W. M. Bugbee
U. S. Department of Agriculture, Agricultural Research Service
Department of Plant Pathology
North Dakota Agricultural Experiment Station
Fargo, North Dakota
Bacterial root inhabitants:
Freshly harvested roots of sugarbeet possess low levels of acid invertase activity.
Acid invertase and invert sugars increase and sucrose decreases during root storage.
Enzymatic destruction of sucrose and accumulation of invert sugars in stored roots of
sugarbeet was caused by at least three forms of microbial invertase and not by an acid
invertase of root origin. Invertase producing bacteria were found within and between xylem
and live parenchyma cells of healthy sugarbeet roots. Invertase activity was not detected
in recently harvested roots, but there was activity in roots stored at 4 to 6 C, 20 to 23
C, or in artificially aged root tissue. Comparison of molecular weight (MOO) of invertases
separated on gel columns showed two acid invertases from stored roots were similar to
those of yeast (205,000) and bacteria (158,000). Acid invertase (presumably of sugarbeet
origin) from aged root tissue had an estimated MW of 33,000. Extracts from roots kept at
20 to 23 C for 21 days also had a neutral invertase, MW 18,200, that was similar to
invertase found in extracts of bacteria isolated from sugarbeet roots.
Scanning electron microscopy showed openings in root cell walls large enough for bacteria to pass through. Some of the holes appeared to originate in primary pit fields. Bacteria were adjacent to these holes. Some of the bacteria that were isolated from roots possessed cellulase but none were pectolytic. The lack of pectolytic activity by these parasitic bacteria makes it difficult to theorize that bacteria cause local wall dissolution because pectinase would be required for dissolution of the middle lamella. Therefore, the mechanism of cell penetration by bacterial parasites was not determined.
Three attempts to grow sterile sugarbeets in a germ free chamber failed because of our inability to completely disinfect seedlings and failure to establish complete sterility within the chamber.
Cercospora and systemic fungicides:
The lack of leaf spot control this past growing season after systemic fungicides were
applied may have been due, in part, to strains of Cercospora that were resistant to the
fungicides. Three problem fields in the Renville district were sampled and all of the 36
cultures of Cercospora were resistant to the systemic fungicides Mertect, Benlate and
Topsin-M. Fifteen Cercospora cultures from four fields in the Min-Dak district were
sensitive to these fungicides.
1981 Sugarbeet Research and Extension Reports. Volume 12, page 155.