Disease control, fungicides

Key words diseases of crop plants, disease symptoms, disease cycles, disease control, fungicide resistance. 

The fungicides are among the chemicals of wide use as plant diseases control agents in intensive agriculture. At the same time these pesticides could have toxic effects, when accumulated in man and animals. Because of general society concern about fungicides use, they should be monitored in waters, soils and crops. 

In the European Union (EU) replacement of copper-based fungicides with other methods of disease control is a priority in organic farming policy (Anonymous, 2002). They were due to be prohibited by law for use in organic farming in the EU from March 2002 but the ban was delayed because of the increased risk of crop diseases and associated economic losses for organic producers in the medium to long term, until effective alternative... 

The detached leaf assays indicated that 625 ppm CAY-1 and sampangine provided effective protectant activity for disease control of anthracnose on the leaf surface (Table 1). These compounds were as effective as the commercial fungicide, azoxystrobin. 

Cultural practices such as irrigation are effective in reducing aflatoxin contamination of peanut and com (54), but this practice is not always available or cost effective to growers. Other conventional disease control practices, such as the use of fungicides, are largely ineffective in controlling A. flavus infection of crops when utilized at concentrations that are cost effective as well as environmentally safe. 

The advances in disease control made in Europe and the associated increases in crop yields prompted further research in the US. Whilst the ensuing collaborative efforts of the French and American pathologists undoubtedly impacted upon the development of, in particular, copper and sulfur fungicides, such opportunities for travel between continents are never missed by Nature and it is likely that the appearance in France at that time of Guignardia bidwellii, another pathogen of vines native to the New World, was no coincidence. 

An important factor in the redistribution of fungicides throughout a target crop is their level of vapour phase activity. Compounds that are relatively immobile in the plant may be extensively redistributed through the vapour phase and effect commercially acceptable disease control. A good example is fenpropimorph. 

Well over 100 plant pathogens have become resistant to various fungicides under field conditions. Failure of the acyl alanines, benzimidazoles, thiophanates, carboxanilides, dicarboximides, hydroxypyrimidines, some organophosphates, and most of the antibiotics has occurred. In other cases, a moderate decrease in sensitivity without a rapid loss of disease control has been observed as in the case of sterol biosynthesis inhibitors (triazoles, pyrimidines, and imidazoles) and organophosphates. The most effective approach is to use fungicides having different modes of action in combination,... 

Pesticides have been widely and heavily (approximately 93 000 tons in 1995) used for the control of weeds (as herbicides), insects (as insecticides), and diseases (as fungicides) in the Great Lakes Basin [2,3], and to a lesser extent in urban settings [4], In terms of mass applied, herbicides (accounting for two-thirds of the total) are used more extensively than insecticides in Ontario [5] and in the USA [4], Corn and soybeans are the major crops receiving large amounts of herbicides in the Great Lakes Basin. 

The organic fungicides of the dithiocarbamate and phthalimide type (e.g. Captan) were a breakthrough in this field in the nineteen thirties and forties. Although they only have protective activity and thus must be used prophylactically, they found broad applications due to their high plant compatibility and broad disease control spectrum. 

The much wider disease control spectrum of the benzimidazole fungicides (eg. benomyl, BCM, thiabendazole) - permitted far wider usage. In the beginning, these were suitable for control of numerous plant diseases, but a new phenomenon soon emerged - resistance Due to the specific mode of action of these fungicides, resistance could appear quite rapidly. The conventional fungicides previously used had a broad biocidal activity and resistance had never been experienced. 

Within the past ten years, the market introduction of several new types of fungicides has significantly improved the prospects of controlling the Oomycetes. They belong to five different chemical classes the carbamates, the isoxazoles, the cyanoacetamide oximes, the etheyl phosphonates, and the acylalanines and related compounds. The chemical structures of those chemicals that have reached the commercial level are shown in Figures 3-5 (29, revised). Trade names, formulations and first reports are summarized in Table II (29, revised). The biological characteristics of these new fungicides and their impact on disease control have been reviewed by several authors (10, 16, 27, 28, 29, 33). 

Schwinn, F.J., and Urech, P.A. 1981. New approaches for chemical disease control in fruit and hops. Proc. Brit. Crop Prot. Conf. Insecticides, fungicides 3, 819-833. 

This paper will consider the basis for the principles of disease control by non-fungicidal compounds, the current research results, and the prospects for their future use. Induction of disease resistance by biotic agents will also be included because non-fungicidal substances might be involved in this process. 

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