temperature would not be lowered beyond that 

 of a cold irrigation water, nor raised above that of 

 a warm irrigation water, a desirable range of tem- 

 peratures would be from 55 to 85 F. 



Effect on plant quality 



There are certain water quality considerations 

 which are not directly concerned with plant 

 growth per se. They are significant, however, in 

 that they adversely affect the quality of the plant 

 for its intended use. For example, water may carry 

 microorganisms either directly pathogenic to the 

 plant, or to animals or humans consuming these 

 plants. Water may contain a material which is not 

 toxic to plant growth, but may be absorbed by 

 and accumulate in plants at levels which may be 

 toxic to animals or humans which consume them. 

 Finally, there are materials such as sediment which 

 affect the appearance and, hence, the marketability 

 of the crop. 



Microorganisms, Pathogenic to Plants, 

 Animals or Humans 



In general, the danger of spread of plant patho- 

 gens in irrigation water is so slight that it is usually 

 ignored. Some plant pathogens, however, can sur- 

 vive and be transported in irrigation water. In 

 irrigated areas where runofif water from infected 

 cultivated fields is used again downstream for ir- 

 rigation, there is definite probability that disease 

 organisms will be spread from one field to another. 

 The importance of this is uncertain compared to 

 other means of spread such as dust storms, farm- 

 to-farm movement of farm equipment, or direct 

 wind transport of spores. 



Faulkner and Bolander {54} confirm that large 

 numbers of nematodes, including plant parasites, 

 are transported in irrigation water. No attempt has 

 been made to ascertain the economic importance 

 of nematodes distributed by water. However, there 

 is little doubt that irrigation water could be a sig- 

 nificant source of nematode infestations. Data indi- 

 cate that each time an acre of land in the Lower 

 Yakima Valley is irrigated, it may receive from 

 approximately 4 million to over 10 million plant 

 parasitic nematodes. 



The most likely situation to cause trouble is 

 where the contaminated water is used for over- 

 head sprinkling. Some bacterial diseases, and dis- 

 eases caused by the so-called watermold group of 

 fungi, may be increased by this practice. Root dis- 



ease organisms in general can probably be intro- 

 duced into clean soils this way also. Recommenda- 

 tions have been made in the tobacco growing 

 areas, where the wildfire disease is a problem, that 

 drainage water from infected tobacco fields not be 

 used to irrigate other fields. Also, fruit growers are 

 advised to avoid using drainage water for sprinkler 

 irrigation in orchards ( 109 ) . 



Lack of efforts to control or eliminate plant dis- 

 ease organisms in irrigation water is partiy due to 

 the difficulty of doing anything effective about 

 them. Usually, any plant disease control based on 

 sanitation is limited to the easiest or least expensive 

 procedures because, at best, they are only a partial 

 answer. The disease organisms are microscopic 

 and cannot be screened from water like weed seeds. 

 Chemical treatment of the water is expensive and 

 has many undesirable consequences. 



Water may be assayed for plant pathogens; but 

 there are thousands, or perhaps millions of harm- 

 less microorganisms for every one that causes a 

 plant disease. While such selective bioassays are 

 valuable in research, they are not practical for 

 monitoring. 



If plant disease organisms are to be included in 

 water quality criteria, they should be framed in 

 terms of preventive measures rather than by any 

 assay procedure. For example, dumping of plant 

 material, which could be diseased, into lakes, 

 streams, or irrigation systems should be prohibited. 

 Water used in washing of fresh produce, such as 

 potatoes, may have to be treated before return to 

 water supplies that will be used to irrigate crops. It 

 is also desirable to prevent storage of irrigation 

 water in a quarantined area for downstream use. 

 Pests such as the soybean cyst nematode or other 

 plant nematodes could easily be spread in this way. 



Plant infection is not considered serious unless 

 an economically important percentage of the crop 

 is affected. The real danger is that a trace of plant 

 disease can be spread by water to an uninfected 

 area where it can then spread by other means and 

 become important. It is unlikely that any method 

 of water examination would be as effective in pre- 

 venting this as would be prohibitions such as those 

 suggested above. 



Many microorganisms , pathogenic for either 

 animals or humans, or both, may be carried in ir- 

 rigation water, particularly that derived from sur- 

 face sources. The list comprises a large variety of 

 bacteria, spirochetes, protozoa, helminths, and 

 viruses, which find their way into the irrigation 

 water from municipal and industrial wastes, in- 

 cluding food-processing plants, slaughterhouses, 

 poultry-processing operations, and feedlots. The 

 diseases associated with these organisms include 

 bacillary and amebic dysentery, Salmonella gastro- 



160 



