Viruses and Mycoplasma-like organisms as plant pathogens
Symptoms of virus diseases — notably flower-breaking and variegation — have been known for several hundred years, but it is only within the past 70 years or so that viruses have been recognized as causing disease in plants and only within the past 40 years that they have been intensively studied. The number of viruses attacking plants is perhaps about 600 and new ones are still being discovered; precise figures are not available since some viruses exist as a number of strains which, in some cases, have been described as separate viruses. Some diseases, previously ascribed to viruses, are now thought to be due to Mycoplasma-like organisms (p. 76).
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