This summer we had an interesting tomato disease in the diagnostic lab.  It’s tomato pith necrosis, caused by the soil-borne bacterium Pseudomonas corrugata

Necrosis and wilt symptoms on tomato plant caused by tomato pith necrosis.

The earliest symptom is chlorosis or yellowing of the younger leaves.  As the disease progresses, leaves may wilt and become necrotic (dead).  Infected stems may or may not have visible dark lesions.  The sample received in the lab this summer didn’t have this symptom.  The primary symptom in this case was wilting and necrosis of the upper part of the plant. 

To investigate further and to check for wilt diseases of tomato, the lower stem was cut in half longitudinally.  The characteristic symptom of tomato pith necrosis, a chambered or hollow pith or center of the stem, was observed.  In some cases, this is white as shown in the photo.

White chambered pith in tomato stem.

More advanced bacterial colonization results in the browning and softening of this tissue.  The vascular system may also be brown.  This tissue is in the outer part of the stem and the sample’s vascular browning is pictured below.  Two wilt diseases of tomato caused by fungi, Fusarium wilt and Verticillium wilt, both cause browning in the outer, vascular tissue of the stem, but not in the pith. 

Stem section with both vascular and pith browning.

White and brown (decayed) chambered pith.

One symptom that is quite distinctive but that was not readily apparent in this case is the development of many adventitious roots on the outside of the stem near the chambered pith areas.  Adventitious roots are roots that develop from above-ground plant parts.  Sometimes, the infected tomato plant is able to grow out of this disease. 

 Conditions that favor tomato pith necrosis include low night temperatures, high nitrogen fertility and high humidity.  It often occurs when the fruits are nearing mature green, or just before they begin to redden. 

To prevent this disease, do not over-fertilize with nitrogen and space, prune and stake tomato plants to promote good airflow around them, reducing humidity.