The control of plant diseases

Puffy seedbeds can increase the risk of take-all in cereals. Over-consolidation or compaction can lead to poor growth and other diseases. 

In plant breeding, although the breeding of resistant varieties is better understood, it is not by any means simple. For some years plant breeders concentrated on what is called single or major gene resistance. However, with few exceptions, this resistance is overcome by the development of new races of the fungns to which the gene is not resistant. 

Some breeding programmes are now concentrating on mnltigene or field resistance , which means that a variety has the characteristics to tolerate infection from a wide range of races with little lowering of yield. Emphasis is now on tolerance rather than resistance. 

Variety resistance is the main method of control for some fungal-transmitted virases snch as barley yellow mosaic vims. 

The risk in any year of a serious disease infection can be reduced on farms growing cereals if a range of varieties are grown that have different disease resistance ratings and resistance to different races of the disease. NIAB produces variety diversification tables for yellow mst in wheat and mildew in bariey. 

---

The avermectins, a family of compounds with potent anthelmintic, insecticidal and acaricidal activity, have vividly demonstrated that fermentation products can have entirely unanticipated activities. Besides their utility in animals, they show great promise for the control of insect pests of plants. Although antibiotics have found only a limited role in the control of plant diseases, the desire to find environmentally acceptable alternatives to the chemicals currently used has prompted new research efforts to discover fermentation products for use as pesticides. 

Fermentation products have played a rather minor role in the control of plant diseases. Table VI gives a classification of agents used on plants. These are divided into pesticides and growth modulators. The pesticides are classified as bactericides, fungicides, insecticides, miticides, nematicides and herbicides. There are fermentation products in each of these categories, and these are listed in Table VII. 

Highly toxic groups will be found. They may be used to step up old nematocides or bring new ones to the fore. However, soil nematocides are contact killers and as such will be supplanted by chemotherapy. Plants will be fed materials toxic to the nematode. Such chemicals (stomach poisons) are already here, as indicated by sodium selenate. Plant parasitic nematodes can bathe in it, but unless it is absorbed by the plant and thus fed to the nematode, no injury is done. The mode of action of parathion is still uncertain. Plant parasitic nematodes placed in a saturated aqueous solution certainly survive for long periods, yet it has been found effective in the control of certain nematodes in living plants 16y 0), The next step is the introduction into the plant of nutriments to replace those taken from the plant by the nematode. The final step is the introduction into the plant of neutralizers of nemic enzymes. This may have already been done in the case of parathion, for it is said to inhibit cholinesterase formation in other animals. Chemotherapy will then have its day, but when that day is done physics may be expected to take over liie burdens of the control of plant diseases. We can expect the use of ultrasonics and radioactive fertilizers. 

Azolyl-0,N-acetals. Significant progress was achieved in the control of plant diseases with the discovery of the highly active class of so-called "triazolyl-0,N-acetal" fungicides at the beginning of the nineteen seventies. These compounds are 0,N-acetals of 2-ketoalde-hydes or 2-hydroxyaldehydes in which the triazole residue forms a part of the 0,N-acetal function. 

ALLELOPATHIC ORGANISMS AND MOLECULES PROMISING BIOREGULATORS FOR THE CONTROL OF PLANT DISEASES, WEEDS, AND OTHER PESTS... 

Resistant of plant pathogens to antibiotics Tolerance or resistance of pathogenic microorganisms to antibiotics has occurred shortly after application of antibiotics for the control of plant diseases as shown in Table VI. In order to reduce or avoid the emergence of tolerant fungi and bacteria in the fields, the alternate or combined application of chemicals with different mechanisms of action is recommended. 

II. They are still used today, primarily for the control of plant diseases and as wood preservatives. They are, however, usually toxic to a wide range of organisms, a characteristic which is often not desirable (except in the case of wood preservatives). They also are generally less effective than many of the organic compounds. Some do have relatively low acute toxicity to humans. 

Since their discovery in the late 1960s several compounds from the chemical class of 1-substituted imidazoles and 1,2,4-triazoles have been developed and successfully used for the control of plant diseases and for the treatment of human fungal infections. The first commercial triazole compound was triadimefon ( 1J, introduced by Bayer in 1973 for the control of powdery mildew, rusts, and seed-borne diseases of cereals. Since that time many other so called "azole-fungicides" have been introduced into crop protection (2) and others are still being developed. 

Forty-six different specific fungicide and bactericide modes of action are actually classified in the FRAC lists [3], including the unknown modes of action (Table 12.1). In addition, numerous multi-site inhibitors and plant defense inducers are available for the control of plant diseases worldwide. Therefore, a sufficient diversity of modes of action seems to be available for the control of plant diseases and for an effective resistance management. However, many fungicides are only available in a restricted number of regions and crops because they may not be registered everywhere or because the market size may not be big enough. 

Although chitin was isolated by Braconnot in 1811 for the first time from the cell walls of mushrooms, and chitosan was found in 1894 by Hoppe-Seyler, they remained unused biological resources for more than 100 years. However, interest in these abundant carbohydrates increased rapidly from the 1960s. The enormons increase in the number of relevant research papers and patents revealed a surprisingly high level of chitin/chitosan research activity from both academic and industrial scientists. Many literatnres reported that CTS (chitin/chitosan and their derivatives) have antimicrobial and plant-defense elicit function (Albersheim and Darvill 1985) therefore, CTS are considered nsefnl pesticides in the control of plant disease. 

---