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

Seed Infection by Bacteria 191in the intestinal tract of corn flea beetle (Chaetocnema pullicaria). Robert (1955)found that 10 to 70% of the beetles emerging from hibernation carried the bacterialwilt organism, and up to 75% of the beetles feeding on corn in midsummer may actas carriers.Vegetative cells of bacteria are not subject to dormancy; still, many are tolerantto desiccation and survive relatively long periods under dry conditions as mentionedin the preceding paragraphs. Do these bacteria have any means of protection in theseed? They occur in varied conditions in seeds as contaminants of the seed surfacein the form of loose cells or embedded in the bacterial slime or ooze, in seed tissuesin intercellular spaces, including the vascular elements (xylem). They may occur inseed galls along with the nematode, e.g., R. tritici or exclusively, R. rathayi. R.rathayi cells are closely packed in a regular fashion along the wall of the gall andalso within the gall (Bird, Stynes, and Thompson, 1980). Bird, Stynes, and Thompson(1980) regard this as an anhydrobiotic state, although they have not carried outstudies on the survival of the bacterium. Chand (1967) observed that R. triticisurvived in soil debris under laboratory conditions for about 7 months only, whereasMathur and Ahmad (1964) had reported that the bacterium remained viable for atleast 5 years in the gall of A. tritici.Bacterial ooze or slime has been considered protective to bacterial cells (Rosen,1929, 1938; Hildebrand, 1939). Erwinia amylovora is believed to be very susceptibleto desiccation, but survives a long time in dry exudate (Rosen, 1929, 1938). Hildebrand(1939) recovered virulent bacterial cells of E. amylovora from dry exudateafter 15 and 25 months, but the organism survived only 13 days in moist exudate.Leach et al. (1957) found an appreciable number of viable X. a. pv. phaseoli cellsin exudate for as long as 1325 days under different conditions.Bacteria present in seed tissues may have better chances of survival than thosepresent on the seed surface. However, environmental conditions, inherent seed factors(structural features), and the inherent characteristics of the pathogen affectsurvival as well as transmission. Schuster and Coyne (1974) have reviewed theliterature on this aspect. It may, however, be indicated that complete information onthe above aspects is not available even for one disease.6.4 CONCLUDING REMARKSGood information is available on initial penetration, multiplication, and spread ofbacteria in plant tissues and penetration of ovule and seed (Zaumeyer, 1930, 1932;Skoric, 1927; Cook, Larson, and Walker, 1952; Wiles and Walker, 1951; Pine,Grogan, and Hewitt, 1955; Getz, Fulbright, and Stephens, 1983; Mukerjee and Singh,1983; Tabei, 1967; Tabei et al., 1988, 1989; Azegami, Tabei, and Fukuda, 1988;Fukuda, Azegami, and Tabei, 1990). There is, however, very little experimentalevidence on penetration and association or establishment of bacterial infection inseeds during the postharvest period. The former may become systemic with longdistance movement through tracheids and vessels or it may be localized, multiplyingand spreading in the intercellular spaces. The course of short distance systemicinfection as shown by Kritzman and Zutra (1983) needs support from histologicalstudies.