WASTES IN RELATION TO AGRICULTURE AND FORESTRY 



103 



is reduced by 12 percent by cotton diseases each 

 year. When such major crops as wheat, corn, and 

 cotton are reduced by plant diseases to the extent 

 noted above, the magnitude of the problem comes 

 into focus, and the benefits to be derived from 

 curtailing these losses become significant. 



A great deal has been accomplished and is be- 

 ing accomplished to reduce the waste associated 

 with plant-disease-causing fungi, bacteria, nema- 

 todes, and viruses. The reduction has been achieved 

 by applying the results of continuing research 

 programs by the U.S. Department of Agriculture, 

 the State agricultural experiment stations, and 

 private industry. 



Chemical controls have been effective for many 

 diseases, especially on high-value crops or when 

 small amounts of the chemical will control the dis- 

 ease on low-value crops. However, this type of con- 

 trol has added to costs of production and in some 

 cases has had undesirable side effects in the form 

 of undesirable residues. 



Many diseases are controlled through the use of 

 resistant varieties, and breeding for resistance to 

 diseases is potentially the most effective and effi- 

 cient method for controlling many crop maladies. 

 However, the evolution of new disease organisms 

 that can attack such varieties remains a constant 

 threat, and control by resistance has not been found 

 or used successfully for numerous destructive 

 diseases. 



Neither chemical control nor plant resistance is 

 available for certain diseases, notably those attack- 

 ing the roots of plants. These are some of our most 

 sinister pathogens because the infected organs are 

 out of sight beneath the soil, and the losses may be 

 attributed to drought, infertility, or anything but 

 disease organisms. The effects of these organisms 

 can be lessened to some extent by various cultural 

 control measures such as crop rotation and sanita- 

 tion, but these provide only small measures of 

 relief. 



Thus, it is obvious that disease coni rols present ly 

 available arc no; sufficient to prevent all of the 

 waste caused by plant disease agents. 



All methods of control discussed should be ren- 

 dered more effective through diligent research 

 efforts. Chemicals that will not contaminate the 

 environment, but will provide better and more 

 economical control of diseases and nematodes 

 should be developed. Additional effort could pro- 



vide pesticides with greater specificity, improved 

 modes of action, and less persistence in soil and 

 water. Methods of application must be improved, 

 especially for nematocides and fungicides applied 

 to (lie soil. Viricides need to be developed to con- 

 trol virus diseases. 



To date, the breeding of crop plants for resist- 

 ance to diseases and nematodes has been largely 

 empirical. Additional research is needed on the 

 nature of such resistance, emphasizing the genetic 

 and biochemical systems of both the host and the 

 pathogen so that resistant crop plants may be 

 bred on a more scientific basis. Extensive explora- 

 tion for additional resistant strains should be 

 made, particularly in the areas of the world where 

 our crops originated. 



Research on the control of plant pathogens that 

 contaminate the soil needs particular attention, 

 since few are subject to control by either chemi- 

 cals or breeding. The use of crop rotation, soil and 

 water management, and other cultural practices 

 can be made more effective by basic research to 

 understand how the organisms survive in the soil 

 and compete with other soil flora and fauna, and 

 where they may be vulnerable to control by man- 

 agement practices. 



Greater attention must be paid to integrated 

 control measures for specific diseases. This type of 

 control brings several methods to bear simulta- 

 neously on the problem. In the past we have tended 

 to concentrate on only one type of control. For 

 example, control of the rusts of grain crops has 

 depended almost exclusively on breeding for resist- 

 ance. This has had disastrous result- from time 

 to time when the resistance, frequently consisting 

 of a single gene in the crop plant, has broken down. 

 We need to have available many types o( rust re- 

 sisi mice, including safe, cheap chemicals and other 

 methods that can he applied for rust control when 

 necessary. 



Although the potato blight epidemic in Ireland 

 occurred more than 100 years ago, we still know 

 very little about plant disease epidemics. We need 

 extensive research programs where plant patholo- 

 gists cooperate with climatologists, meteorologists, 

 and ecologists to work out the basic principles o{ 

 plant disease development and spread. 



Finally, in these days of vastly increased air- 

 plane travel and with the supersonic air transport 

 age almost upon us. we must he able to cope with 



