Induction of Disease Symptoms in Barley by Powdery Mildew

Powdery millews usually grow superficially on their hosts but produce symptoms of disease throughout the underlying host tissues. The symptoms include marked increase in respiratory rate (1, 3, 15. 24, 26, 30. 35); accumulation of organic and inorganic substances (1,29, 36): increase in dry weight (10. 26) ; increase in wet weight (3) decrease in photosynthesis when the disease is well developed (1. 26. 29) chlorosis at mildew colony centers with redevelopmient of chlorophyll at those centers in older colonies (1), and retention of green pigment around colonies on (letache(d or senescent leaves (10, 29, 33). Simiilar symptoms occur in rust infected leaves (25. 29. 30, 31. 35. 36). The processes leading to the prodiuction of these symptoms are unknown. It has been suggested that diffusible substances produce the respiratory increase (1) and the accumulation phenomena (29), but there has been little evidence that such substances are involved. The respiratory rates of mildewed and rusted tissues are quite high and several workers have prepared extracts of such diseased tissue in the hope of finding substances which produce large respiratory increases in healthy tissue. Gretschushnikoff (11) reporte(l respiratory stimulation by crude extracts of several varieties of rusted plants. He attributed the activity to urea andl ammonia. More recently, Millerd and Scott (14) reported that phosphate buffer extracts of mildewed barley would produce respiratory stimulation of healthy tissue. Others have used similar methods and have found no consistent differences between extracts of healthy and diseased tissues in ability to stimulate respiration of healthy tissue (8, 9). While respiratory increase is a symptom of many other plant diseases, only two compounds have been identified as the causative agents of such increaseethylene (5, 37) and victorin (12, 13). Both compounds probably act as uncoupling agents and. as Allen concluded in review (2), the alterations in metabolism of plant tissue under attack by obligate parasites do not conform to the changes produced by uncoupling agents.