In view of the necessity for the international exchange of plant products especially wheat. It is evident that significant losses from disease in one area may have serious adverse effects, not only locally but internationally as well.
Moreover, the presence of epidemic disease in a single area poses threats to other localities. Where the crop in question is widely grown even though a pathogen may not destroy the host crop.
It may seriously affect the quality of the product; or agricultural products may be damaged in transit and storage by other pathogens. All of this leads to dissatisfaction and economic loss. Thus, plant protection is necessarily an international concern and responsibility.
Wheat (Triticum vulgare)
In any discussion of crops of primary significance to society, the cereal crop must necessarily hold a leading position. They occupy nearly half of the 2.6 billion acres of cultivated land throughout the world. Wheat contribute roughly 80 per cent of the calories necessary for society. Many cereals are widely cultivated, but wheat, rice, and corn have been selected. Because of their major significance as international food crops.
Although wheat and the other cereals are all susceptible to attack by many kinds of plant pathogens. The rust fungi cause greater annual losses in wheat production than all the other pathogenic organisms attacking wheat.
There are three principal rusts of wheat, namely, stem rust, Puccinia graminis var. tritici; stripe rust, Puccinia glumarum; and leaf rust; Puccinia rubigo-vera. Each of these may be a limiting factor in wheat production in areas where conditions are favorable for their development. Although stem rust is by far the most damaging throughout the world.
The organism responsible for this disease is well known and has been extensively studied by plant pathologists throughout the world. Perhaps the greatest contributions to the present understanding of the problem of Puccinia graminis var. tritici on wheat and other grasses have been made by investigators at the University of Minnesota.
It is clearly recognized that the stem rust fungus is a complex consisting of over 250 physiologic races and biotypes. Which have been derived by mutation. But the vast majority are the result of the hybridization of races of stem rust growing on their alternate host. The common barberry and its allies.
Although there is great variation in the known distribution of various races of Puccinia graminis var. tritici. The spores are air-borne, and conceivably any race could become distributed and established throughout the world if climatic and other circumstances were favorable.
Where there are major natural barriers, such as mountain ranges, extensive bodies of water, broad crop barriers, and unfavorable prevailing winds, the natural movement of rust spores is limited.
Fundamentally the control of stem rust is a procedure requiring international cooperation. Local control measures, including the eradication of alternate hosts, i.e., Berberis species and Mahonia species. The use of resistant varieties are of great importance in reducing the number of naturally occurring biotypes and the production of inoculum.
However, successful international control requires careful study of the races and biotypes of stem rust wherever they occur. The regular exchange of information resulting from these studies. Concerted effort to develop breeding stock with factors for resistance to rust. This is a continuous process in which human intelligence is pitted against the challenge of nature.
Chemical treatments for the control of stem rust have long been sought. Elemental sulphur and compounds of sulphur are known to be effective in the control of stem rust when applied as dust, but these have not proven economic.
Recently, interest has centered on the use of organic compounds such as the carbamates which might act systematically within wheat plants to protect them from the attack of stem rust throughout critical periods.
Although this approach to the control of stem rust has not reached the stage of being practical, there is sufficient evidence to justify further research in the hope that systematic compounds may one day become important aids to the control of Puccinia graminis var. tritici.
Stripe rust is severe where low temperature prevail during the growing period, prior to heading. The same varieties grown at lower altitudes or in warm climates may show little evidence of stripe rust, and the disease is unknown as a major factor in wheat production in most of the wheat belt in the United States and northwestern Mexico, and in wheat –producing areas in India and Pakistan.
The primary symptoms of stripe rust are elongate pustules or a series of pustules oriented parallel to the leaf veins. On a single leaf several of these orange-yellow stripes may occur, separated by green leaf tissues; in consequence, stripes rust is sometimes called flag rust. Heads of wheat and stems may also be attacked.
But the characteristic stripes are typical of the disease. The control of Puccinia glumarum can only be accomplished successfully by the use of resistant varieties in those areas where stripe rust is important. Factors for resistance are well known and can be incorporated into locally adapted varieties.
Leaf rust is widespread and under favorable combinations of climatic conditions may do considerable damage, particularly to the winter varieties of bread wheat. Rust attacks principally the leaves and the leaf sheaths and is readily recognized by the typical orange pustules that are produced. It is fairly tolerant to temperature variation but is most damaging under conditions of high humidity and relatively high temperatures.
Although stem, leaf, and stripe rust are the most important limiting factors in wheat production, a considerable number of the other pathogens attacking wheat are also of international importance.
Prominent among these are several species of smuts-loose smuts, Ustilago tritici; stinking smut, Tilletia spp., flag smut, Urocystis tritici and scab, Gibberella zeae; blotch, Septoria nodorum; and take-all, Ophiobolus graminis.
As might be expected, the other small grains, including barley, oats, and rye, are hosts to many of the same or allied forms of the pathogens which attack wheat. Most prominent are the rust and smuts, although root rots, blotches, and blight also do considerable damage each year to these and other small grains.