Spongospora subterranea , an aquatic fungus now classified as Protozoa, is best known to cause powdery scab in potatoes, but it is also capable of attacking many other nightshades, whether cultivated or not. Its symptoms on tomato roots are quite rarely reported. However, this plant is sensitive to this obligate parasite which first produces white pustules on its roots which rapidly develop into tumors. The latter gradually turn brown and become superficially corky (Figures 1 and 2).
  

Although sometimes very spectacular, these root tumors do not seem to have a significant impact on the development of the tomato. On this plant, this fungus bears fruit quite poorly and does not perpetuate there. Mature zoosporangia can still be observed in some cells of the cortex and in rootlets (Figure 3), which confirms the identification of S. subterranea .

For additional information on this fungus, you can consult the sheet Spongospora subterranea fact .