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Invasion Mechanisms Of Cronartium ribicola in Pinus monticola Bark. Bruce L. Welch, Biological Laboratory Technician, U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station, Ogden, Utah 84401; Neil E. Martin, Research Plant Pathologist, stationed in Logan, Utah, and Moscow, Idaho. Phytopathology 64:1541-1546. Accepted for publication 19 July 1974. DOI: 10.1094/Phyto-64-1541.

In Pinus monticola bark infected with Cronartium ribicola, the main mode of pathogen invasion in the peripheral and midcanker areas of the rust canker is by “passive” growth through host intercellular spaces; there was no evidence of mechanical force being involved. Scanning electron microscopy of P. monticola bark revealed numerous large intercellular spaces in the cortical parenchyma. Intercellular space accounted for about 30% of bark volume. Evidence is provided of pectinase activity and mechanical force in the total invasion of the aecial area. Scanning electron microscopy also revealed that C. ribicola hyphae are frequently affixed to host cell walls. An hypothesis concerning the physiological significance of this phenomenon is discussed.