fenarimol, nuarimol, etc.), imidazoles (prochloraz), triazoles (bitertanol, triadimefon, propiconazole, penconazole, flutriazole, flusilazole, cyproconazole, difenoconazole) and piparazines (triforine).
(2). The second group includes fungicides which interfere with the C-14 reductase such as morpholines (fenpropimorph and tridemorph).
The SU fungicides combine protective and curative properties and thus interfere with target fungi throughout their infection cycle after penetration. They are very good for use in a disease warning systems. Combination of elemental sulphur improves efficacy of DMI fungicides in the control of pathogen strains that have developed resistance.
Also read: Multiple Choice Questions On Plant Pathology
Pyrimidines
This group includes diamethirimol (Milcurb), ethirimol (Milstem), bupirimate (Nimrod), fenarimol (Rubigan), nuarimol (Trimidal) and triarimol. Diamethirimol was first reported to be specifically systematic in cucurbits and fungitoxic to the powdery mildew fungus Sphaerotheca fuliginea. These fungicides translocate from seed (seed treatment) or roots (granular application in soil) and reach the leaves to control powdery mildews. Protected plants are more vigorous with greater root weight than the unprotected plants. Ethirimol is more effective as soil or seed treatment fungicide than as foliar spray material. Fenarimol and nuarimol are effective against powdery mildews and also several leaf spot fungi as well as rusts and smuts. Triarimol has sytemic and curative effect on apples scab disease. It is also effective against Ustilago striiformis and Urocystis agropyri.
Triazoles
Triazoles are the largest class of fungicides. Some of the best SI fungicides belong to this group. These include triadimefon (Bayleton), triadimenol (Bayton), bitertenol (Bacor), boutrizol (Indar), propiconazole (Tilt), etaconazole (Vangard), and many others such as flusilazole, myclobutanil (Trade name eagle, nova, rally, prohane and systhane), defenoconazole (Ciba Geigy ), penconazole and prothiaconazole (Byer). They show long term protective and curative activity (thus facilitating lesser number of applications and longer intervals between sprays) against many powdery mildews, rusts, smuts, leaf spots and blights. Theya are applied as foliar spray and as seed and soil treatment.
Bayleton is used against ceral rusts and powdery mildew in India. Tilt is very effective against cereal rusts even with one spray. Boutrizol (4-butyl-4H-1, 2, 4-triazole) gives complete control of leaf rust of wheat. In many countries it is extensively used against yellow or stripe rust.
Morpholines
This group includes the fungicides dodeporph (Meltatox) and tridemorph (Calixin). They are preventive and eradicant foliar fungicides effective against powdery mildews and leaf spots on cereals, ornamental and tropical plants. Tridemorph is available in India as emulsifiable concentrate and is used against powdery mildew of wheat, pea and cucurbits. The fungicide is also effective against the stripe or yellow rust of wheat and barley, sigatoka or leaf spot of banana, pink disease of rubber trees and many other diseases of tea, coffee, cereals, etc. The uptake and translocation is acropetal. Dimethomorph, though a morpholine, is quite distinct because of its activity against oomycetes.
The imidazole SI fungicide prochloraz is effective against Ascomycetes and Fungi-imperfecti causing powdery mildews, leaf spots blight and fruit rots. Triforine, a piparazine SI fungicide is a foliar spray fungicide (Cella). It is effective against powdery mildews, leaf and fruit spots, fruits rots, anthracnose, and some rusts.
Also read: Vectors of Plant Diseases
Organic Phosphates
The systemic fungicides based on organic phosphate include fosetyl-AI, Kitazin (IBP), edifenphos (Hinosan) and pyrazophos (Afugan). The alkyl phosphonate fungicide, aluminium-O-ethyl phosphonate, more commonly known as fosetyl-AI is a fungicide against oomycetes after metalaxyl. It has a narrower spectrum of biological activity than metalaxyl but is more effective against some fungi. It has low in vitro and in vivo activity against late blight of potato, blue mold of tobacco, and phytophthora roit rot of soybean but is highly effective against phytophthora diseases of pineapple, avocado, citrus and walnut. The fungicide breaks down in soil or in plant tissue into carbon dioxide and phosphonic acid, the latter being the fungitoxic principle in fungicide. It persists in soil for several months. The fungicide is highly phloem mobile being translocated from leaves to roots. The breakdown product of fosetyl-AI is highly inhibitory to sporangia formation of Phytophthora citrophthora and Phytophthora parasitica.
Fosetyl-AI is very effective against foliar, root, and stem disease caused by Oomycetes such as Pythium. It can be applied as foliar spray, soil drench, root and post harvest dip, as well as soil mix. Treatment remains effective for 2-6 months depending on the crop. Kitazin and Hinosan are highly effective against the blast disease of rice. Afugan is effective against powdery mildews and Helminthosporium diseases.