Histopathology of Seed-Borne Infections - Applied Research Center ...

174 Histopathology of Seed-Borne Infectionsfor Rhodococcus facians (Corynebacterium fascians) in strawberry and cauliflowerare reported to be essential as vectors of these pathogens (Hunger, 1901; Carne,1926; Vasudeva and Hingorani, 1952; Crosse and Pitcher, 1952; Pitcher and Crosse,1958).6.1.2 SPREAD IN PLANT AND COURSE OF ENTRY INTO OVARYAND FRUIT AND OVULE AND SEEDAfter entering the plant tissue, the bacteria multiply in a substomatal cavity(Figure 6.1A, C, E) or space beneath the site of entry. Within the host, the bacteriafollow two courses for further spread: they invade xylem elements and move verticallyafter coming in contact with vascular elements and they multiply and spreadin intercellular spaces (Figure 6.1B, D). The latter course usually results in arestricted spread. Vascular bacteria can move over long distances, and they can alsospread laterally after dissolving the walls of the vessels (Smith, 1911; Yu, 1933;Pine, Grogan, and Hewitt, 1955) or after escaping through openings (pits) in thewalls of vessel elements (Ivanoff, 1933; Wolf and Nelson, 1969). Nelson and Dickey(1966) have also suggested lateral movement of bacteria through plasmodesmatalconnections in the pit membranes of vessels. Association of bacteria with phloemis observed, but it probably plays no significant role in their spread (Pine, Grogan,and Hewitt, 1955).Several seed infecting bacteria are systemic vascular pathogens (Zaumeyer,1930; Pine, Grogan, and Hewitt, 1955; Cormack and Moffat, 1956; Mukerjee andSingh, 1983; Du Plessis, 1990). Pine, Grogan, and Hewitt (1955) have given adetailed account of the spread of C. m. subsp. michiganensis, a bacterial cankerpathogen, in young tomato plants and provided evidence of longitudinal and lateralmovement of the organism in xylem elements (Figure 6.4A to D). Bacterial pocketswere formed in phloem and pith, but they gave no indication of vertical movement.During earlier studies, C. m. subsp. michiganensis was described primarily as aphloem parasite (Smith, 1914, 1920). However, Zaumeyer (1930) has also observedinvasion of xylem elements in leaf (Figure 6.1F) and stem (Figure 6.1G) by X. a.pv. phaseoli, and its subsequent spread throughout the organism. X. oryzae pv. oryzaehas a similar course in rice (Mukerjee and Singh, 1983).The systemic vascular infection often enters the reproductive shoot and causesovary and fruit infection (Zaumeyer, 1930; Cormack and Moffatt, 1956; Mukerjeeand Singh, 1983; Du Plessis, 1990). Zaumeyer (1930) traced the passage of X. a.pv. phaseoli from plant to pods in beans and noted that the bacteria in the xylemvessels of the stem travels up and through the vascular elements of the pedicel andpasses into the vascular supply of the pod. Mukerjee and Singh (1983) observedthat X. o. pv. oryzae reaches the rice husk through vessels of the infected rachilla.Skoric (1927) reported that Pseudomonas syringae pv. pisi is largely a parenchymainvader and only occasionally may enter the vessels under field conditions.The bacterium penetrates pods through wounds and spreads in intercellular spaces,forming abundant slime on the inner side of the pod. After artificial inoculation byP. s. pv. lachrymans of the stem 1.5 m from the fruit, Kritzman and Zutra (1983)observed systemic invasion of the cucumber fruits. No histopathological studies were