Storage Decay

W. M. Bugbee
Plant Pathologist
USDA, ARS, PSRD, Fargo

It has been observed for years that immature sugarbeets do not store as well as mature sugarbeets. It has been assumed that the low sucrose content of immature roots accounts for their susceptibility to storage decay. Convincing data to support this assumption is not available. Part of my research for 1971 was directed toward collecting this type of data.

Sugarbeet samples of three varieties were harvested at 2-week intervals beginning August 16. Six of the roots were inoculated with Phoma betae. This fungus is present in our soils and causes storage decay after the roots are harvested. It is potentially dangerous because it can decay sugarbeets at low temperatures (about 50°F) and most other pathogens can not. The inoculated roots were incubated at 50°F for 4 weeks then the decayed areas were measured.

The results in table 1 show that as the sugarbeets matured, the sucrose content increased, as expected. The drop in sucrose during October can be attributed to cool, dark, rainy weather. As sucrose increased in the sugar varieties, the roots became more resistant to Phoma. This was not true for A58, a fodder variety used here because of its susceptibility to decay. A58 remained susceptible up to harvest. The sucrose content of A58 reached a maximum on September 27 as did the sugar varieties.T he disease ratings on that date for 2B and A58 were nearly equal but the sucrose content of A58 was much less than 2B. Also, the drop in sucrose during October was not accompanied by an increase in the disease rating as would be expected if the statement is true that sucrose content governs susceptibility to decay.

A second experiment was performed to precisely correlate the disease rating with sucrose. Sucrose was measured in the most susceptible and resistant root from each of the above varieties. The results in table 2 show that a high disease rating is not always associated with a low sucrose content. For example, in the variety 2B that was harvested on October 27, the sucrose percentage in the two roots was nearly the same but one root was considerably more susceptible than the other.

Another test was designed to see how well Phoma could utilize the cell wall material as a carbon source. The data in table 3 show that the variety as well as the harvest date influence the utilization of this cell wall material. The variety 2B was slightly more resistant from the August 30 and September 13 harvest dates (table 1) and this reflected here as less growth on what is apparently resistant cell wall material. Sucrose is not a factor because it was removed along with other cell components during the extraction process.

One other test was used to examine the maturity-decay relationship. Phoma produces enzymes which break down cell walls and allows the fungus to invade the root tissue. These enzymes were separated from the fungus. Thin slices of root tissue were placed in the enzymes or in water and the loss of strength of the slices placed in the enzymes was expressed as a per cent of the water check. The results in table 4 suggest that as the roots matured, the tissue slices became more susceptible to the fungal enzyme. This does not agree with the fact that as roots mature in the field they become more resistant to the fungus. However, this test is not complete. The results from the final harvest date may clear up or add to the confusion.

One conclusion from these experiments, pending completion of all tests, is that as sugarbeets mature in the field, they become more resistant to an important storage rot fungus, Phoma betae. However, this resistance is not controlled entirely by the sucrose content but perhaps by the composition of the cell wall material of the storage root. If this proves to be true, it means that sugarbeets can be selected for resistance to P. betae without regard to sucrose content; one less factor to consider in a selection program. Once sources of resistance are identified, the factors possibly could be incorporated into sugar varieties in a breeding program.

Evaluation of the World Collection of Beta sp. for resistance to P. betae

One way of alleviating storage losses is to use a sugarbeet variety that is resistant to P. betae. A search of sources of resistance was begun with the world collection. Seeds from 298 collections were planted at the Fargo experiment station, Roots were harvested from 181 of these collections, inoculated with Phoma, incubated at 50°F for five weeks, and then examined. Those that appeared resistant were saved as mother beets. Some of the selections were manger or table as well as sugar types (table 5). If possible, seed will be obtained first from the best specimens of the sugar types. Progeny will again be evaluated for resistance.

Similar testing is being done with all commercial varieties being grown in the Valley and those being introduced. This work is not yet complete.

Fungicide seed treatment and space planting

In the event the practice of planting to a stand comes to the Valley, it would be desirable to know what effect fungicidal seed treatments might have on the establishment of stands. Seed of the variety 2B were treated with some common and successful fungicides listed in table 6. The seed planted with a precision plot planter at a rate of about 1 seed per 4 inches. This was later thinned to 1 per 6 - 8 inches. Another plot was planted at the rate of 1 seed per 8 inches. This was considered as being "planted-to-a-stand". A stand count was taken 30 days later, prior to thinning.

The weather was ideal for emergence. There were no moisture or crusting problems, therefore very little seedling disease. The benefit of the seed treatments: was not evident because of the lack of disease pressure. The stand counts at least suggest that phytotoxicity was not a problem among the various treatments. This would have been especially apparent in the spaced planted plot.


1971 Sugarbeet Research and Extension Reports. Volume 2, pages 32 - 39.


Red River Trade Corridor
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