This section is from the book "The Potato: A Compilation of Information from Every Available Source", by Eugene H. Grubb, W. S. Guilford. Also available from Amazon: The Potato: A Compilation Of Information From Every Available Source.
Dry rot, due to Fusarium oxysporum, has been known in the potato world for a good many years, but the real cause was not understood until recently. A great deal of research work has been done in Germany.
This dry rot is a fungous disease that attacks the potato plant through the root system, which not only destroys the root hairs and secondary roots, but penetrates the main roots, the tubers, and later, when the plant is practically destroyed, the stem is wilted. It also causes dry rot in potatoes in storage.
The disease is well distributed over the potato districts of practically all potato producing countries.
A vast amount of experimenting has been done with the idea of finding the best way of combating the pest. In "Bulletin 229" of the Ohio Agricultural Experiment Station Thos. F. Manns summarizes the results of their work. The things he says about Ohio apply to any similar conditions:
1. The dry-rot fungus (Fusarium oxysporum, Schlecht) of potato proves to be a field trouble common in Ohio, which causes a blight and wilt of the crop.
2. It produces a sick soil condition in potato districts.
3. The field symptoms are characterized by a cessation of growth, a yellowing of the foliage, with an upward and inward rolling of the upper leaves, accompanied by wilt during the heat of the day.
4. The sick soil conditions may reduce the yield to 50 per cent. or more of an average crop.
5. The casual fungus is carried within the tubers.
6. The internal infection is characterized by brown or blackened areas usually in the vascular ring; occasionally it specks the flesh in other areas.
7. Internally infected tubers are the chief means of distributing the disease.
8. The presence of the disease in the tubers may be made known by cutting knife sections from the stem end.
9. The infection may be removed from slightly infected seed by clipping away the stem end and following by external treatment with formaldehyde.
10. No attempt should be made to use deeply infected seed, as the infection cannot be cut away.
11. Slightly infected seed will not materially reduce the yield the first season. It is a means, however, of infecting the soil, which may later result in sick fields.
12. Spraying will not control the disease.
13. Proper storage prevents the progress of the disease as a dry rot.
14. Careful inspection of the seed should be made before placing it in storage. Cellar storage under dwellings should be avoided when seed is infected. Proper pit storage will give better results.
15. A seed plot on non-infected soil planted with carefully selected healthy seed will offer a means of getting a sound seed supply.
16. Sick fields should not be planted in potatoes again for at least five or six years, and even longer time may be required to work the parasitic fungus from the soil. Grass and grain crops will undoubtedly eliminate the fungus from the soil quicker than will manuring and cultivated crops.
Hill of potatoes partly diseased by the dry-rot fungus (Fusarium). From Bulletin 55, Bureau of Plant Industry, United States Department of Agriculture.
Various stages in destruction of potato tubers by Fusarium. From Bulletin 55, Bureau of Plant Industry, United States Department of Agriculture.
17. Longer than a three-year potato rotation should be practised.
18. Storage litter and sick seed should not be allowed to reach the manure pile, as this will be a sure method of distributing the disease and infecting the fields.
19. The disease demands further study. The Department of Botany invites cooperation with potato growers. Examination of seed potatoes and plants will be made and the results reported.
This Department, in cooperation with the Bureau of Plant Industry of the U. S. Department of Agriculture, has the franking privilege on diseased plant material; the franks will be sent to those having diseased material to be forwarded."
Prof. B. O. Longyear of the Department of Botany and Forestry of the Colorado Agricultural College says:
One of the most widespread and common diseases of the potato caused by fungi is that commonly known as the Fusarium disease, or Fusarium blight. This trouble first manifests itself in the field by the wilting and yellowing of the lower leaves of plants that have reached the height of ten to twelve inches. In bad cases the entire foliage appears to suffer as though the plant were not getting sufficient moisture. Later on, the tips of the leaves turn brown and dry up, leading to the trouble commonly known as tip burn. The edges of the leaves commonly roll inward during the heat of the day, although they may partially revive during the night.
Badly affected plants will be found to have the root hairs and rootlets rotted away and often the larger roots appear sickly. Cross sections of the main root often appear brownish in the region of the vascular bundles or woody part. Under the microscope thin sections of such roots and of the lower part of the stem will show the delicate filaments of the fungus which pass upward through the water-conducting tubes of the plant and eventually clog them to such an extent that the flow of sap is greatly obstructed. This is what causes the wilting and eventual drying of the foliage of the plant.
The fungus also passes into the tuber-bearing stems under ground and frequently enters the stem end of the tuber for some distance. In bad cases the stem end of the tuber may be rotted away and the presence of the fungus deeper in is indicated by the browning of the vascular ring shown in* a cross section of the tuber. The fungus may also enter the tuber from the soil through any bruise, crack, or other break in the skin. Attacks of insect larvae upon the tubers are often followed by this disease through the wounds which the ' worms' produce.
Under conditions of plenty of moisture and high temperature, this disease makes its most rapid progress and may reach its culmination at about the time when the tubers are ordinarily half to two thirds grown. When a plant once shows the infection to any marked degree, all further growth ceases. The plants seem to stand still and eventually wilt down entirely or else struggle along in a dwarfed and sickly condition for some time.
A common source of infection in newly planted fields is through the use of tubers for seed that already contain the fungus. Another common source of the trouble is from planting the potato in fields that have previously shown the disease within two or three years. Such soils are said to be 'sick.'
A second period of destruction due to this disease comes during storage. Tubers infected in the field when stored under conditions of moderate temperature are apt to show a high percentage of dry rot. In such cases the fungus causes a blackening of the tuber, with a final outbreak of a whitish mold, and may serve to infect the wounds in other tubers.
In the control of this widespread and destructive disease much emphasis should be placed upon the use of tubers free from the disease. ('Bulletin 229/ Ohio Agricultural Experiment Station.) Experiments have been tried with diseased tubers which indicate that if the diseased portion is largely cut away and the tubers treated with formaldehyde solution in water such seed will give nearly as good results as tubers from healthy plants. Potatoes should not be grown on the same soil immediately following a crop of the same kind which showed the disease. Experiments along this line indicate that even a three-year rotation is too short to make it safe.
Storage of the tubers is also an important matter. It has been found that storing in outdoor pits is preferable to cellar storage, as a rule, and the lower temperature at which the tubers are kept under this method usually prevents, to some extent, the spread of the dry rot. Being a soil fungus and capable of living for several seasons in the soil of an infested field, no spray which can be applied to the part of the plants above ground is effective in controlling the disease."