Resistant varieties

Chemical treatment of diseases is not common, because of legislative controls and costs caused by the difficulty of appHcation through the leaf canopy. Breeding of resistant varieties is the main weapon for disease control. Some diseases, chiefly ratoon stunting disease, are controlled by hot water treatment of cane (6,8). 

Figure 29.10. Estimates of softening or decomposition temperature (whichever is lower) of most heat-resistant varieties of different polymer species...

From a practical review, perhaps it can be stated that buildings and construction materials are exposed to the most severe environments on earth, particularity when the long time factor is included. The environments include such conditions as temperature, ultraviolet, wind, snow, corrosion, hail, wear and tear, etc. Basically the following inherent potentials continue to be realized in different plastics ease of maintenance, light weight, flexibility of component design, combine with other materials, corrosion/abrasion/weather resistance, variety of colors and decorative appearance, multiplicity of form, ease of fabrication by mass production techniques, and total cost advantages (combinations of base materials, manufacture and installation). 

Urethane Liquid Exceptional abrasion, cut, and tear resistance. Poor moisture and heat resistance. Variety of formulations leading to different properties including range of durometers without plasticizers. Antistatic rollers and tires, hose for transfer of flammables, strain gages, pressure transducers. 

Despite vast efforts over 20 years, rice remains one of the most salt-sensitive of crops. Efforts have centred on screening the existing germplasm, and the more resistant varieties have proved to be older, traditional (and therefore non-dwarfed) land races. Although some of their resistance has been incorporated into modern varieties, these have not increased in resistance beyond that of their parents, and do not achieve the resistance of barley or wheat (Ponnamperuma, 1984). 

While it is conceivable that an excess of bases in the cell solution might be protective against mild sulfur burn, this possibility has not yet been tested. On the other hand, a small increase in buffer capacity might reduce sulfur burn. An example of this effect may be seen in the buffer curves of the leaf sap in two of the United States Department of Agriculture s muskmelon varieties. No. 5, which is susceptible to sulfur burn, has a buffer curve which lies 0.2 to 0.3 pH unit closer to the acid side than the buffer curve of the sulfur burn-resistant variety, No. 11353 (Figure 1). 

Disease-resistant varieties are attractive because they should pose little or no risk to the environment and enable growers to reduce and in some cases eliminate the need for pesticides. In some host-pathogen systems, resistance may persist for many years, but in others it may be short-lived (Koike et al., 2000). Unfortunately, resistance is not available to counter every disease and for some of the most damaging ones, such as tomato late blight (P. infestans) and white rot (Sclerotium cepivorum) of alliums, no acceptable resistant varieties are currently available. 

In bean hypocotyl, the increase of resistance in the course of ripening was found to be accompanied292 by a decrease in the meth-oxyl content from 0.5 to 0.2%, while the content of calcium increased from 0.38 to 1.92%. An increased content of calcium was observed in a resistant variety of cucumber hypocotyl.293 These results suggest that one of the protective mechanisms of the plant against infection is the conversion of pectic acid into calcium pectate, which is resistant to the action of pectic enzymes. 

While insect control is more often a limiting factor in tropical bean production, there are several diseases that are serious in certain seasons and locations. Bean rust is one of the most widespread diseases, and it can be controlled effectively with protective fungicides, such as elemental sulfur and certain of the carbamic acid derivatives. The economics of bean production usually preclude any costly applications, however, and the problem has generally been turned over to the plant breeder to solve with resistant varieties. At present, the most practical control of bean anthracnose and the bean blights is through the use of clean seed and resistant varieties. Control with fungicides has always proved difficult and of doubtful value. 

The time-honored method of controlling insect-borne virus diseases is by breeding resistant varieties. This has been practical in annual crops, but is hopelessly slow in tree crops, where it may take 20 years or more to test a new variety. What is needed desperately is some sort of treatment which will control the virus, probably a systemic treatment, as the virus works within the plant cells. This is not a new idea and work has been done along this line by many workers. A sense of urgency is inevitable, however, when 500 to 600 acres of citrus can be wiped out completely in 3 to 5 years time, followed by an expensive replanting job and a wait of 5 to 6 years to get back into production. This is the outstanding problem at the present time and may need years to answer. 

Viruses are too small to be seen with conventional microscopy, so they are usually named after the plant In which they were first discovered and the symptoms they cause. Control is restricted to planting virus-free material, the use of resistant varieties, and controlling the insect vectors of the virus. 

Above) Seed potatoes Virus-free certification programs exist for seed potatoes and some fruits look for these, and for disease-resistant varieties. 

What to do Grow in a well-drained site. Grow a resistant variety such as Avonresister. Earth up in summer to stop the spores of black canker from reaching the roots. Use close spacing to produce smaller roots that may be less susceptible to some cankers. Control carrot fly (q.v.). Use a crop rotation. 

Oxidant problems are starting to be of concern to plant-breeders. Extensive varietal screening of tomato, petunia, and other plants has permitted some resistant cultivars to be recommended for use in high-oxidant areas. None of the varietal screens have involved breeding experiments in which resistant lines are developed and used in the development of resistant varieties for new introductions. 

Earlier studies indicated that wheat cultivars responded differently to these organisms (15). If necessary, it should be possible to develop resistant varieties, especially When we know the mechanisms of plant uptake and mode of action of the toxin within the plant. One biological control may be the development of a TOX-negative inoculum for seed treatment. These bacteria may act like other rhizobacteria, which are known to increase plant growth, apparently by displacing nonbeneficial bacteria in the plant rhlzosphere (14). 

Caballero, P. and Smith, C.M., Isoflavones from an insect-resistant variety of soybean and the molecular structure of afrormosin, J. Nat. Prod., 49, 1126, 1986. 

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